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Chapter 13: Sexual behavior.

CHAPTER OBJECTIVES

* Define sexual behavior.

* Discuss sexual behavior from the perspective of behavioral ecology.

* Describe general patterns of sexual behavior in male and female mammals.

* Discuss theories describing the regulation of sexual behaviors.

BACKGROUND

Previous chapters emphasized the physiologic mechanisms regulating production of gametes in males and females. Although the production of spermatozoa and oocytes is necessary for sexual reproduction, it is not sufficient on its own. In order for sexual reproduction to occur, a spermatozoon and oocyte must fuse and form a zygote. This aspect of reproduction depends largely on the behaviors of males and females. Any behavior that facilitates conception is commonly referred to as sexual behavior. It should be emphasized from the outset that the sexual behaviors of males and females are interdependent. In other words, behaviors expressed by one member of the mating pair are influenced by and influence the behaviors expressed by the other member. This makes it difficult, if not impossible, to provide completely independent accounts of the sexual behaviors of males and females. With this in mind, it is useful to differentiate between behavioral and ethologic approaches to studying behaviors. The former deals with the behaviors of individual animals, often studied under experimental conditions. The latter field of study deals with expression of behaviors under the social and environmental conditions in which the behavior is normally expressed. In the case of sexual behavior, an ethologic approach focuses on the behaviors expressed when two or more individuals interact sexually. A derivative of ethology is the field of behavioral ecology; that is, the study of the ecologic and evolutionary bases for behavior. The goal of behavioral ecology is to provide an account of how a behavior contributes to an animal's ability to adapt to its environment. The information provided in this chapter will emphasize an ethologic perspective. However, in order to appreciate why animals express various types of sexual behaviors, it is important to have an appreciation for the insights derived from behavioral ecology.

Definition and Causes of Behavior

A behavior is commonly defined as a response to a stimulus. This general definition may seem insufficient because various stimuli induce responses that are not generally regarded as behaviors. For example, stimulation of the cervix of a queen will induce ovulation, but ovulation is typically regarded as a physiologic response, not as a behavior. We generally regard behaviors as the responses of whole organisms to stimuli. Behaviors can be understood from the level of individual animals, groups of animals, or entire species of animals. For example, howling is a behavior expressed by individual wolves as well as packs of wolves. It is also trait that helps characterize the species of Canis lupus. From ecologic and evolutionary perspectives a behavior can be understood to mean an animal's responses to its environment. Sexual behaviors are responses to various stimuli in the context of mating.

What causes behavior? This may seem like a silly question. An obvious, but superficial, answer would be that a stimulus causes a behavior. This question is much more complex than it might first appear to be. Consider howling in canids. What causes these animals to howl? Behavioral ecologists address this issue of causation by differentiating between ultimate and proximate causes. Proximate causes of behavior include so-called ontogenetic variables and mechanistic variables. Ontogenetic variables refer to the development of behavior; that is, how an animal learns to perform a behavior. These variables include the animal's life experiences; for example, interactions with other dogs that howl. Mechanistic variables include the physiologic mechanisms regulating behavior; for example, the physiologic ability to produce this type of vocalization.

Behaviors are also attributed to ultimate causes. These include phylogenetic and adaptive causes. Phylogenetic causes can be viewed as constraints related to the animal's lineage. For example, dogs can howl but they can't vocalize like a cow. Adaptive causes deal with selection of a particular behavior; that is, the extent to which a behavior enables an animal to adapt to its environment. To put this in simpler terms, howling is caused by genes that allow dogs to do so as well as selection pressure that favored this type of vocalization.

The question, "What causes dogs to howl?" is the same as, "Why do dogs howl?" Based on the aforementioned analysis, an appropriate answer would be as follows. Dogs howl because they express a particular set of genes (an ultimate cause) that give rise to physiologic mechanisms (a proximate cause) that allows dogs to vocalize in this way. Moreover, they howl because this trait was selected for (an ultimate cause) and they live in an environment in which they learn how to do it (a proximate cause). The same philosophical framework can be applied to any behavior, including sexual behaviors. It may be useful to keep this discussion in mind as we explore the various sexual behaviors of mammals.

Behavioral Ecology

Behavioral ecologists are primarily concerned with the ultimate causes of behaviors. Because the focus of this book is on physiology, our main concern will be with the proximate causes of sexual behavior; that is, how animals develop and express sexual behaviors. Nevertheless, it is valuable to consider some of the fundamental principles of behavioral ecology. The general goal of this field is to understand behaviors in the context of the relationships between an animal and its environment (both living and nonliving components). From a Darwinian perspective, an animal expresses a particular behavior because that behavior allowed it to adapt successfully to its environment. Consider the homosexual mounting that characterizes estrus in domestic cows (Bos taurus) that evolved in Europe. Not all members of the Bovidae family express this behavior. For example, in addition to Bos taurus, yaks (Bos grunniens), a domestic bovine that inhabits central Asia and the American bison (Bison bison) express this behavior as well whereas water buffalo (Bison bubalis) do not. It is important to keep in mind that the environment in which an animal evolves need not be natural. For example, in the case of domestic animals, human influence can have profound effects on what types of behaviors have been perpetuated. It appears that the selection pressure from humans might have enhanced expression of mounting behavior in Bos taruus. The exact reason for this is unclear. Some people have suggested that this trait was perpetuated by humans because it proved to be useful in identifying when a cow is reproductively active, especially in cases where farmers were sharing the use of a bull for breeding.

When evaluating a sexual behavior from an evolutionary perspective, one should consider its cost. In other words, what impact does the expression of a particular behavior have on the overall fitness of the animal? Consider the behavior of the feral Soay sheep (mentioned in the discussion section of Chapter 8). These animals exist in an extremely harsh environment and the breeding season occurs during the autumn, a time when it is important to feed and build up body fat to help survive the brutal winters. The rams of this breed engage in a violent and exhausting rut at a time when it is important for animals to build up a reserve of metabolic fuels. These circumstances make the energetic costs of this rutting behavior tremendous. Not only does the behavior itself require an expenditure of energy, it reduces the amount of time these animals have to feed. Such behavior severely reduces the fitness of rams. The winter mortality rate of Soay rams under natural conditions often exceeds 90 percent. The reason such behavior persists is that these animals attain puberty at an extremely early age, which allows them to mate before the onset of winter. This example illustrates that concept that an understanding of why a certain sexual behavior exists requires consideration of the environment in which the animal must live as well as knowledge of its other reproductive traits.

PATTERNS OF SEXUAL BEHAVIOR IN MAMMALS

It is difficult, if not impossible, to discuss the physiologic control of sexual behavior without first describing some general features of these behaviors in representative species. This section includes an overview of the patterns of sexual behavior in rodents, humans, domestic carnivores, and domestic ungulates. It is appropriate to discuss the sexual behavior of rodents because these species have been studied more extensively and intensively than any other type of mammal. It is also appropriate to consider the sexual behavior of humans. In addition to being a topic with which most people are fascinated, conflicting ideas regarding what constitutes normal sexual behavior underlie several extremely contentious social issues (e.g., should certain sexual practices, such as homosexuality, be condoned?). A biological understanding of such behaviors provides a useful context for analyzing such issues. Finally, an understanding of sexual behavior in domestic animals is justified because such information is often used to manage the reproductive activity of these animals. Moreover, such understanding helps us to better understand the animals with which we interact and upon which we depend.

