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Sexual selection and fitness variation in a population of small mouth bass, Micropterus dolomieui (Pisces: Centrarchidae).

Key words. -- Centrarchidae, fitness variation, mating systems, Micropterus dolomieui, phenotypic evolution, sexual selection.

Sexual selection on males operates through male-male competition for access to females, and female preference for some males over others (Darwin, 1871). These processes are generally presumed to result in stronger selection under polygyny than monogamy. Competition among males for mates or female mate choice within a polygynous population can result in many females being mated by relatively few males. In contrast, the action of either process may produce little or no variance of mate number among males in a monogamous system, as nearly all males are expected to mate given sufficient time and numerical equality of the sexes (Bateman, 1948; Brown, 1975). Thus, relative to polygyny, monogamy is argued to constrain fitness differences among males and restrict the strength of sexual selection (Huxley, 1938; Bateman, 1948; Mayr, 1972; Kirkpatrick et al., 1990).

The mating constraint imposed by monogamy, however, does not confine the action of sexual selection on males to nonmonogamous mating systems, or monogamous populations with male-biased sex ratios. Factors other than the number of females in a population may limit the number of males that acquire a mate (Baylis, 1981). Furthermore, competition among males for females or female mate choice can effect a correlation between a male trait and the number of mates males acquire, or the fertility of acquired mates. Darwin (1871) proposed that the evolution of elaborate male traits in many monogamous organisms especially birds) resulted from the combined early breeding of the most fecund females and female preference for males bearing some particular character(s) (see Fisher, 1930; O'Donald, 1980a, 1980b; Kirkpatrick et al., 1990). When all males obtain a mate, however, the efficacy of this pairing process is expected to be limited because the strength of sexual selection is constrained by variation of female fecundity (Kirkpatrick et al., 1990). Nevertheless, the variance of relative mate number among males may severely underestimate the potential impact of sexual selection on males of monogamous species in which female fecundity is highly variable (see Wade, 1979).

The total variance of relative fitness among males sets an upper limit on the force of directional selection that can act on male phenotypic traits, and the magnitude of fitness variance can be directly compared among populations (Crow, 1958; Arnold and Wade, 1984a; Arnold, 1986; Wiegmann, 1990; see Payne, 1984). Demonstrating that selection has acted on a specific male character, however, requires measuring the association between the male trait and fitness. A number of field studies of sexual selection have revealed such correlations, but the evolutionary interpretations of many of these findings are compromised because selection was estimated from age-structured data on male characters that change with ontogeny (see Howard, 1979; Downhower and Brown, 1980; Schmale, 1981; Price, 1984). Simple correlations between fitness and male traits that vary with age are not sufficient to demonstrate the action of selection when measurements include data on males of more than one cohort (Baylis, 1982; Arnold and Wade, 1984b). Establishing that selection acts on an age-dependent male trait during the particular segment of the life cycle from which age-structured data are collected requires that analyses take into account potential fitness differences among males due to age.

In the present study we examined factors related to within-season differences of reproductive success of females and known-age males in a monogamous population of smallmouth bass (Micropterus dolomieui). We examined whether selection on males resulted from the acquisition of a mate or differences of mate fertility, and estimated the extent of fitness variation due to differences of mate number and female fertility for each sex (Wade, 1979; Arnold and Wade, 1984a, 1984b; Wiegmann, 1990).

MATERIALS AND METHODS

The Study Species. -- Adult male M. dolomieui move into shallow spawning areas and excavate nest sites in the spring, as water temperatures approach 15.5 [degrees] C (Coble, 1975). Nests typically contain sand, gravel, rubble, or some combination of these substrates (Hubbs and Bailey, 1938; Latta, 1963; Mraz, 1964), but are also constructed of vegetation or woody debris (Turner and MacCrimmon, 1970; Goff, 1984). A single territorial male may excavate several nests in close proximity, but generally constructs nests at distances greater than 1.2 m from nests of neighboring males (Mraz, 1964; Pflieger, 1966). When more than one nest is constructed by a single male, however, spawning occurs in only one nest (Cleary, 1956; Mraz, 1964).

