Comparative demography of black rat snakes (Elaphe obsoleta) in Ontario and Maryland

Abstract

This study investigated how demographic characteristics of black rat snakes Elaphe obsoleta are affected by the length of the active season, and also used the resulting demographic data to determine the proximate factors responsible for male-biased sexual size dimorphism (SSD) in the species. Demographic data collected from 1981 to 1998 in Ontario (ON; 583 males and 588 females) and from 1942 to 1976 in Maryland (MD; 180 males and 150 females) were used to develop growth models with the von Bertalanffy growth equation. Instantaneous growth rates declined signi®cantly with increasing snout±vent length (SVL) in males and females of each population. The growth models predicted age using SVL satisfactorily, but the accuracy of the model decreased signi®cantly with increasing SVL. As predicted, based on the brevity of their active season (ON % 135 days, MD % 190 days), rat snakes of both sexes from Ontario had lower and more variable instantaneous growth rates and matured at a more advanced age (ON % 9 years, MD % 4 years) than snakes from Maryland. However, the rapid growth and early maturation in Maryland snakes occurred at the expense of longevity (maximum: ON % 30 years, MD % 20 years). Slower growth and later maturation will make rat snakes in Ontario less capable of recovering from population declines. Within each population, males grew faster than females. Survivorship for Ontario snakes did not vary by sex, but increased signi®cantly with increasing SVL. The demographic consequence of sex differences in growth was a male-biased sex ratio among larger snakes. Because SSD is a function of males growing faster than females, SSD in black rat snakes is probably a product of sexual selection, suggesting that large size confers a mating advantage in males.

Drought survival and reproduction impose contrasting selection pressures on maximum body size and sexual size dimorphism in a snake, Seminatrix pygaea

“…Third, in reptiles, survivorship generally increases with age or is independent of age after the first year (Turner 1977;Parker and Plummer 1987). Age and body size are often not highly correlated in adult reptiles because of substantial individual variation in growth trajectories (Madsen and Shine 2000b;Blouin-Demers et al 2002). Thus, although we were not able to age adult S. pygaea in our population, it is unlikely that all of the largest individuals were also the oldest individuals.…”

Potential environmental influences on variation in body size and sexual size dimorphism among Arizona populations of the western diamond-backed rattlesnake (Crotalus atrox)

“…In general, body sizes of endotherms are negatively (i.e., Bergmann's Rule) and ectotherms positively correlated with temperature (Ashton and Feldman, 2003;James, 1970; but see: Olalla-Tárraga et al, 2006), but there is no general agreement on the proximate determinants responsible for these patterns. In terrestrial ectotherms, for example, regions of warmer climate provide extended periods of activity and potentially more foraging opportunities, which may lead to accelerated growth and, ultimately, larger body size (Blouin-Demers et al, 2002;Bronikowski and Arnold, 1999). Alternatively, high environmental temperatures may limit diurnal activity and cause increases in body temperature, leading to higher maintenance metabolism and a resulting smaller body size (Beaupre, 1995b;Grant and Dunham, 1988).…”

Phenotypic plasticity of life history traits in relation to reproductive strategies in Boa constrictor occidentalis

“…In snakes, several factors such us gender and size modulate detection probabilities (Christy et al 2010). However, as we used the same sampling method in both populations, we assume that size class distributions between sites can be compared (Velasco andAyarzaguena 1995, Rohr 1997;Vitt et al 1997;Shine et al 1999;Waye 1999;Wapstra et al 2001;Blouin-Demers et al 2002;Aponte et al 2003;Boback 2006;Hoare et al 2007). Statistical analyses were performed with InfoStat version 2007p (Grupo Infostat, Facultad de Ciencias Agropecuarias -Universidad Nacional de Córdoba, Córdoba, Argentina) at a = 0.05.…”

“…Our results showed that this optimum body size is shaped by trade-offs in life-history parameters to maximize not only clutch size but also other reproductive parameters such as reproductive frequency. Furthermore, since sexual size dimorphism is probably a product of sexual selection and might reveal the intensity of competition for mates (Shine 1994;Blouin-Demers et al 2002;Karsten et al 2009), size of males could be also variable. Large body size in males would enhance male reproductive success in mating systems that reward physical strength and in species that are not highly malebiased, as B. c. occidentalis.…”