The following descriptions of sexual behaviors reflect general commonalities for various species. However, behavioral ecologists remind us that there is tremendous variation in behavior among individual members of a particular species. It is this variation in behavior that allows a species to adapt to its environment.

Rodents

Most research concerning the sexual behavior of rodents has been done with rats and has relied on methods consistent with a behaviorist perspective; that is, experiments involving an isolated mating pair in laboratory conditions, usually confined to a cage or arena. More recently investigators have embraced an ethologic approach where the rats are studied in the company of other males and females, and where animals can retreat to other locations to avoid interactions with each other. These conditions correspond to conditions that prevail in settings outside the laboratory. The following description reflects an ethologic perspective of the sexual behaviors of rodents such as rats and hamsters.

Rats and hamsters are opportunistic copulators. In other words, they will engage in sexual activity under a variety of circumstances. These might involve a dyad (one male and one female), a triad (one individual of one sex and two of the other sex), or even groups. Unlike humans and domestic animals, where an uninterrupted period of copulation is followed by orgasm, rodents engage in multiple bouts of copulations during which the male undergoes a series of ejaculations.

When a female becomes sexually receptive (i.e., expresses estrus or heat) it begins to mark its surroundings with its scent in order to attract males to its domain. Once the male encounters a female, the pair engages in mutual anogenital investigation. When a female is in heat, she will face the male and then run away in order to make the male chase after her. If the male chases the female, she will stop abruptly and assume what is known as presentation posture, or "pre-lordosis crouch" (Figure 13-1A). If the male mounts the female and palpates her flanks with his front legs, she will assume the full lordosis position (Figure 13-1B) and the male will attempt intromission. The female will then dart a short distance away from the male and then crouch again to solicit the male. This cycle of solicitation, runaway, and lordosis may be repeated several times and represents a "bout of copulation" from the male's perspective. The interval between intromissions is determined solely by the female. Only after several bouts does the male ejaculate. After ejaculation the male enters a "postejaculatory refractory period," an interval of approximately 5 minutes during which the male is not receptive to a soliciting female. The female also enters a postejaculatory refractory (approximately 1 minute in duration) during which she will not solicit a male.

[FIGURE 13-1 OMITTED]

As estrus terminates, the female solicits males less often and the inter-intromission interval increases. Females may still exhibit lordosis, but they begin to fight with the males rather than copulate with them. The males usually withdraw from their pursuit at this time and seek other receptive females.

There appears to be a limit to the number of ejaculations a male can experience during mating. After several ejaculations, the male becomes sexually exhausted and cannot initiate or complete a bout of copulation with the same or different female. This usually lasts an average of 4 days. During group matings, female rats will compete for males. In other words, one female might attempt to intercept a male that is chasing after another female. Males, on the other hand, typically express cooperative behavior; that is, a male will allow other males to mate with the female after the original male has engaged in several copulations.

Domestic Carnivores (Dogs and Cats)

When male dogs and cats first encounter females, they sniff and lick the area surrounding the perineum (area around the anus and vulva) of the female. Males of both species determine if the female is in heat by pheromones that are present in the urine and secretions of the anal glands and vulva. A pheromone is a chemical substance secreted by one individual that will induce a behavioral response in another. In this case the pheromones produced by the female trigger male sexual behavior, which, in turn, evokes female sexual behaviors.

When the bitch is in heat, she is likely to accept investigations by the male and may engage in soliciting behavior. This includes licking the male's genitalia, presenting her perineal region to the male and then running away (teasing). If the female expresses this behavior, rather than hostile behavior, the male will attempt to mount. The female will permit the male to mount only when she is in heat. The mounting behavior of the male consists of clasping his forelimbs just anterior to the bitch's hind limbs. As this occurs, the male's penis becomes erect and he attempts intromission. If successful, the male will thrust vigorously for several minutes. During copulation, the penis continues to enlarge and cannot be withdrawn. This is known as the copulatory lock or tie (Figures 13-2 and 13-3). The male typically dismounts the female by stepping off of the female, lifting one of its hind legs over her back such that the two animals are standing "rear to rear." The pair of dogs usually stands quietly for approximately 15 minutes until the penis becomes flaccid. During the copulatory lock, the female's vagina contracts around the penis while it releases pulses of prostatic fluid. This behavior appears to facilitate movement of ejaculatory fluid into the vagina. Disunion normally occurs in a quiet fashion, but the bitch might express aggressive behavior when the tie is broken. In many cases, mating occurs in the presence of several males. If there are more than three males present, the duration between male-female contact and copulation increases and the amount of time spent copulating decreases. This may be due to increased aggression among males, as well as expression of mate preference by the bitch. Male dogs rarely attempt mating more than once per day, whereas females will copulate with different males daily. Therefore, it is possible for a bitch to give birth to pups from different males.

[FIGURE 13-2 OMITTED]

[FIGURE 13-3 OMITTED]

[FIGURE 13-4 OMITTED]

Sexual contact between a queen and a tom is typically due to the female entering the male's territory. Male cats are extremely territorial and will not usually breed away from their homes. When a queen is in heat, she will express soliciting behavior. This includes assuming lordosis, treading their hind limbs, and presenting their perineum to the male (Figure 13-4). In response to an estrual female, toms might caterwauler (make a harsh cry). When the tom encounters a queen expressing soliciting behavior, he will grip the scruff of the queen's neck with his teeth and then mount. Intromission and ejaculation proceed quickly once the male mounts the female. In most cases, these behaviors last less than 60 seconds. As the male ejaculates, the female will cry out, roll over, and bat at the male with her from paws. This causes the male to disengage and jump away. Once the male separates the female will roll frantically and lick at her vulva, a response known as the "after reaction." Following the after reaction, which lasts from 1 to 7 minutes, the female will allow the male to approach and the process is repeated. Domestic cats will mate at a rate of approximately twice per hour, but rarely mate more than 15 times each day. Under feral conditions, a male will share its territory with a harem of females and can service as many as 20 queens.

Domestic Ungulates

Detailed accounts of the sexual behaviors of domestic animals can be found in the scientific literature and will not be presented herein. This chapter will focus only on the general principles concerning expression of sexual behavior in these animals, specifically in cattle, sheep, swine, and horses. Like rodents, the females of these species express estrous cycles. Therefore, the focal point of the sexual behaviors of males and females is expression of sexual receptivity of females.

Precopulatory Behavior

Table 13-1 summarizes the general characteristics of behaviors that precede copulation in farm animals. In general these behaviors are more variable (both within and between species) than those expressed during copulation.

