Growth rates and intraspecific variation in body weights of raccoons (Procyon lotor) in southern Texas.
The size of a mammal species is closely related to, and influences, virtually every aspect of its biology (McNab, 1971; Clutton-Brock and Harvey, 1983) from its physiology and behavior to its life history and ecology. Intraspecific variations of patterns in postnatal growth rates and adult body size are particularly useful for elucidating aspects of the interaction between environmental factors and a species' physiology and behavior that may have influenced the evolution of geographic variation in its life history (Millar and Zammuto, 1983; Stearns and Koella, 1986; Gittleman and Oftedal, 1987; Geffen et al., 1996). Raccoons (Procyon lotor) represent an excellent species with which to investigate these relationships because of their widespread geographic distribution (Hall, 1981) and array of morphologic and behavioral variation (Ritke and Kennedy, 1988). However, despite the importance of postnatal growth rates for understanding evolutionary patterns and life history strategies (Case, 1978; Prestrud and Nilssen, 1995), few studies have reported such information for raccoons (Mech et al., 1968; Montgomery, 1969). Although adult raccoons are sexually dimorphic in body size, little is known about the ontogeny of this dimorphism. One might predict that the sexual dimorphism in body size of raccoons is manifested in different postnatal growth rates; however, growth rates have not been compared between sexes.
In contrast to the paucity of data on growth rates of raccoons, numerous studies have reported on weights of adult raccoons for populations east of the Mississippi River. In temperate climates, weights of raccoons fluctuate from season to season; raccoons deposit fat in autumn, and then metabolize it for energy during the winter (Stuewer, 1943; Mech et al., 1968). Thus, in northern areas where winter temperatures are extreme, raccoon body weight may exhibit a two-fold variation between late spring and late autumn (Mech et al., 1968; Moore and Kennedy, 1985). In southern latitudes where winter temperatures are mild, raccoons remain active throughout the year (Goldman, 1950; Johnson, 1970). Thus, Mugaas and Seidensticker (1993) suggested, based on the smaller range of variation in body weight reported for raccoons in the southeastern United States, that seasonal weight fluctuations may be graded geographically, and relatively little seasonal weight change may occur among southern populations. However, seasonal weight data have not been published for raccoons residing in subtropical climates west of the Mississippi River (Goldman, 1950).
We addressed two objectives by measuring weights at different seasons for a population of raccoons in southern Texas, a geographic area for which there are few data for raccoons. The first objective was to determine if postnatal growth rates of males and females differed, and the age at which the sexes diverged in weight. Second, we addressed the hypothesis that seasonal weight fluctuations for raccoons are graded latitudinally. Because raccoons in southern Texas do not exhibit a reduction in home-range size during winter (Gehrt and Fritzell, 1997) and are active nearly year-round, we predicted seasonal weight fluctuations should be minimal relative to that of raccoons in temperate climates.
Study area. - Fieldwork was conducted from February 1990 to July 1992 on the Welder Wildlife Refuge, San Patricio County, Texas (28 [degrees] 6[minutes]N, 97[degrees]25[minutes]W). The refuge is bordered on the north by the Aransas River and is situated in a transitional zone between two major vegetational types: the South Texas Plains and the Gulf Prairies and Marshes. Within the refuge the habitat mosaic is complex; 16 vegetational communities are represented (Drawe et al., 1978), with Mesquite-mixed grass and Chaparral-mixed grass communities predominant.
The climate is subtropical Drawe et al., 1978; Jones, 1982), characterized by long hot summers and cool winters. Low temperatures fall below 0 C only a few days each year. The average daily minimum temperature during winter months is 7.8 C. Average monthly minimum temperature during our study ranged from 5.3 to 24.1 C. The length of the growing season is usually over 300 days annually, with a probable first freeze by 14 December and last freeze by 13 February. Snowfall rarely occurs; there is no measurable snowfall in 9 out of 10 winters (National Oceanic and Atmospheric Administration, 1992).
Annual precipitation varies considerably from year to year. This study was initiated during the third consecutive year of below-average rainfall. Beginning in September 1991, the refuge experienced above-average rainfall for 8 of the following 9 mo. By December 1991 much of the refuge was flooded and all natural impoundments were full.