A male leads a female to the nest site where courtship either terminates, or the female deposits a circular mass of eggs (Goff, 1984). The entire spawning sequence can last more than two hours, after which the female leaves the site (Reighard, 1906; Schneider, 1971). The male remains on the territory and guards the developing embryos from potential nest predators. Eggs hatch in 2 to 10 days depending on water temperature (Webster, 1948), and the male continues to guard the nest and offspring until the larvae disperse (Coble, 1975).

Most descriptions of spawning behavior of M. dolomieui include only a single male and female, and sneaker males have not been observed in natural populations (Ridgway et al., 1989). Polygynous mating in this species has been documented (Webster, 1954; Neves, 1975), and upon disturbance of courtship a female may spawn in the nest of more than one male (Reighard, 1906; but see Beeman, 1924). Time-lapse photography of nesting males and comparisons of actual egg deposition with the expected fecundities of known-size spawning females, however, indicate that both males and females of the study population are generally monogamous (Raffetto, 1987).

The Study Population. -- Field observations of the reproductive patterns of M. dolomieui were conducted daily from 18 May to 6 June 1988 on Nebish Lake, located in Northern Highland State Forest in northcentral Wisconsin (latitude 46 [degrees] 04'; longitude 89 [degrees] 35'). The lake is typical of northern hemisphere freshwater bodies, being a small, relatively shallow, closed basin (Ruttner, 1966; Wetzel, 1975), and has a shore perimeter of approximately 5.2 km (Serns, 1984). In addition to M. dolomieui, the lake supports yellow perch (Perca flavescens), central mudminnows (Umbra limi), bluntnose minnows (Pimephales notatus), and horny-head chubs (Nocomis biguttatus). Nebish Lake is one of several undeveloped research lakes studied by the Wisconsin Department of Natural Resources (WDNR). The WDNR has followed the demographic profile of M. dolomieui in the lake since 1967. Angling on Nebish Lake is subject to a special permit, and the WDNR continuously enforces a mandatory creel census. During the course of this study a 25.4 cm total length (TL) minimum size limit on M. dolomieui was in effect.

Population Estimates. -- M. dolomieui were captured in fyke nets and by electro-fishingjust prior to the onset of the breeding season. Captured individuals 20.32 cm TL and larger were tagged with uniquely numbered Floy FD-67C anchor tags, sexed, and examined for maturity (Benz and Jacobs, 1986). Tagged M. dolomieui subsequently harvested by anglers were internally examined to determine the proportions of inaccurately field-sexed individuals.

Abundance estimates were calculated by age for individuals 25.4 cm TL and larger and, due to sampling constraints imposed by the 25.4 cm TL minimum size limit, by 2.54 cm TL intervals for individuals less than 25.4 cm TL (Ricker, 1975). The age compositions of abundance estimates of individuals less than 25.4 cm TL were assigned in proportion to the length-group age composition of individuals sampled in fyke nets and during electrofishing. The length-specific proportions of mature individuals and the age-specific sex ratios of mature individuals captured in fyke nets and during electrofishing were used to estimate the number of mature individuals and the number of adult males and females within each cohort, respectively. The numbers of mature males and females of each cohort were then adjusted using the length-specific proportions of tagged males and females in the creel found to be incorrectly sexed when examined internally. Annual mortalities of males and females were estimated from the abundance estimates of mature individuals within each cohort (Ricker, 1975).