[FIGURE 13-5 OMITTED]

Precopulatory Behaviors

Precopulatory behaviors include those that begin with searching for a mate and end with mounting. Some of the behaviors leading up to copulation appear to be necessary for copulation. In other words, such behaviors may be necessary for sexual arousal. For example, some wild mammals undergo extensive and elaborate courtship rituals consisting of complex sequences of motor patterns and multisensory stimulation lasting several hours. In general, the precopulatory behaviors of domestic animals consist of only a brief courtship period. For example, when placed in a small herd of cows (_30), a bull will move among the females and sniff the perineal region of each individual. Once the bull encounters an estrual female, the pair may engage in mutual anogenital investigation and move in a circular fashion. Within a few minutes the bull will rest its head on the rump of the female. Once the bull makes contact, he will attempt to mount.

In addition to investigatory behavior, males and females express other types of behaviors including vocalizing, urinating, and licking. It is not uncommon for the female to initiate sexual activity. For example, in cattle, estrual females may approach bulls and push against them or even mount them to arouse sexual interest.

One of the more peculiar sexual behaviors is the homosexual mounting expressed by domestic cattle and some other bovids (Figure 13-5). A cow that is in heat will attempt to mount other cows, and will also allow other cows to mount her. In a herd setting, groups of estrual cows form sexually active groups and engage in mutual mounting. It is difficult to classify this behavior in terms of copulation. Nevertheless it seems plausible to suggest that homosexual mounting in cows is a precopulatory behavior because it may play a role in attracting males from a distance. Once the male makes contact with this sexually active group he then engages in the precopulatory behaviors described previously.

[FIGURE 13-6 OMITTED]

A behavior that is frequently associated with mating in ruminants and horses is the flehmen response (Figure 13-6). Although males and females may express this behavior at any time, it is typically expressed by males during the precopulatory period. During the flehmen response an animal's head is raised and its upper lip is curled as though it were grimacing. The behavior is usually triggered when the male is exposed to urine or when he makes direct contact with the female's perineal region during anogenital investigations. Such behavior restricts air flow through the nasal passages and exposes the paired nasopalatine ducts that are continuous with the vomeronasal organs, chemoreceptors located in the basal region of the nasal cavity. These paired organs are enclosed by a bony or cartilaginous capsule and are divided by the nasal septum. The epithelial lining of the vomeronasal organs is lined with pseudostratified cells, including sensory neurons. Microvilli protrude from these nerve cells into the lumen of the organ. Axons emanating from these cells merge to form vomeronasal nerves that enter the accessory olfactory bulbs, which are located at the posterior-dorsal portion of the main olfactory bulbs. Presumably these neurons interact with other nerve cells that project to other parts of the central nervous system, including those that regulate sexual behavior. The vomeronasal chemoreceptors are stimulated by pheromones that are be present in bodily fluids aspirated into the nasopalatine ducts.

Copulatory Behaviors

As noted earlier, copulatory behaviors are very similar across species. The only copulatory behavior typically described for females is the mating posture. The willingness of a female to stand firm for mounting is the tell-tale sign that she is in heat.

The copulatory behaviors of males include erection, mounting, intromission, and ejaculation. The former three behaviors can be easily observed, whereas ejaculation is difficult to confirm.

Once the male becomes sexually aroused, its penis will become erect and mounting soon follows. Mounting behavior is quite similar among the various species of four-legged mammals. In all cases the male rises on its hind legs with its chest resting on the rump of the female and its front legs clasping each side of the female's hips. Almost anyone who has contact with nonhuman mammals knows that mounting behavior is not necessarily expressed in the context of mating. For example, young, sexually immature animals frequently mount each other regardless of sex. In addition, homosexual mounting has been observed in all wide variety of both domestic and wild mammals (see Box 13-1).

As the male mounts, its penis enters the female's vagina (intromission). In each of the species of domestic ungulates, the penis protrudes just before mounting. In bulls, seminal fluid dribbles from the penis before intromission. This behavior has not been observed in rams, stallions, or boars. Once intromission has been achieved, the males exhibit pelvic thrusting. Bulls and rams typically exhibit only one deep thrust during ejaculation. In contrast, stallions will thrust multiple times. Boars will exhibit multiple shallow thrusts that serve to position the penis in the sow's cervix. Ejaculation begins once the penis is positioned appropriately. The duration of mounting lasts only several seconds in bulls and rams, 20 to 60 seconds in stallions, and between 5 and 20 minutes in boars. The physiologic regulation of penile erection and ejaculation will be described in detail in a subsequent section of this chapter.
BOX 13-1 Focus on Fertility: The Kinsey Scale

No discussion of human sexual behavior is complete
without mentioning the work of famed
sexologist Alfred Kinsey (1894-1956). By 1937,
Kinsey was a renowned zoologist who made important
scientific contributions to taxonomy and
evolution theory. Some of his work dealt with the
mating patterns of the gall wasp. This work might
have provided the incentive for Kinsey to initiate
studies of human sexuality. Before Kinsey's work,
there were no detailed accounts of how human
beings expressed their sexualities. In 1939,
Kinsey assumed responsibility for teaching a
marriage course at Indiana University and began
collecting case histories of human sexual behavior.
He and his colleagues conducted over 18,000
interviews, which served as the basis for two
important books: Sexual Behavior in the Human
Male, published in 1948, and Sexual Behavior in
the Human Female, published in 1953. Perhaps
the most noteworthy, and controversial, aspect of
Kinsey's work is the idea that although men and
women engage in a variety of sexual activities
that can be classified as homosexual or heterosexual,
individuals themselves should not be
strictly classified in this dualistic manner.
According to Kinsey, "Males do not represent two
discrete populations, heterosexual and homosexual.
The world is not to be divided into sheep
and goats. It is a fundamental of taxonomy that
nature rarely deals with discrete categories ...
The living world is a continuum in each and every
one of its aspects." Kinsey developed a seven-point
scale to characterize human sexual behavior
(Table 13-2) and reported distributions of
scale ratings for populations of males and females
during a particular period of time. For
example, within a given year approximately
75 percent of males were rated as completely
heterosexual, whereas 7 percent could be
classified as exclusively homosexual. Around
15 percent of men had a rating of 3.

Are Kinsey's ideas applicable to other
mammals? It is clear that homosexual behavior
occurs in a large number of animal species
including mammals. For example, such behavior
has been documented in all of the domestic
livestock species. The most detailed studies have
been done with rams. According to recent
research, an average of 8 percent of rams are
male-oriented, meaning that when given a
choice between males and females they choose
to engage in sexual contact with males. Are
there degrees of homosexuality in rams? Some
evidence suggests that this might be the case.
For example, within all-male groups of Bighorn
sheep, the dominant ram typically mounts
subordinate rams, but also copulates with
females during the breeding season.

