Richard B. King

COLOR-PATTERN VARIATION IN LAKE ERIE WATER SNAKES : THE ROLE OF GENE FLOW

In an effort to clarify the evolutionary processes influencing color‐pattern variation in Lake Erie island water snake (Nerodia sipedon) populations, rates of gene flow among island and mainland populations were estimated from patterns of allozymic variation detected using electrophoresis. Rates of gene flow were high with Nm, the number of migrants per generation, averaging 25.5 among island sites, 9.2 between the Ontario mainland and the islands, and 3.6 between the Ohio mainland and the islands. Based on estimates of current population size from mark‐recapture work and of past population size extrapolated from the extent of shoreline habitat, values of m between island and mainland populations ranged from 0.0008–0.01. Synthesis of estimates of the rate of gene flow with information on inheritance of color pattern, the strength of natural selection, and population history supports the hypothesis that color‐pattern variation in island populations results from a balance between gene flow and natural selection. However, depending on the mode of inheritance of color pattern, stochastic processes such as drift may have been important in the initial stages of differentiation between island and mainland populations.

Population Ecology of the Lake Erie Water Snake, Nerodia sipedon insularum

Population ecology of the Lake Erie water snake, Nerodia sipedon insularum, is described based on a 5 yr capture-mark-recapture study involving 1449 captures of 1247 individuals. Water snakes are widespread in the island area of Lake Erie but have declined in numbers and have disappeared from one island within the last 50 yr. Population estimates for adult snakes range from 25 to about 500 individuals on seven islands. Snakes are active from late April until early October. Males are caught most often during the breeding season in May and June, while females are taken more often later in summer. Females appear to feed over a longer portion of the active season than males, grow at a faster rate (0.014 vs 0.012 cm/d mean growth rate) and attain a larger body size (82.1 vs 62.5 cm mean snout-vent length). Weight gain occurs throughout the active season in adult females but is restricted to mid-summer in adult males. Some females reproduce annually but smaller females may skip opportunities to reproduce. Number and size of offspring are positively correlated with female size. Comparisons with data from mainland populations elsewhere in the range of this species indicate that island water snakes differ in having larger adult body sizes (mean snout-vent length is 10-16 cm greater in males and 13-14 cm greater in females), lower growth rates (maximum growth rate is 0.06 cm/d vs 0.13 cm/d in mainland populations) and shorter tails (by 2-5% in males, 1-3% in females). In addition, litter size is less strongly correlated with female body size in island populations (r2 = 0.17 vs 0.34-0.72 elsewhere). Differences may also exist in size of newborns (mean newborn SVL of mainland population ranges from 12% shorter to 4% longer than that of island populations), diet (mainland populations typically consume more amphibians and fewer fish) and intensity of predation (frequency of stub tails is two times greater in mainland populations).

Effects of body size and melanism on the thermal biology of garter snakes (Thamnophis sirtalis)

Abstract Both color (i.e., reflectance) and body size should influence body temperature in heliothermic ectotherms. We compared heating rates and equilibrium temperatures between the normal striped morph and the solid black melanistic morph of the garter snake, Thamnophis sirtalis, from polymorphic populations in the Lake Erie region. Snakes ranging in mass from 2.6–216.2 g were divided into three size classes (small, medium, and large). Using temperature-sensitive PIT tags, we tested pairs of snakes under artificial light in an environmental chamber, then used curvilinear regressions to estimate heating rate and equilibrium temperature. In small and medium-sized snakes, we found no difference in heating rate nor equilibrium temperature between morphs, suggesting that melanism does not provide a thermal advantage in young snakes. In large snakes, morph affected equilibrium temperature, but not heating rate. Melanistics averaged 1.3 C warmer than striped snakes. This difference may represent a thermal advantage for adults, of which gravid females are the largest and may have the most to gain from a thermal advantage.