Nest Site Characteristics. -- We attempted to locate all M. dolomieui nests constructed during the spawning season, employing a combination of observers viewing from boats, snorkeling, and using SCUBA. Each nest was marked with a unique number-tag placed near the nest perimeter, and periodically examined throughout the breeding season; a circuit of the entire nesting area was completed every three to four days. We recorded whether a nest was empty or contained eggs (or more advanced offspring) on each examination. On the initial observation of a nest the primary substrate was recorded. The remaining nest characters were measured following the dispersal of larvae from all nests (Table 1). [TABULAR DATA 1 OMITTED]

Number of Breeding Individuals. -- The sum of the total number of nests constructed in the lake (including those in close proximity) and the number of nests reused after an initial spawn produced dispersed larvae or failed was used to estimate the maximum number of males attempting to breed. The number of males acquiring at least one mate was estimated as the sum of the number of nests found to receive eggs and the number of reused nests. We assumed that a male guarding a reused nest was not the male guarding the initial spawn, unless data on captured males indicated other-wise (see below).

Approximately 10 eggs were collected from various locations within each nest found to contain eggs. These samples were examined under a dissecting scope for within-nest differences of embryo development (McElman and Balon, 1985). Nests containing eggs differing in development stage were presumed to have been spawned in by more than one female, the number of differing stages representing the number of females spawning in a nest. A female was assumed to spawn with only one male (Beeman, 1924; Raffetto, 1987), and the number of females breeding ([F.sub.b]) was estimated as [Mathematical Expression Omitted] where [p.sub.i] equals the proportion of sampled nests spawned in by i females, and N, is the total number of nests found to contain eggs or offspring over the duration of the season.

Number of Eggs Deposited in Nests. -- A clear acrylic sheet was placed over nests containing eggs, and a wax pencil was used to outline the egg mass. The number of eggs in 1 [cm.sup.2] was counted at approximately the center, halfway between the center and perimeter, and perimeter of each egg mass. Each egg-mass outline was copied onto tracing paper and digitized to obtain the area. The number of eggs deposited in a nest was estimated as the product of the average density count and the egg-mass area (Raffetto et al., 1990).

Guarding Males. -- Males attending nests found to contain offspring were captured with a long-handled net. A diver placed the net over the nest and instructed a nearby assistant to start a stopwatch. When the male moved onto the nest the net was lowered slowly and moved laterally to direct the male off the nest. The net was then inverted and the male was lifted from the water, at which point the assistant stopped the watch. The capture-time (s), weight (g), standard length (cm), and TL (cm) of each captured male were recorded. Tag numbers were recorded for males previously captured by the WDNR; untagged individuals were tagged with uniquely numbered Floy FD-67C anchor tags, and scales were removed for aging. Males were then returned to their nests.

Number of Larvae Dispersing from Nests. -- The number of larvae successfully reared in nests that acquired eggs was assessed as zero, unknown, or volumetrically estimated. Larvae were collected from nests with a live-capture suction device just prior to dispersal, at which time larvae are jet black and swim short distances within the nest (Raffetto et al., 1990). The total volume of captured larvae (V), the number of individuals contained in a 1 ml subsample (S), and the number of individuals not captured by the suction device (R) were recorded for each nest sampled. The live larvae were then returned to their nest. The number of larvae dispersing from each sampled nest (L) was estimated following Raffetto et al. (1990) as L = VS + R.

Likelihood that Nesting Males Obtained a Mate. -- The effects of nest characteristics on the likelihood that territorial males obtained a mate were examined by performing a logit regression in which the response variable was dichotomous (Aldrich and Nelson, 1984), 0 and 1 indicating that a male failed or was successful in acquiring a mate, respectively. Except for spawning date, all variables in Table 1 were entered as predictors in the initial model and the final model was chosen using nested analysis (Aldrich and Nelson, 1984). The analysis (and all others) was performed on LIMDEP (Green, 1990).

Sizes of Mated and Nonmated Males. -- The mean size of mated males will differ from the average size of males in the population when the mean sizes of mated and nonmated males differ. The mean TL of mated males and males captured in fyke nets and during electrofishing were compared for each cohort using t-tests (Draper and Smith, 1981).