TABLE 13-2 Descriptions of Kinsey scale ratings for human sexual
behaviors and distribution of scale ratings for single and married
men in any single year (1)

Kinsey Scale   Description                                      % men

0              Exclusively heterosexual with no homosexual.     75

1              Predominantly heterosexual, only incidentally    22
               homosexual.

2              Predominantly heterosexual, but more than        20
               incidentally homosexual.

3              Equally heterosexual and homosexual.             15

4              Predominantly homosexual, but more than          10
               incidentally heterosexual.

5              Predominantly homosexual, but only               8
               incidentally heterosexual.

6              Exclusively homosexual.                          6

(1) From Kinsey, et al. (1948)


Postcopulatory Behaviors

Following copulation both the male and female enter a so-called refractory period during which no sexual contact occurs. The length of this refractory period varies among species, but usually lasts no longer than several minutes. The period of time between copulations is often referred to as the postejaculatory interval. Once the individuals have recovered they will resume mating behaviors. Females will engage in sexual activity so long as they are in estrus. However, it is not uncommon for females to terminate sexual contact with a particular male following several copulations. On the other hand, males may attempt to mate repeatedly with the same female and ignore other sexually active individuals. Complete cessation of sexual activity in males occurs after repeated ejaculations. This phenomenon is known as sexual exhaustion. Boars, stallions, rams, and bulls will exhibit exhaustion after approximately 8, 20, 70, and 35 ejaculations, respectively.

Humans

Research on human sexual behavior under laboratory conditions has been limited due to ethical constraints and social taboos. Nevertheless, sex researchers including A. Kinsey, W.H. Masters, V.E. Johnson, and more recently S. Hite, have compiled a massive amount of data based on observations, interviews, and surveys. Such studies reveal that humans, like rodents, are opportunistic; that is, they will engage in sex in a myriad of circumstances and in an almost limitless combination of partners and sexual orientations. This realization lead Alfred Kinsey (see Box 13-1) to remark, "the only unnatural sex act [in humans] is the one that cannot be performed."

In spite of the difficulties studying human sexual behavior, there have been objective measurements of sexual arousal and response in both men and women. Masters and Johnson conducted landmark studies on the human sexual response and identified several distinct phases: 1) increasing levels of sexual excitement and arousal, 2) plateau, 3) orgasm, and 4) resolution (the aftermath of a dramatic release of tension). The basic response is similar for men and women, but the occurrence of multiple orgasms is more frequent in women.

Compared to the rat, human copulatory behavior is less stereotypic (more likely to be learned), less sexually dimorphic (aside from the male's pelvic thrusting the behaviors of men and women are quite similar), and more likely to be continuous (continuous genital simulation until orgasm rather than bouts of copulation leading to refractoriness).

It is important to emphasize that in most primates, including humans, the female does not express estrus. Estrus refers to the behavioral state during which a female is sexually receptive to a male; that is, will accept a male. This state is caused by high circulating levels of estradiol. It would be incorrect to claim that female primates are always sexually receptive. It is more appropriate to say that their sexual activities are not confined to a particular time during the menstrual cycle. Interestingly, the rate of sexual activity in women is highest during the middle of the menstrual cycle. However, this does not appear to be due to a direct effect of estradiol on the brain, as is the case in animals that express a true estrus. The increase in sexual activity at this time during the menstrual cycle seems to be attributed to an increase in the sexual interests of males, not an increase in female receptivity per se. This response may be due to changes in aromatic compounds produced by microorganisms in the female's reproductive tract, which act as pheromones to stimulate the sexual interest of males.

THEORETICAL FRAMEWORK FOR UNDERSTANDING SEXUAL BEHAVIOR

One approach to learning the sexual behaviors of mammals is to become familiar with the specific behaviors expressed by males and females of each species during sexual encounters. The obvious disadvantage of this approach is that it would quickly become overwhelming. The patterns of sexual behavior described in the previous section illustrate that there is considerable variation among species. A more manageable approach is to determine if there are some general guidelines or principles that apply to all mammals. Decades of studies involving laboratory rodents have provided a wealth of information about the sexual behaviors of these animals. These data have been used to construct a theoretical framework for understanding the sexual behavior of other species including humans. In the next several sections, we shall consider this framework and then use it to provide an account of the sexual behaviors of several representative species.

Sex as Incentive

Most ethologists regard sex as a consummatory act. Consummatory acts are those that are associated with the termination of some goal-directed behavior; that is, consummation. Sex is a consummatory act in the sense that it terminates behaviors directed toward the goal of finding and having sex with another individual. Sex is also an intrinsically rewarding act. In other words, it is rewarding independent of whether or not it reduces a biological need. Thus it is as incorrect to say that an animal seeks sex to reduce some deprivation of sex as it is to say that a person seeks a piece of chocolate in order to alleviate some deprivation of chocolate. In both cases the things that are sought are the incentives for seeking them. These examples are quite different from the case where an animal searches for food in order to alleviate a deficit in nutrients. In the latter case, food has instrumental value; that is, it is used for a particular purpose. The main point to remember is that sex is the incentive that motivates an animal to express behaviors directed at seeking and having sex with a potential mate.

The best evidence to support the claim that sex is a powerful incentive for certain behaviors comes from experiments where rats have to traverse an electric grid to in order to copulate. Male rats will cross the grid and risk being shocked only when estrual females are used as stimuli. Likewise, female rats will cross the grid when intact males are present, but not when castrated males are used.

Sexual Behavior as Adaptive Behavior

Having established that the act of sex itself provides the incentive that motivates animals to seek sexual encounters, it is now possible to consider how this incentive activates sexual behaviors. The theory of adaptive behavior provides a useful framework for understanding this issue. The central assumption of this theory is that the physiologic mechanisms regulating an animal's behavior can adjust (adapt) to a particular environment such that the animal can better cope with that environment. The behavior an animal expresses in a particular environment is known as a "state." For example, a ram that is grazing on pasture is said to be in the grazing state. If the ram encounters a sexually receptive ewe, the ram will enter the reproductive state, and its behavior will change accordingly. The extent to which the ram will complete the sexual act depends on two variables: 1) how motivated it is and 2) previous experiences with receptive ewes (learned behaviors). These ideas are central to "the sexual incentive motivation model," an hypothesis proposed by Dalbir Bindra during the 1970s (Figure 13-7) to explain how motivation and learning activate the transition from one behavioral state to another.

According to Bindra's theory, transitions in behavioral states involve two regions in the brain: 1) the "central motive state" and 2) the "central representation of the incentive." The name of the former region may be confusing because the term "state" usually is used to refer to an animal's behavioral condition, not a physical location in the brain. It seems more appropriate to think of this region as a neuronal system that becomes more active during a particular behavioral state. The central representation of the incentive should be understood as a neuronal system that integrates and provides a physiologic representation of the inherent features of the goal object; for example, the visual, olfactory, gustatory, auditory, and tactile signals generated by the potential mate. These two regions are mutually excitatory. At some point both systems reach some threshold level of excitation and interact to promote a transition from observing to acting on its motivation. The behaviors involved with acting are divided into those that bring the animal into contact with the goal object (appetitive behaviors) and those that are performed once the animal has made contact with the goal object (consummatory behaviors). Bindra's model also addresses the fact that the execution of the goal can feed back on the aforementioned regions of the brain to affect subsequent sexual activity. This explains how learning can affect behavior.