Analyzing the Relationship between Clutch Size and Female Body Size in Reptiles

Ecological studies of reptiles often include linear regression analyses of the relationship between number of eggs or offspring (clutch size) and female body size. Recent issues of Copeia (vol. 1997-1998), Herpetologica (vol. 53-54) and Journal of Herpetology (vol. 3132) include 19 publications in which the relationship between clutch size and female size in reptiles is reported. Length was used as a measure of female size in 16 analyses and mass was used as a measure of female size in three analyses. In only two cases did authors log transform clutch size and female size prior to analysis. Here, I suggest that analysis of log transformed variables is preferred for meeting the assumptions of linear regression and for facilitating biological interpretation and comparisons among studies. Log transformation of variables such as clutch size and

Determinants of Offspring Number and Size in the Brown Snake, Storeria dekayi

Determinants of reproductive variables in the brown snake, Storeria dekayi, are examined using path analysis, a method which reveals both direct effects (e.g., of female size on offspring number) and indirect effects (e.g., of female size on offspring size through offspring number). In S. dekayi, there is a clear trade-off between offspring number and offspring size. Part of this trade-off is mediated through the timing of parturition suggesting that placental nourishment contributes to offspring development. However, placental nourishment is not sufficient to result in an uncoupling of offspring number and size. Another clear result is that female size and female condition at ovulation (a size-corrected measure of mass) both influence offspring number. As a result, female size and female condition have indirect effects on offspring size such that overall, larger females and females in better condition produce smaller offspring. Larger females also produce offspring in poorer condition. Additional reproductive mass (mass lost by females at parturition beyond that accounted for by offspring) is determined directly by offspring number suggesting it represents a per-offspring allocation of resources. Captivity has marked effects on repro- duction; females held captive longer produced offspring which were smaller and in poorer condition. Comparisons among species reveals that female size consistently has positive effects on offspring number and offspring size in viviparous snakes but that the effect of offspring number on offspring size varies among species.

COLOR-PATTERN VARIATION IN LAKE ERIE WATER SNAKES: PREDICTION AND MEASUREMENT OF NATURAL SELECTION

A classic example of natural selection, that of color‐pattern variation in Lake Erie island populations of water snakes, was reexamined to overcome shortcomings resulting from classification of snakes into discrete color‐pattern categories and use of cross‐sectional data. Four continuously varying color‐pattern components (DB, the number of dorsal blotches; LB, the number of lateral blotches; ROWS, the height of lateral blotches measured in scale rows; and VEXT, the extent of ventral pigmentation) were analyzed. Patterns of natural selection were predicted from the relationship between color‐pattern scores and independent measures of relative crypsis. Tests for natural selection were carried out using longitudinal data on neonate to juvenile‐aged snakes and cross‐sectional data on juvenile to adult‐aged snakes. As predicted, the form of selection differed between younger and older age classes of snakes: selection resulted in a reduction in DB and LB among neonate and juvenile snakes but had little influence on color‐pattern components in older snakes. The correspondence between observed patterns of natural selection and predictions based on the relationship between color‐pattern scores and relative crypsis supports the hypothesis that differential predation by visual predators on younger age classes of snakes is the mechanism of selection. Gene flow from mainland populations or the initial lack of an allele necessary for reduced pattern may explain why selection has not resulted in greater differentiation between island and mainland populations.

Evidence for Phenotypic Plasticity in Snake Body Size and Relative Head Dimensions in Response to Amount and Size of Prey

To test whether variation in body size and relative head dimensions may be a phenotypically plastic response to feeding experience in snakes, we assigned neonate water snakes, Nerodia sipedon, from four litters (n = 48) to two feeding treatment groups. One group was offered one large minnow twice weekly; the other group was offered two small minnows twice weekly. Body size (snout-vent length, mass) and head dimensions (head length, head width, jaw length, interocular distance) were measured at the beginning of the experiment and upon completion of the experiment 20 weeks later. Repeated-measures multivariate analysis of variance revealed significant effects of feeding treatment, sex, and family on change in body size. Repeated-measures MANCOVA (with body length as covariate) also revealed a significant effect due to family on change in head dimensions. The multivariate effect of feeding treatment on head dimensions approached statistical significance (P = 0.102); the univariate effect of feeding treatment was significant for jaw length (P = 0.010). Females increased in body size more than males did, and snakes offered large fish increased in body size and jaw length more than snakes offered small fish. These results suggest that body size and jaw length respond plastically to amount and size of prey.