Timing of Reproduction. -- Stepwise regression was performed to examine the effects of male size and age on date of nesting and date of spawning (Table 1; F-enter = F-remove = 4; Draper and Smith, 1981). Male age was coded as a series of 1-0 dummy variables (in all analyses) and interacted with size to permit the effect of size to differ among males of different ages (Draper and Smith, 1981). We did not directly measure the size of breeding females. However, Raffetto (1987) found that the number of eggs deposited in nests corresponded with the expected fecundities of known-size spawning females, and female fecundity increases with size in M. dolomieui (Vogel, 1981; Raffetto et al., 1990). Thus, the relationship between timing of reproduction and female size was indirectly examined by regressing spawning date on the number of eggs deposited in nests.

Factors Influencing the Number of Eggs Spawned in Nests. -- Stepwise regression was used to determine the variation of egg number explained by male size and age, and the measured nest characteristics (Table 1; F-enter = F-remove = 4). A variable indicating the number of females spawning in a nest was also included as a candidate predictor.

Factors Related to Within-Nest Larvae Production. -- The variation of larvae number in nests at dispersal was examined by the stepwise regression of larvae number on egg number, male size, age, and capture-time, and the measured nest characteristics (Table 1; F-enter = F-remove = 4). Nests from which the male was harvested by an angler prior to larvae dispersion were excluded from the analysis.

Selection Opportunities. -- The variance of total relative fitness (I) sets the upper limit to the rate of fitness increase that selection can effect (Crow, 1958). Arnold and Wade (1984a) provided a means of partitioning I into components corresponding to separate selection episodes, and showed that the square root of selection indices sets the upper limit to the number of standard deviations that the mean of a character can shift as a result of the corresponding selection process (see also Arnold, 1986; Crow, 1989).

The total variance of relative egg number among males and females was partitioned into indices corresponding to differences of (1) mate number, and (2) female fertility (Wade, 1979; Arnold and Wade, 1984b). The former index for males was separated into components attributable to nest site acquisition, and differences of mate number among territorial individuals (Wiegmann, 1990). [TABULAR DATA 2 OMITTED]

RESULTS

Population Estimates. -- Estimates of the number of mature males and females in the population by age are given in Table 2. The estimated total humber of adult males in the population was 405, similar to the estimate of 403 for adult females. The annual mortalities of adult males and females were estimated as 0.83 and 0.79, respectively.

Number of Breeders. -- A total of 321 nests were excavated during the breeding season, and eggs were deposited in 223 of these nests. Three nests acquired a second spawn after an initial spawn was successful or failed. The male guarding the second spawn on one of these nests was not the individual guarding the initial spawn; males guarding initial and subsequent spawns were not captured from the other reused nests. Thus, a maximum of 324 adult males were estimated to have established a territory, and 226 of 405 males were estimated to have acquired a mate during the breeding season.

Within-nest comparisons of collected egg samples from 86 nests ([Chi] [bar] [+ or -] SD = 10.8 [+ or -] 5.4 eggs per nest) revealed only two nests containing embryos differing in developmental stage. One nest contained both embryos in step [C.sup.2]2 and 4, and the other contained embryos in both early and late stages of step [E.sup.1]4 (McElman and Balon, 1985). Thus, the estimated number of females breeding was 231, and 172 mature females were estimated to have failed to breed during the season.

Likelihood that Nesting Males Mated. -- The best-fit logit model included nearest-neighbor mating success, distance to shore, distance to structure, nest substrate, distance to nearest neighbor, and the interaction of nest substrate with distance to nearest neighbor (Table 3). Territorial males were more likely to mate when nests were constructed near a nonmated nearest neighbor, near shore, and 1 m or less from a structure. The likelihood of mating increased with distance to nearest neighbor for males on nests of pebble, gravel, or vegetation. Distance to nearest neighbor had no apparent effect on the likelihood of mating for males on rock or sand nests (Table 3, Fig. 1). [TABULAR DATA 3 OMITTED]

Sizes of Mated and Nonmated Males. -- Mated males of age three were larger than the average size of age-three males in the population, indicating that mated males were larger than nonmated males of the cohort (Table 4). The mean TL of age-four and age-five mated males did not differ from the mean of same-age males in the population. [TABULAR DATA 4 OMITTED]

Timing of Reproduction. -- Heavier males constructed nests and spawned earlier in the season than less heavy males (Fig. 2). The stepwise selection procedure incorporated neither male age nor interactions of weight with age into either regression. Thus, timing of nest construction and spawning did not depend on male age after taking into account the main effect of male weight. The regression of date of spawning on number of eggs deposited in nests suggests that larger females also spawned earlier than smaller females (Fig. 3).