Sampling and data analyses. - We livetrapped raccoons during February-March and August-October in 1990, January-March and September in 1991, and January-March and June-July in 1992. During each trapping session, 30 traps were distributed across the refuge and placed in areas of raccoon activity (Gehrt and Fritzell, 1996a). Upon their first capture, unmarked raccoons were immobilized with an injection of ketamine hydrochloride and acepromazine (Bigler and Hoff, 1974). Raccoons were measured, weighed to the nearest 0.1 kg with a spring scale with 10 kg capacity, aged according to tooth wear (Grau et al., 1970) and reproductive characteristics (Sanderson and Nalbandov, 1973) and eartagged with individually-numbered monel #3 eartags (National Band and Tag Go., Newport, Kentucky). Scales were calibrated with known weights before each trapping period. Recaptured raccoons were released without handling at the capture site, unless it was their first capture of a new trapping session. Rarely were individuals immobilized more than once in any trapping session. All captured raccoons were released at the trapsite within an hour after checking the trap. Raccoons usually were weighed only once per trapping session, although a few were weighed in different months within the same trapping session.
We captured the young of radiocollared females either by removing them from the natal den after the mother began foraging, or while they rested with their mothers outside the den. Each juvenile captured in this manner was restrained by hand, and we recorded sex, pattern of tooth eruption and weight. If juveniles were large enough they were eartagged. Juveniles captured by hand were generally processed and returned to their den or released at the capture site within 5-10 min.
We recorded 248 weights for 167 raccoons. These data were partitioned into three age groups: juvenile ([less than]12 months), yearling (12 to [less than]24 months) and adult ([greater than or equal to]24 months). Growth rate was estimated for juveniles and yearlings with the Gompertz growth model (Zullinger et al., 1984; Prestrud and Nilssen, 1995):
M(t) = A x [e.sup.-[e.sup.k(t-I)]]
where M = body weight at age t (months), A = asymptotic body weight, k = growth-rate constant ([months.sup.-1]) and I = the age at the inflection point (months). Growth curves were fitted by nonlinear regression with the Marquardt-Levenberg algorithm in Sigmaplot (version 4.11, Jandel Scientific, Inc., 1992; Kunz and Robson, 1995). Regression parameters were compared using t-tests.
Mean weights were calculated by month for each sex/age group. We considered successive weight measurements for each individual as independent observations. Preliminary analyses indicated there were no year effects on weights, therefore we pooled data across years. Some adult raccoons were weighed in the same month in different years, and for those individuals we calculated individual mean weights for that particular month. Thus, each individual was represented only once each month. Because of small sample sizes for juveniles and yearlings, we compared their weights between month, sex and age groups with Mann-Whitney U or Kruskal-Wallis H tests. To test for sex and seasonal differences among adult raccoons, we employed a fixed two-way analysis of variance with sex and month as main effects with an interaction term.
Growth rate. - Juveniles of both sexes rapidly increased in weight during their first six months of age, but parameters of the Gompertz model differed between the sexes ([ILLUSTRATION FOR FIGURE 1 OMITTED]; Table 1). Asymptotic weight (A) and growth rate constant (k) were greater for males than females (A: t = 6.28, df = 139, P [less than] 0.05; k: t = 15.84, df = 139, P [less than] 0.01). Age at inflection (I) was younger for males than females (t = 2.53, df = 139, P [less than] 0.05).
Juvenile male (n = 56) and female (n = 42) weights were similar until nine months of age when males were heavier (U = 49.0, P = 0.02) than females. However, this divergence diminished during the second year and yearling weights for both sexes were similar (February test, U = 8.0, P = 0.39). Mean monthly weights for yearling females (n = 18) were 98% of mean adult female weights by September (16 mo of age, U = 13.5, P = 0.68). Yearling males (n = 25) were slower than females to reach adult size; in September yearling male weights were only 80% of adult weights. By January yearling male weights approached (U = 7.5, P = 0.09) adult male weights, but then dropped below (U = 15.5, P [less than] 0.01) adult weights again in February.