Physiological Color Change in the Green Treefrog: Responses to Background Brightness and Temperature

In an effort to better understand the function of physiological color change in anurans, we measured color change responses of the green treefrog, Hyla cinerea, to background brightness and temperature. We used our results to compare color and color change responses of males and females and to test two hypotheses for the function of physiological color change in anurans: (1) that color change serves to prevent detection by visual predators, and (2) that color change functions in thermoregulation or water balance. We found that (1) males are generally darker than females but the sexes exhibit similar color change responses, (2) treefrogs become lighter on brighter backgrounds, (3) treefrogs become lighter at higher temperatures, and (4) there is a background-by-temperature interaction such that color change responses to one variable depend on the level of the other. Our results suggest that color change may function both in predator avoidance and in thermoregulation or water balance. However, further investigations of color change responses to differently hued backgrounds, of the magnitude and biological significance of color-induced changes in body temperature and rates of evaporative water loss, and of color change in freeranging animals are needed.

Growth, Survival, and Reproduction in a Northern Illinois Population of the Plains Gartersnake, Thamnophis radix

Abstract A six-year mark-recapture study, consisting of 567 captures of 216 individually marked snakes, and the birth of 889 young in 53 litters born to 39 wild-caught females were used to estimate demographic and life-history parameters of a Northern Illinois population of Thamnophis radix. Using the von Bertalanffy growth model, males were found to differ from females in asymptotic size but not in the rate at which they approached this size. Size of known age individuals together with growth rate estimates were used to assign age at first capture. Age and sex were then used as grouping variables to obtain age- and sex-specific survival using program MARK. The results suggest that males and females have approximately equal survival in the 0 and 1 age classes but that females have higher survival than males as adults (0.45 vs 0.35). Population estimates calculated using Schumacher-Eschmeyer and Jolly-Seber methods indicate an adult population size of 64 and 172 adults, respectively, corresponding to densities of 40 and 107 adults per hectare. Average female fertility increased from 6.4 in one-year-old females to 21 among six-year-old females. Detailed demographic studies such as this have utility in development of management strategies and theories concerning life-history evolution.

Lake Erie water snakes revisited: Morph- and age-specific variation in relative crypsis

SummaryPopulations of water snakes (Nerodia sipedon insularum) on islands in western Lake Erie are variable in colour pattern, consisting of unbanded, intermediate, and banded morphs. In contrast, mainland populations (N. s. sipedon) consist solely of banded morphs. Previous investigators hypothesized that natural selection favoured unbanded morphs on exposed island shorelines and banded morphs in overgrown mainland habitats and that gene flow from mainland populations was responsible for the persistence of banded morphs on islands. To clarify the potential role of natural selection, I quantified relative crypsis among morphs and age classes of water snakes by comparing the size of patches making up their colour patterns with the size of patches in island and mainland backgrounds. This analysis reveals that if unbanded morphs are more cryptic than intermediate and banded morphs on islands, it is only in the young-of-the-year age class. For older snakes on islands and for all snakes on the mainland, unbanded morphs are consistently less cryptic than intermediate and banded morphs. Given these results, the net direction of selection in island populations should depend on the intensity of predation on different age classes of snakes. Overall, selection may favour unbanded morphs (e.g. if predation occurs primarily on young-of-the-year), intermediate and banded morphs (e.g. if predation occurs primarily on older snakes), or be weak or absent (e.g. certain combinations of predation on young-of-the-year and older snakes). Using estimates of relative crypsis to guide reanalysis of morph frequency data, I find support for the hypothesis that unbanded morphs are favoured by natural selection in island populations.