Number of Eggs Spawned in Nests. -- Nesting males obtained between 1,298 and 19,942 eggs ([Chi] [bar] [+ or -] SD = 6,300 [+ or -] 4,252 eggs per nest). Regression analysis indicated that the number of eggs acquired by males of each age was negatively related to date of spawning, but that the effect of male weight on egg number depended on male age (Fig. 4). The number of eggs spawned in nests of age-four males was positively related to male weight after controlling for date of spawning, but weight had no apparent effect on the number of eggs acquired by age-three or age-five males.

Larvae Production. -- The number of larvae in nests at dispersal depended on nest substrate, male capture-time, and the initial number of eggs spawned in nests (Table 5). The stepwise selection procedure incorporated the substrate dummy variables but not interactions of substrate dummies with other variables, indicating that some substrate intercepts differed and that the slopes for nests of different substrate types were not heterogencous after accounting for other factors in the model. Comparisons of substrate intercepts indicated that the mean number of larvae produced in rock nests was larger than in nests of any other substrate type, and greater in pebble nests than nests of vegetation. Larvae number was positively related to the initial number of eggs in nests. Coefficients of factors related to larvae number after taking into account egg number measure effects on within-nest larvae survivorship. Thus, the differences of substrate intercepts correspond to differences of young survivorship within nests. Capture-time of guarding males was negatively associated with larvae survivorship, and more site attentive males were generally large (Fig. 5). However, survivorship of young did not depend on male age after accounting for other factors; neither male age nor interactions involving male age were incorporated into the regression by the stepwise selection procedure. [TABULAR DATA 5 OMITTED]

Variation of Relative Egg Number. -- Results of partitioning the variation of relative egg number within each sex are given in Table 6. The total variance of relative egg number was similar for males and females. Differences of mate number contributed about two times more than differences of female fertility to the total variation of relative egg number within each sex. Among males, the variation of relative mate number due to the acquisition of a territory was about half that due to differences of mate number among territory holders.

DISCUSSION

Variance of Relative Fitness. -- Variation of female fecundity is often presumed to contribute more than differences of mate number to the total reproductive variation among males of monogamous populations in which males and females are equal in number (Darwin, 1871; Bateman, 1948; Kirkpatrick et al., 1990). Our findings suggest, however, that this assumption may not be justified. The ratio of males to females in the study population was 405:403. Differences of mate number nevertheless contributed about two times more than differences of mate fertility to the total variation of relative egg number among males. Variation of relative mate number accounted for a similarly large fraction of the total index of selection on females (Table 6). [TABULAR DATA 6 OMITTED]

Only 2 of 86 males in our sample mated bigamously, indicating that the magnitude of the variance of relative mate number among males was largely due to nonbreeding individuals. Many adult males failed to establish a territory and many adult females did not spawn (see also Raffetto et al., 1990). Some evidence suggests that the number of nesting male M. dolomieui is limited by the establishment of home ranges prior to the breeding season, but timing of eproduction in M. dolomieui may also be under bioenergetic control (Ridgway et al., 1991). We found that larger males of all ages nested earlier in the season than smaller males (Fig. 2a). The observed relationship between spawning date and the number of eggs deposited in nests suggests that timing of reproduction among females was similarly size-dependent (Fig. 3). Thus, the relatively short breeding seasons of northern populations may preclude small male and female M. dolomieui from nesting and spawning, respectively.