Adult weights. - Individual adult weights ranged from 5.8 to 11.0 kg for males (n = 64) and 4.6 to 8.6 kg for females (n = 43). Mean weights were larger (F = 51.18, df = 1, 82, P [less than] 0.01) for males than females during all months [ILLUSTRATION FOR FIGURE 2 OMITTED]. Adult weights fluctuated seasonally (F = 10.52, df = 5, 82, P [less than] 0.01), with an increase during autumn and a decline during spring. A second order interaction term was nonsignificant (F = 0.24, df = 5, 82, P = 0.94), indicating that patterns in monthly weight fluctuation were similar between males and females [ILLUSTRATION FOR FIGURE 2 OMITTED]. Mean maximum weights occurred during January (males: [Mathematical Expression Omitted], SD = 0.51, n = 6; females: [Mathematical Expression Omitted], SD = 1.03, n = 4). All individuals (n = 9) that were [TABULAR DATA FOR TABLE 1 OMITTED] weighed in February and again in March lost weight (25-.35% for males and 14-26% for females). There was a decline of 28% (females) and 26% (males) from mean maximum weights to minimum weights in June. Mean monthly adult weights were negatively correlated with mean monthly temperature (male: r = -0.73, df = 7, P = 0.04; female: r = -0.90, df = 6, P [less than] 0.01).
For raccoons in South Texas, sexual dimorphism of body size appeared to be a manifestation of differential growth patterns as measured by body weights. Juvenile male raccoons on the Welder Wildlife Refuge grew more rapidly than females during their first year, as evidenced by the earlier inflection point and growth rate constant for males than for females. Although growth rates were greater for males than for females, males did not necessarily complete their growth earlier than females. Yearling males nearly reached adult size during winter but dropped below adults during spring, whereas yearling females had already attained adult size prior to winter. The asymptote estimate for the male growth curve was similar to the seasonal lows for adult male weights in June and July, when yearlings would graduate to the adult class. Montgomery (1969) provided weights for captive juvenile raccoons, but only until 16 weeks of age, which would affect estimates of growth trajectories, particularly for males. Therefore growth rates estimated for that data set (Zullinger et al., 1984) are not comparable to the results of this study.
Like raccoons in southern Texas, young raccoons of both sexes in Alabama also usually attain adult size during the autumn of their second year (Johnson, 1970). In contrast to southern populations, raccoons of northern populations do not attain adult size until after their second year of age, or possibly even later (Stuewer, 1943). This delay in development may be a result of constraints during severe winter conditions. For raccoons in our study and those in Alabama (Johnson, 1970), both sexes exhibit a small decrease in average weight during their first and second spring seasons, but these spring declines are relatively minor compared to northern populations where 50% weight loss over winter is not uncommon (Stuewer, 1943; Mech et al., 1968). Thus, young raccoons at southern latitudes may attain adult size earlier than those in northern populations.
Weights of adult raccoons in this study were greater than weights reported for other raccoon populations at southern latitudes. Mean maximum weights for raccoons at a similar latitude in Florida (Mugaas and Seidensticker, 1993) were only 56% (males) and 53% (females) of mean weights for raccoons in Texas. Mean maximum and minimum weights for raccoons in Alabama (Johnson, 1970), Tennessee (Moore and Kennedy, 1985) and Virginia (Mugaas and Seidensticker, 1993) also were less than the same measurements for raccoons in Texas. Raccoons in Michigan (Stuewer, 1943) and Minnesota (Mech et al., 1968) had published maximum weights close to, or greater than, mean maximum weights for South Texas raccoons.
Goldman (1950) speculated that raccoons in southern climates that remained active year-round might not exhibit seasonal weight fluctuations, and Mugaas and Seidensticker (1993) suggested that the amplitude of annual weight fluctuation might depend on the severity of winter. In northern climates, raccoons often spend a portion of the winter in a dormant state, particularly when there is snow cover and low temperatures (Stuewer, 1943; Mech et al., 1968). These dormant periods may continue for days or months, and fat deposited during the previous autumn provides the energy essential to maintain basal metabolism and endothermy during these periods (Mugaas et al., 1993). Thus, fat deposition is an important function for survival of raccoons at northern climates, and presumably is less important for raccoons at southern climates.