[FIGURE 13-7 OMITTED]

The Central Motive State

Bindra and other behavioral scientists view this region as a hypothetical entity consisting of neuronal circuits that promote goal-directed actions in response to particular stimuli. The so-called state is generated by hormonal inputs (sex hormones such as testosterone, estradiol, and progesterone) as well as sensory inputs. The main physiologic role of this area is to regulate the degree of motivation generated by a stimulus. For example, the region will generate appetitive behaviors under the influence of appropriate sex hormones and positive sensory inputs (e.g., pleasure). In contrast the region will generate aversive behaviors when there is little to no stimulation from sex hormones and/or the sensory inputs are negative (e.g., pain).

In the case of sexual behavior, the medial preoptic area (MPOA) of the hypothalamus appears to play an important role in generating the central motive state. Receptors for estrogens and androgens are found in the MPOA and when this region is destroyed, the motivation of male and female rats to seek mating partners is diminished.

Bindra suggested that sexual motivation was regulated in part by gonadal hormones, but did not propose specific roles for these hormones. A later section of this chapter provides a discussion of the role of gonadal steroid hormones in generating the central motive state. At this point it is only necessary to point out that these hormones exert two types of action. First, these hormones play a role in the sexual differentiation of this region, much like the actions of these hormones on differentiation of the genitalia. Some scientists refer to this action as organizational or morphogenic. Such effects are permanent and irreversible. The embryonic brain appears to start out as inherently female or indifferent. During the fetal and early prenatal stages of development portions of the brain that regulate sexual behavior either remain feminine in the case of females or become defeminized (suppression of a behavior that is characteristically female) and/or masculinized (acquisition of a behavior that is characteristically male) in the case of males. A second role of these hormones becomes important once the animal becomes pubertal. In sexually mature animals, gonadal steroids are necessary for expression of sexual behavior. A familiar example is the induction of estrus behavior by estradiol in females. These types of actions are often referred to as activational. These effects are reversible; that is, they are expressed only when the particular steroid hormone is present. It is important to remember that both the organizational and activational actions of steroid hormones are necessary for an adult to express sexual behaviors. In other words, the adult must have the appropriate neuronal architecture to generate the behaviors as well as a stimulus (steroid hormones) to induce the behavior.

The Central Representation of the Incentive

Although the central motive state is involved with activating sexual motivation, it does not act alone. Sexual motivation also requires an incentive stimulus, which is represented somewhere in the brain. This region, like the central motive state, should be viewed as a hypothetical system of neuronal circuits. It is likely located somewhere in areas of the brain that deal with integration of sensory information as well as memory. The physiologic role of this region is to provide the animal with an accurate image of the stimulus such that a particular behavior is expressed in the appropriate context. For example, if a male lacked or had an erroneous central representation of a receptive female, he might not express sexual behavior in the presence of a receptive female and/or might express such behavior in the presence of inappropriate stimuli (e.g., presence of nonreceptive females, or other species).

Transition from Approach to Consummatory Behaviors

As noted earlier in this section, the central motive state and the central representation of the incentive act together to induce approach (appetitive behavior). In other words, when there are positive inputs to the motive state and the incentive stimulus is clearly represented, an animal will initiate behaviors that bring it into contact with the incentive. Once contact is made, the animal engages in consummatory behaviors. Details of these types of behaviors will be considered in the next section.

Incentive Sequence Model

Bindra's model of sexual behavior provided a foundation for research that is aimed at characterizing the sequence of behavioral events that characterize the transition from the nonsexual to the sexual state. As noted earlier, this sequence of events involves appetitive and consummatory behaviors. This transition period can be thought of as a cascade of behavioral events that begins with initiation of approach behavior and ends with completion of copulation. Figure 13-8 illustrates a common sequence of events that is associated with copulation in mammals. This model divides sexual behavior into several phases including: 1) courtship, 2) precopulation, 3) mounting, 4) intromission, 5) ejaculation, and 6) postejaculatory. The courtship and precopulatory behaviors are generally regarded as appetitive behaviors, whereas mounting, intromission, and ejaculation are usually classified as consummatory behaviors. The major problem with this account of sexual behavior is that it is overly simplistic and fails to capture the interactive nature of sex. Moreover, it fails to account for the fact that appetitive and consummatory behaviors are not necessarily distinct categories. The transition from one behavioral phase to the other is more subtle and involves behaviors that are truly transitional in nature; that is, they fit into both categories. For example, once the male rat comes into close contact with a female rat, the female begins solicitation and copulation quickly follows. It is difficult to classify solicitation as either appetitive or consummatory. On the other extreme is the so-called postejaculatory interval; that is, the pause between consecutive copulations. Does it occur at the end of the consummatory phase or the beginning of the appetitive phase? It seems more appropriate to classify this behavior as transitional. Figure 13-9 illustrates a more detailed account of sexual behaviors using rats as a model. This view is known as the "incentive sequence model." In this model appetitive and consummatory behaviors are depicted as two overlapping Venn diagrams. The overlapping areas represent transitional behaviors. Figure 13-9 also depicts how male and female behaviors can be mutually excitatory. In general, appetitive behaviors are not as stereotypic as consummatory behaviors. In other words, appetitive behaviors are not as sexually differentiated; that is, they are expressed by both males and females. Common appetitive behaviors in the rat include: grooming, investigation, motor activation (increased activity), instrumental (i.e., behaviors that have proved to be effective), and preference (showing a preference for certain traits of partners). The former three behaviors are sometimes classified as excitement or anticipatory behaviors, whereas the latter two are sometimes called preparatory behaviors. Consummatory behaviors expressed by rats are sexually differentiated, but are common to most mammals. Transitional behaviors include those that can occur during the transition from the appetitive and consummatory phases, or between the transition from consummatory to appetitive phases.

[FIGURE 13-8 OMITTED]

Human sexual behavior can also be understood in terms of this incentive sequence model (Figure 13-10). As noted earlier, there are few if any differences in sexual behaviors expressed by men and women. Nevertheless, human sexual behaviors can be classified into appetitive, transitional, and consummatory. There have been no attempts to classify the sexual behaviors of livestock in terms of this model. However, it seems reasonable that this model could be easily applied to these species.

[FIGURE 13-9 OMITTED]

PHYSIOLOGY OF SEXUAL BEHAVIOR

The ability to execute behaviors directed toward copulation in response to appropriate sexual stimuli is dependent on the presence of a particular neuronal circuitry as well as the ability of these neuronal circuits to become activated. For example, the behaviors required for a bull to approach and copulate with an estrual cow are dependent on neuronal pathways that convey information about the cow as well as those that control erection, mounting, intromission, and ejaculation. Comparable neuronal pathways are necessary for the cow to interact sexually with the bull. A comprehensive understanding of these physiologic aspects of behavior requires an appreciation of how the brain develops the neural structures necessary for sexual behavior as well as how these structures function in adults.

[FIGURE 13-10 OMITTED]

Sexual Differentiation of the Brain

The fact that males and females express different types and patterns of sexual behavior suggests that the neuronal structures regulating these behaviors are also different. An extensive amount of research done in rodents suggests that at some time during development, the nervous system becomes sexually differentiated via mechanisms that are similar to those controlling sexual differentiation of the genital organs. In other words, sexual differentiation of the nervous system depends on the actions of gonadal hormones (see Chapter 3). The sexual differentiation of the brain is expressed both behaviorally and physiologically. Behavioral differences were described in earlier sections of this chapter. Physiologic differences include mechanisms controlling release of reproductive hormones, particularly LH. Some of the best-documented examples of sex-related differences in LH release include 1) ability of estradiol to induce an LH surge in rats and 2) timing of the prepubertal increase in pulsatile LH release in sheep. Our investigation of the sexual differentiation of the brain will emphasize behavioral differences between males and females.