Many nesting males did not obtain a mate, and the likelihood that a territorial male mated depended on the site chosen for nest construction (Table 3). Males nesting near mated neighbors were less likely to obtain a mate than males nesting near bachelors (see also Winemiller and Taylor, 1982). Males nesting in the absence of a nearby structure were also less likely to mate than males nesting in close proximity to a large object. Male M. dolomieui defend nest sites more vigorously after egg deposition (Coble, 1975), and this change of behavior likely deters females from spawning with males neighboring mated individuals. Such interference of courtship among nesting male M. dolomieui has been observed in hatchery ponds at inter-nest distances of more than 9.1 m (Beeman, 1924). The presence of a structure near the nest may impede these interactions (Reighard, 1906).

Females were more likely to mate with males nesting near shore. The likelihood of mating among territorial males also depended on nest substrate. Males constructing nests of rock were likely to acquire a mate at any distance from a neighboring nest, whereas males defending sand nests were relatively unlikely to mate at any inter-nest distance. The selectivity of females for males guarding nests of predominantly pebble, gravel, or vegetation, however, depended on the location of other nesting males. Our logit model predicted that males guarding these nests are more likely to obtain a mate when located far from other nests, and that the probability of mating becomes more similar among these males at large inter-nest distances, suggesting that females may search a relatively small area before selecting a site for egg deposition (Fig. 1).

Selection on Male Size. -- Population age-structure complicates detecting the action of selection on phenotypic traits that change with ontogeny. A correlation between an age-dependent male trait and fitness measured across cohorts may indicate that selection acted on the trait within each cohort during the period of data collection, but it is also possible that the fitness differences among males were due to age and that there was no selection on the trait per se (Arnold and Wade, 1984b).

Because male age was incorporated into our analyses we were able to distinguish between effects of ontogeny and selection on male size. The mean size of mated males was larger than the average size of same-age adult males in the population among age-three males, but not among age-four or age-five males (Table 4; see also Raffetto et al., 1990). Thus, selection resulting from the acquisition of a mate within the season favored large size (fast growth) among only age-three males.

Larger males mated earlier in the season than smaller males irrespective of male age (Fig. 2b), and the number of eggs obtained by mated males of all ages was negatively related to date of spawning (Fig. 4). Thus, fertility selection favored early reproduction within the season by males within each cohort. Male size was positively related to the number of eggs acquired by mated males of age four after taking into account spawning date, but there was no apparent effect of male size among mated males of age three, or among age-five mated males. Fertility selection within the season, therefore, did not favor larger males at all ages as may have been construed if male ages were not known. Selection presumably also favored larger females as fecundity increases with female size (Vogel, 1981; Raffetto et al., 1990). This conclusion is compromised, however, because female size is correlated with age and spawning females were likely of several cohorts.

Darwin (1871) proposed that sexual selection on males of monogamous species cohort. Male size was positively related to the number of eggs acquired by mated males of age four after taking into account spawning date, but there was no apparent effect of male size among mated males of age three, or among age-five mated males. Fertility selection within the season, therefore, did not favor larger males at all ages as may have been construed if male ages were not known. Selection presumably also favored larger females as fecundity increases with female size (Vogel, 1981; Raffetto et al., 1990). This conclusion is compromised, however, because female size is correlated with age and spawning females were likely of several cohorts. Darwin (1871) proposed that sexual selection on males of monogamous species would result from the earlier breeding of more fecund females and female mate choice. The effects of reproductive timing among females and male size on the number of eggs deposited in nests of age-four males suggest that this pairing process may occur to some extent in M. dolomieui. Large size is often of advantage to males in attracting females (Noonan, 1983; Downhower and Brown, 1980), but we can not directly eliminate the possiblity of male mate choice of more fecund females (see Manning, 1975; Gwynne, 1981; Downhower and Brown, 1981). Females are likely more selective of mates than territorial males, however, because many males that established a territory failed to obtain a mate (see Fisher, 1930), and the fertility of breeding females declined over the season. The large number of nonmated territorial males indicates that males failing to mate, given the opportunity, risk postponing reproduction until a future season. The high estimated annual mortality for adult males (0.83) suggests that such an option is indeed precarious. Furthermore, the negative relationship between spawning date and the number of eggs deposited in nests indicates that delayed reproduction by males within a season is likely to result in the reception of fewer eggs, irrespective of male size or age.