We did not record weights during all months of the year, therefore we may have underestimated the magnitude of seasonal weight fluctuations for our study population. Nevertheless, adult raccoons in southern Texas exhibited seasonal fluctuations in body weight, with annual weight losses of 26-28% of mean maximum weights. These results appear to support the hypothesis that seasonal weight fluctuations exhibited by raccoons are scaled to respond to the severity of the climate (Mugaas and Seidensticker, 1993). Weight reductions from maximum to minimum annual weights of 31% to 50% have been reported for raccoon populations located at latitudes between 36 and 45 [degrees] N (Stuewer, 1943; Mech et al., 1968; Moore and Kennedy, 1985; Mugaas and Seidensticker, 1993), which are greater than the annual weight loss recorded for this study. However, variation in these patterns may occur among populations within climate zones. Annual percentages of weight lost from maximum body weights for Texas raccoons were greater than the 14-24% annual weight loss recorded for other populations at comparable latitudes (28-32 [degrees] N; see Mugaas and Seidensticker, 1993).
It is important to note, however, that the timing of the annual pattern of weight fluctuations in this study differed from that of northern populations. Raccoons of more northern populations typically lose weight during winter, reach a nadir in spring, and begin gaining weight in late spring and summer (Stuewer, 1943; Moore and Kennedy, 1985). Appreciable weight reduction in our study did not occur until March. We did not measure weights during November and December when raccoons may have been at their heaviest, but there was little difference in size between January and February, and most raccoons we captured or examined post-mortem in February appeared to be in excellent physical condition.
The sudden weight loss during March may have been partially a result of mating behavior. The temporal pattern of mating exhibited by our study population was bimodal, with a strong peak of activity during March and a smaller peak of activity during May and June (Gehrt and Fritzell, 1996b). During this study, nearly all adult females came into estrus during a two-week period in March or late February, which coincided with March weight loss. Adult males competed for access to estrous females, and some females consorted with multiple males (Gehrt, 1994). Given a polygynous/promiscuous mating system based on direct competition and the timing of most matings (i.e., during late winter or early spring), it is likely that mating behavior had an energetic cost associated with it. Similar patterns of weight loss associated with mating behavior have been observed for Virginia opossums (Didelphis virginiana), with up to 23% weight loss for adult males during the mating season (Ryser, 1992). The mating system of opossums is similar to that of raccoons, with direct competition among males for access to estrous females.
Energetic costs of mating may explain some weight loss in March, but it is unlikely that this factor alone accounts for the annual pattern of weight loss in both sexes. Summers in South Texas are long, humid and hot (Drawe et al., 1978), and the weight loss exhibited by raccoons during spring and early summer may have been a response to high ambient temperatures and a reduced need for energy provided by subcutaneous fat. Because raccoons do not seasonally adjust their basal metabolic rate, they must change their thermal conductance to reduce the potential for heat stress (Mugaas et al., 1993). In addition to molting (Irving et al., 1955) and evaporative cooling (Mugaas et al., 1993), weight loss may be an adaptive physiological response by raccoons to increasing ambient temperatures. The negative correlation between weight and average monthly temperature, and the similar seasonal patterns of weight changes between males and females, suggest that spring weight loss by raccoons in southern Texas may be a response to changes in ambient temperature.
Acknowledgements. - This study was supported by the Rob anti Bessie Welder Wildlife Foundation and the Missouri Cooperative Fish and Wildlife Research Unit (National Biological Service, Missouri Department of Conservation, University of Missouri, and Wildlife Management Institute, cooperating). S. D. Gehrt was supported during this research with Welder and Edward K. Love fellowships. Numerous students and staff at Welder assisted with fieldwork. Comments by M. L. Kennedy and J. N. Mugaas improved the manuscript. This is Welder Wildlife Foundation contribution 515, and contribution 12,760 of the Missouri Agriculture Experiment Station Journal Series.
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|Author:||Gehrt, Stanley D.; Fritzell, Erik K.|
|Publication:||The American Midland Naturalist|
|Date:||Jan 1, 1999|
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