Hormonal Control

Early studies of the sexual differentiation of the genital organs provided the basis for investigating the sexual differentiation of the brain. An important underlying assumption of these studies is that gonadal steroids play a central role in regulating the sexual differentiation of the brain. Table 13-3 provides a summary of results from these early studies which were done in laboratory animals (rats and guinea pigs primarily). It is clear from these results that gonadal steroids influence development of sexual behavior. Removal of the testes in males during the early postnatal periods leads to expression of feminine behaviors and patterns of gonadotropin release (tonic or pulsatile as well as surge modes) during adulthood. In contrast, exposure of females to testosterone during the same periods leads to expression of masculine behaviors and gonadotropin patterns (tonic only) during adulthood. The hypothesis that emerged from these studies is that the early brain is indifferent or inherently female and that exposure to testosterone at a critical time in development induces structural changes that are necessary for expression of male behavior. Testosterone can bring about these effects by masculinization (acquisition of behaviors characteristic of males) and defeminization (suppression of behaviors characteristic of females). According to this view, the absence of androgens causes the brain to feminize. The extent to which feminization requires estrogen is unclear. Male rats that are castrated neonatally (during a critical period of sexual differentiation) will express lordosis as adults. Complete development of the female behavior also requires exposure to estrogens early in life.

Although it is well-accepted that gonadal steroids play important roles in the sexual differentiation of the brain, other nonendocrine mechanisms play a role as well. These mechanisms involve neural expression of genes located on the X and Y chromosomes. For example, sexually differentiated patterns of cell migration within the central nervous system occur early in embryonic development before these cells express receptors for gonadal steroids.

The results presented in Table 13-2 reflect the organizational and activational actions of gonadal steroids that were described previously. The neonatal effects of testosterone in males are organizational in the sense that they influence the organization of neuronal structures that regulate sexual behavior in the adult. In contrast, the sexual behaviors and LH patterns expressed in adult rats require exposure to androgens and estrogen, respectively.

Compared to the rat, our knowledge of the sexual differentiation of the brain in other mammalian species is incomplete. In all cases, there is anatomic, physiologic, and behavior evidence to suggest that the central nervous system is sexually differentiated. Major differences are associated with which traits are different as well as the degree of difference between males and females. For example, regulation of gonadotropin secretion does not appear to be sexually differentiated in primates; that is, both male and female rhesus monkeys will express cyclic gonadotropin patterns characteristic of the ovarian cycle when provided with ovarian transplants or given injections of estradiol and progesterone in patterns that mimic the ovarian cycle. The extent to which sexual behavior is differentiated in primates is unclear. Most of the data concerning this issue has been derived from individuals that express disorders that affect the production and/or response to gonadal steroids. Moreover, the fact that the sexual behaviors of some primate species (e.g., humans) are not as stereotypic as those observed in the rat makes it difficult to clearly differentiate masculine and feminine behaviors. Nevertheless some clinical information from human patients has been interpreted to support the hypothesis that testosterone has a masculinizing effect on sexual behavior. For example, men who are androgen insensitive develop female gender identities, whereas women exposed to high levels of androgen during development have a tendency towards a masculine identity. Such results should be viewed with caution since sexual identity in humans is a complex trait and is likely influenced by social as well as developmental factors.

Our understanding of the sexual differentiation of the brain in domestic livestock is superficial compared to laboratory rodents. The major difficulty associated with studying sexual differentiation in livestock is that a significant portion of sexual differentiation occurs prenatally (Recall that the critical period of sexual differentiation in the rat and guinea pig occurs during the first few days after birth). Therefore, it is extremely difficult, if not impossible, to study the effects of gonadectomy on development of sexual behavior in larger animals. However, it is possible to treat pregnant females with testosterone and study its effects on the behavioral development of their offspring. Much of our understanding of the hormonal control of sexual differentiation of the brain in these species is based on this experimental approach.

Normal adult ewes appear to retain the potential to express either male or female behaviors. In addition to expressing estrous behavior in response to estrogen, they will show strong masculine behaviors when treated chronically with testosterone. In contrast rams express only masculine behaviors and will not express estrous behavior when treated with estradiol. Ewes that were exposed to androgens prenatally do not express estrus as adults. These results support the hypothesis that testosterone promotes development of male behavior by defeminizing as well as by masculinizing neural structures that control sexual behavior.

Sexual differentiation of the brain in cattle and swine appears to involve defeminization more than masculinization. Cows and sows will express male mounting behavior when treated chronically with testosterone. Moreover, cows routinely display mounting behavior during estrus. In contrast neither bulls nor boars express estrous-like behavior when treated with estradiol. One plausible explanation is that the females of these species retain the neural structures necessary for both male and female behaviors, whereas males lose the ability to express female behaviors due to defeminization. Presumably the trigger for defeminization is prenatal exposure to testosterone.

Critical Periods

As noted earlier, gonadal steroids exert their organizational effects at certain critical periods during development. In rodents, the critical period for testosterone-induced masculinization of the brain is between embryonic day 18 (E18) and up to 30 days after birth, depending on the trait studied. Testosterone-induced defeminization of behavior can occur from E18 up to 7 days postpartum. With respect to masculinization, there may be two critical periods for sensitivity to testosterone, one spanning the neonatal and prepubertal periods and another coincident with onset of puberty. Each of these critical periods coincides with surges of testosterone production. The male rat experiences surges in circulating concentrations of testosterone at E18 and on the day of birth. A third surge of testosterone occurs at puberty. Recent evidence suggests that testosterone produces organizational effects on the brain of male rats throughout adolescence. Castration of male rats after the neonatal period, but before onset of puberty permanently impairs expression of sexual behavior.

A critical period for sexual differentiation of the human brain has not been precisely identified. Regions of the brain that show sexual dimorphisms do not become fully differentiated after birth (in some cases not until adulthood). Gender identity is believed to become established within the first year of life, but it is unclear if this is related to particular structural changes in the brain.

It is generally agreed that the critical period for sexual differentiation of the brain in mammals with long gestation lengths occurs during the prenatal phase of development. This appears to be the case for cattle, and sheep, but not for swine. The critical period for masculinization and/or defeminization in cattle may be between days 80 and 100 of the 284-day gestation period. This is a time when androgen concentrations increase in male fetuses. Ewes that were exposed to androgens between days 50 and 80 of pregnancy (gestation length _ 180 days) do not show sexual receptivity suggesting that a critical period of sexual differentiation occurs during this time window. In swine, there is little evidence to support the idea of a prenatal critical period. Sows exposed to androgens from days 29 to 35 or days 39 to 45 of the 113-day gestation period express normal estrous cycles as adults. Moreover, boars that are castrated before 2 months of age express sexual behaviors similar to those expressed by females. Taken together these data support the idea that a critical period for de-feminization of the brain occurs postnatally in male pigs.