Larvae Production among Mated Males. -- Males that mated more fertile females produced a larger number of larvae. The number of larvae produced by mated males also depended on factors related to within-nest survivorship of young (Table 5). Bain and Helfrich (1983) found that survivorship of young in nests of a related centrarchid (Lepomis macrochirus) was positively related to the proportion of coarse substrate in nests. In this study, survivorship of young was greater in nests of predominantly rock than in nests constructed primarily of any other substrate type, and greater in pebble nests than nests of predominantly vegetation. In combination with the probabilities of mating among males guarding nests of different substrate types (Table 3, Fig. 1), these results suggest that the distribution of females among territorial males may in part be determined by differences of nest-site quality (Verner, 1964; Orians, 1969; see Pleszczynska, 1978).

Within-nest survivorship of offspring did not depend on male age after taking other factors into account. Survivorship of young, however, was related to the behavior of a male upon disturbance of the nest. Males returning to a nest sooner (or never leaving) were more easily captured, and capture-time was negatively related to within-nest survivorship of young. Capture-time of guarding males was also negatively related to male size (Fig. 5), suggesting that male size may be used by females as an indicator of parental ability (see Downhower and Brown, 1980). More data are required to test this hypothesis, however, as a number of factors are expected to influence the level of care provided by guarding males (Ridgway, 1988, 1989; Clutton-Brock, 1991).

Constraint on Mating Variance. -- Relative to polygyny, monogamy constrains the distribution of females among males and is thus presumed to constrain the strength of sexual selection (Bateman, 1948; Brown, 1975; Kirkpatrick et al., 1990). Although differences of mate number contributed substantially to the reproductive variation among males in this study, the estimated index of opportunity for selection on males due to variance of mate number (0.83) is relatively small in contrast to estimates from comparable studies of polygynous systems (see Downhower et al., 1987). Previous index estimates due to differences of mate number among males of the study population, however, ranged between 2.0 and 8.58 (Wiegmann, 1990). The latter value is exceeded by only one of the 13 estimates for males from field studies listed in Downhower et al. (1987).

Because relative fitness was partitioned across age classes, and lifetime variation was not measured, strict evolutionary interpretations of the above comparisons can not be applied (Crow, 1958, 1989; Arnold and Wade, 1984b). The contrasts suggest, however, that sexual selection on males need not be more restricted when mating is monogamous rather than polygynous. The maximum possible evolutionary change attributable to differences of mate number among males in any population can be obtained either under monogamy (when breeding is restricted to a single pair), or polygyny (when a single male mates with two or more females). Furthermore, the evolutionary change that selection can effect is a function of the demography of a population (Crow, 1989). Determining the actual association between mating systems and constraints on male phenotypic evolution due to sexual selection is an empirical issue and will require measurements of selection indices from a large number of organisms.

ACKNOWLEDGMENTS

We thank Trout Lake Station and the University of Wisconsin Department of Zoology for providing accommodations to D.D.W. and J.R.B. during the study period, and the WDNR for support of the project. We are especially indebted to C. Annett, M. Bloomer, L. Kellner, N. Raffetto, and P. Samerdyke for helpful hands in the field. W. Manning and P. Tiemeyer provided useful insights into logit analyses. Helpful comments on the manuscript were provided by J. Hailman, A. Ives, J. Kitchell, E. Martins, and reviewers S. Arnold and N. Metcalfe. This project was supported in part by funds from the Wisconsin Alumni Research Foundation and the Electric Power Research Institute. D.D.W. performed this study in partial fulfillment of a Master's degree at the University of Wisconsin-Madison.

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Author:Wiegmann, Daniel D.; Baylis, Jeffrey R.; Hoff, Michael H.
Publication:Evolution
Date:Dec 1, 1992
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