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

Abstract

The causes and consequences of body size and sexual size dimorphism (SSD) have been central questions in evolutionary ecology. Two, often opposing selective forces are suspected to act on body size in animals: survival selection and reproductive (fecundity and sexual) selection. We have recently identified a system where a small aquatic snake species (Seminatrix pygaea) is capable of surviving severe droughts by aestivating within dried, isolated wetlands. We tested the hypothesis that the lack of aquatic prey during severe droughts would impose significant survivorship pressures on S. pygaea, and that the largest individuals, particularly females, would be most adversely affected by resource limitation. Our findings suggest that both sexes experience selection against large body size during severe drought when prey resources are limited, as nearly all S. pygaea are absent from the largest size classes and maximum body size and SSD are dramatically reduced following drought. Conversely, strong positive correlations between maternal body size and reproductive success in S. pygaea suggest that females experience fecundity selection for large size during non-drought years. Collectively, our study emphasizes the dynamic interplay between selection pressures that act on body size and supports theoretical predictions about the relationship between body size and survivorship in ectotherms under conditions of resource limitation.

Effects of even-aged timber harvest on stream salamanders: Support for the evacuation hypothesis

“…Retreat may also lead to a skewed population size distribution following recovery from persistence in suboptimal conditions due to differential survival. Previous studies with salamanders (Ash, 1997;Sattler and Reichenbach, 1998) and turtles (Bodie and Semlitsch, 2000) suggest that larger individuals may better survive or recolonize following habitat perturbations, although a study by Winne et al (2010) on aquatic snakes found that intermediate-sized individuals were most likely to survive. Again, only with time will these alternative scenarios play out, but we do know that southern Appalachian salamander assemblages are predictably distributed from the stream into the surrounding forest with larger species tending to be more aquatic and smaller species more terrestrial (Hairston, 1949(Hairston, , 1980Organ, 1961;Crawford and Semlitsch, 2007).…”

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

“…In deserts, higher primary productivity, and subsequent increases in rodent abundance, results from higher precipitation (Beatley, 1969;Brown and Ernest, 2002). This precipitation-driven resource input could influence snake growth and therefore size through a bottom-up pattern of regulation (Madsen and Shine, 2000;Winne et al, 2010). Beaupre's (2002) model, however, predicted that only male rattlesnakes would increase in body size in resource-rich environments, because adult females allocate nearly all available resources to reproduction, thus additional food had little effect on body size.…”

“…If the allocation strategy in females is constant, then increased food resources would increase reproductive output, but not increase growth or body size. Thus, in populations with more available resources, SSD is likely to be pronounced (Beaupre, 2002;Winne et al, 2010).…”

Adult survival selection in relation to multilocus heterozygosity and body size in a tropical bird species, the Zenaida dove, Zenaida aurita

“…Indeed, directional (positive or negative) (Szulkin et al 2010) or stabilizing selection on body size has been evidenced in several vertebrate species. For instance, both juvenile and adult survival can be size dependent in fishes and reptiles (Hurst and Connover 1998;Bodie and Semlitsch 2000;Munch et al 2003;Carlson et al 2008;Johnson and Hixon 2010;Winne et al 2010). Differential survival according to body size has also been observed in mammals, although the evidence is limited to a few species (Milner et al 1999;SchulteHostedde et al 2002).…”