Hormonal Control

Although surges of testosterone production coincide with critical periods of masculinization and defeminization of the brain, testosterone is not the principal hormone responsible for these changes. Estradiol, a metabolite of testosterone, is responsible for most of the organizational changes necessary for masculine behavior in most mammals. Regions of the brain that are involved with sexual behavior express the aromatase enzyme, which converts testosterone to estradiol. Estradiol then acts via its receptors to induce structural changes in these areas. These effects are mediated by two types of estrogen receptors. The alpha form of the receptor is involved with masculinization, whereas the beta form is involved primarily with defeminization. In addition to these effects, there are a number of regions in the rat brain that become masculinized by testosterone or dihydrotestosterone acting on androgen receptors.

Does estrogen have any effect on sexual differentiation of the brain in females? Although much of feminization occurs independent of estrogen, there is evidence to suggest that complete feminization of the rat brain requires estrogens. When an anti-estrogen is administered to female rats during the neonatal period, these animals fail to express normal estrous cycles and sexual receptivity. This raises an interesting question; how can estrogen both masculinize and feminize the brain? The answer may lie in the dosages required to produce these changes. It seems likely that low amounts of estrogen cause the brain to feminize, whereas higher doses cause it to masculinize. Thus sexual differentiation of the brain may be more of a continuum than a dichotomous process.

In species where estradiol serves as the primary masculinizing agent, there are mechanisms that protect the female brain from exposure to high levels of estrogen. For example, throughout the prenatal and prepubertal periods rats produce [alpha]-fetoprotein, a serum protein which circulates in high concentrations and has a higher affinity for estradiol than for testosterone (Figure 13-11). The protein binds estradiol thereby preventing the hormone from entering the brain. Testosterone, on the other hand, does not bind the protein and freely enters the brain.

In some species (e.g., guinea pig, rhesus monkey, and human) testosterone acts directly to masculinize the brain (i.e., it doesn't have to be converted to estradiol to be effective). In these cases, there are no mechanisms protecting the female brain from estrogens (females produce insignificant amounts of androgen under normal circumstances).

Neuroanatomic Changes Associated with Sexual Differentiation of the Brain

Although the hormonal mechanisms controlling sexual differentiation of the brain have been well documented, we are only beginning to understand where these hormones act as well as what effects these hormones have on neural structures. The search for where steroid hormones act can be limited to those regions of the brain that express high densities of steroid receptors. Of these areas, those that are sexually dimorphic are of particular interest. Although there are as many as 18 brain structures that meet these criteria, the sexually dimorphic nucleus of the pre-optic area (SDN-POA) appears to be particularly important in regulating sexual behavior, at least in rodents. The volume of this area is six times larger in males than in females. This larger size is attributed largely to a difference in number of neurons. The prevailing hypothesis linking this structural difference to the physiologic effects of steroid hormones is illustrated in Figure 13-12. Briefly, it is proposed that the masculinizing effects of steroid hormones (e.g., estradiol) prevent apoptosis in SDN-POA neurons thereby causing the volume of this area to be larger in males than females. Exactly how this structural feature is related to expression of behavior is unclear. One idea is that estrogen promotes development of a particular neuronal circuitry that is necessary for expression of male behaviors.

[FIGURE 13-11 OMITTED]

Behavioral Reflexes

In this final section, we return our attention to the actual behaviors expressed before and during mating. However, our focus now will be on the physiologic regulation of such behaviors rather than descriptions of them. It is useful to view sexual behaviors as component of behavioral reflexes which are analogous to the neural reflex arc described earlier in Chapter 6. In other words, various stimuli are detected by sensors that activate afferent neuronal pathways that impinge upon efferent neurons that induce responses in one or more effector organs.

[FIGURE 13-12 OMITTED]

We have a detailed physiologic understanding of only a few of the behavioral reflexes associated with reproduction. Penile erection and ejaculation have been studies more extensively than any other sexual behaviors. Our most complete understanding comes from studies with men.

Physiologic Control of Penile Erection

Your understanding of erection will be facilitated by reviewing the anatomy of the penis described in Chapter 5. Of primary importance are the penile erectile tissue (corpus cavernosum and corpus spongiosum) and the smooth muscle layers of the arteriolar and arterial walls. During the flaccid state, the smooth muscle cells are tonically contracted, thus restricting blood flow to the erectile tissue. Although blood flow is reduced during the flaccid state it is sufficient to supply nutrients to the penile tissues. During sexual arousal, neurotransmitters are released from efferent nerves that innervate the smooth muscle and induce relaxation of this tissue. This allows blood flow to the penis to increase, leading to the following events that cause erection:

* Due to increased blood flow, blood becomes trapped in the expanding sinusoids of the erectile tissue.

* The venous plexuses that allow blood to flow from the penis become compressed, thus reducing venous outflow.

* The increase in blood volume causes an increase in pressure within the cavernous tissues, causing the penis to elevate.

* Contraction of the ischiocavernosus muscle leads to a further increase in intracavernous pressure causing a rigid erection.

In recent years our understanding of the molecular mechanisms controlling penile erection has been advanced. Figure 13-13 illustrates how the smooth muscle cells of penile smooth muscle are regulated. The tonic contraction of smooth muscle cells during the flaccid state (Figure 13-13A) is maintained by norepinephrine, a neurotransmitter released by sympathetic neurons that innervate this tissue. Norepinephrine binds to membrane receptors to enhance production of inositol triphosphate, which acts as an intracellular messenger to increase intracellular concentrations of calcium. Calcium interacts with another protein (calmodulin) which promotes the interaction between actin and myosin that is necessary for contraction. During erection, the mechanisms that promote muscle contraction are suppressed (Figure 13-13B). Of primary importance is the activation of parasympathetic neurons, which release acetylcholine, a neurotransmitter that inhibits the sympathetic neurons that release norepinephrine. The reduction in norepinephrine release removes the stimulus that is necessary to sustain smooth muscle contraction. It has recently become apparent that other neurons play important roles in erection. These neurons innervate smooth muscle cells and release nitrous oxide, which enters smooth muscle cells and promotes the synthesis of cyclic GMP. This second messenger acts on a protein which promotes the uptake of calcium from the cytosol. This decrease in calcium promotes relaxation of the smooth muscle cells.

[FIGURE 13-13 OMITTED]

Physiology of Ejaculation

Although erection and ejaculation are usually thought of as separate processes, the two behaviors are closely related. In fact, it may be useful to view erection as part of the ejaculatory process because it is a necessary condition for ejaculation under normal circumstances. Ejaculation should be viewed as a complex behavior consisting of two components: emission and expulsion. Emission refers to the entry of seminal fluid into the urethra. In contrast, expulsion refers to release of the seminal fluid from the urethra. Ejaculation is typically portrayed as a spinal reflex, and this may be the case in some mammals. However, it is clear that ejaculation includes cerebral components as well, especially in humans. Interestingly it appears that ejaculation in human males can be regulated by both spinal and cerebral reflexes. In normal men, ejaculation appears to be primarily a cerebral process which inhibits the spinal reflex. However, men with spinal cord injuries (i.e., lack descending input from the brain) retain the ability to ejaculate suggesting that they retain spinal ejaculatory centers. Figure 13-14 illustrates the concepts of spinal and cerebral ejaculatory reflexes. In each case stimulation of the penis leads to activation of somatic sensory neurons which enter the central nervous system. In ways that are not fully elucidated, these neurons interact with other nerve cells, leading to activation of autonomic efferents. Sympathetic neurons innervate smooth muscles cells in the accessory sex glands, the ductus deferens, and neck of the bladder and cause these tissues to contract. This produces a two-fold response: 1) entry of seminal fluids into the urethra (emission) and 2) closure of the bladder to prevent retrograde flow of ejaculate into the bladder. The entry of fluids into the urethra stimulates other sensory neurons which enter the central nervous system and ultimately activate parasympathetic efferents that innervate the bulbospongiosus muscle, ischiocavernosus muscle, and muscles of the pelvic floor. These nerves cause rhythmic contractions of these skeletal muscles to expel semen from the urethra (expulsion).

[FIGURE 13-14 OMITTED]

SUMMARY OF MAJOR CONCEPTS

* A sexual behavior is a response of an animal to a stimulus in the context of mating and can be classified on the basis of when the behavior occurs relative to copulation.

* The causes of sexual behavior are ultimate (how the traits of a species allow it to adapt to its environment) and proximate (the physiologic and psychologic mechanisms regulating behavior).

* All mammals express a particular sequence of behaviors leading up to and following copulation and these can be understood as incentive-based behaviors.

* According to the prevailing theory, sexual behavior is the result of an animal having an incentive to engage in sex, as well as a clear depiction of the sexual stimulus. Various regions of the brain regulate the motivation and perception of sex and these areas are sexually differentiated.

DISCUSSION

1. The interaction between mares and stallions can be violent. During estrus, mares will allow a male to approach, but often show defensive behavior toward stallions when they are not in heat. Typically a mare will kick at a stallion when she is not sexually receptive. Based on this observation and your understanding of the causes of sexual behavior, explain why experienced stallions approach mares from the side during the appetitive phase of copulation.

2. Using the incentive sequence model of sexual behavior, categorize the sexual behavior of males and females of your favorite species of mammal.

3. What type of sexual behavior would you expect to observe in an adult female rat that was treated with extremely high doses of estradiol during the first 3 days after birth? Explain your answer.

4. Homosexual behavior is not uncommon in males of many species of mammals. There is intense disagreement over the cause of such behavior. Some argue that it is the result of how an individual brain is structured, whereas others assert that it is a learned behavior. Using your understanding of sexual behavior develop hypotheses to support each point of view.

REFERENCES

Agmo, A. 1999. Sexual motivation--an inquiry into events determining the occurrence of sexual behavior. Behavioural Brain Research 105:129-150.

Bindra, d. 1974. A motivational view of learning, performance and behavior modification. Psychological Review 81:199-213.

Dean, R.C., M.D. and T.F.Lue. 2005. Physiology of penile erection and pathophysiology of erectile dysfunction. Urologic Clinics of North America 32:379-395.

Ford, J.J. and M.J. D'Occhio. 1989. Differentiation of sexual behavior in cattle, sheep and swine. Journal of Animal Science 67:1816-1823.

Gorski, R. A. and C.D. Johnson. 1982. Sexual differentiation of the brain. Frontiers in Hormone Research 10:1-14.

Katz, L.S. and T.J. McDonald. 1992. Sexual behavior of farm animals. Theriogenology 38:239-253.

Kinsey, A.C., W.R. Pomeroy, and C.E. Martin. 1948. Sexual Behavior in the Human Male. Philadelphia: W.B. Saunders, pp. 636-659.

McDonnell, S.M. 2000. Reproductive behavior of stallions and mares: comparison of free-running and domestic in-hand breeding. Animal Reproduction Science 60-61:211-219.

Morris, J.A., C. L. Jordan and S.M. Breedlove. 2004. Sexual differentiation of the vertebrate nervous system. Nature Neuroscience 7:1034-1039.

Motofei, I.G. and D. L. Rowland. 2005. Neurophysiology of the ejaculatory process: developing perspectives. BJU International 96:1333-1338.

Owens, I.P.F. 2006. Where is behavioural ecology going? Trends in Ecology and Evolution 21:356-361.

Pfaus, J.G. 1996. Homologies of animal and human sexual behaviors. Hormones and Behavior 30:187200.

Resko, J.A., A. Perkins, C.E. Roselli, J.A. Fitzgerald, J.V.A. Choate, and F. Stormshak. 1996. Endocrine correlates of partner preference behavior in rams. Biology of Reproduction 55:120-126.

Root-Kustritz, M.V. 2005. Reproductive behavior of small animals. Theriogenology 64:734-746.

Wilson, C.A. and D.C. Davies. 2007. The control of sexual differentiation of the reproductive system and brain. Reproduction 133:331-359.

Wood, R.I. and D.L. Foster. 1998. Sexual differentiation of reproductive neuroendocrine function in sheep. Reviews of Reproduction 3:130-140.

Keith K. Schillo, PhD

Department of Animal and Food Sciences

University of Kentucky

Lexington, Kentucky
TABLE 13-1 Common Behaviors Expressed by Males and Female Livestock
Prior to Mating

Sex      Searching                       Courtship

Male     * Approaches group of           * Investigates, licks, and
           sexually active females         nuzzles anogenital region
           (bull)                          (bull and boar)

         * Approaches and moves          * Grinds teeth and salivates
           among females and               (boar)
           investigates (stallions,
           bulls, rams, and boars)       * Rests chin on females
                                           (bull)
         * Exhibits flehmen (bulls,
           stallions, and rams)          * Becomes excited and
                                           restless (stallion)

                                         * Stretches neck and holds
                                           head horizontally (ram)

Female   * Increases locomotion (cows    * Grooms other individuals
           and mares)                      (cows)

         * Becomes restless (ewes and    * Attempts to mount other
           sows)                           females (cows)

         * Increases vocalization        * Urinates in presence of
           (cows)                          male (mares and ewes)

         * Twitches (cows) or elevates   * Assumes immobile stance
           tail (cows and mares)           (sow)

TABLE 13-3 Effects of Neonatal Castration and Steroid Replacement on
Sexual Behavior and Patterns of LH Release in Laboratory Animals (1)

                                Sexual Behavior   Adult Pattern
Type of Animal                  as Adult (2)      of LH Release (2)

Non-treated Male                Masculine         Tonic
Non-treated Female              Feminine          Tonic + Surge
Castrated Female                Feminine          Tonic + Surge
Female + Testosterone           Masculine         Tonic
Castrated Male                  Feminine          Tonic + Surge
Castrated Male + Testosterone   Masculine         Tonic

(1) Treatments imposed within 7 days after birth.

(2) In the presence of appropriate gonadal steroids.
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Author:Schillo, Keith K.
Publication:Reproductive Physiology of Mammals, From Farm to Field and Beyond
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Date:Jan 1, 2009
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