Resource availability and sexual size dimorphism: differential effects of prey abundance on the growth rates of tropical snakes
Broad phylogenetic patterns in sexual size dimorphism (SSD) are shaped by sex differences in net selection pressures (e.g. sexual selection, fecundity selection, survival selection), but environmental and ecological factors can also affect the expression of SSD. Discussions of proximate ecological influences on SSD have focused on niche divergence; for example, increase in a prey type used by only one sex can elevate growth rates of that sex but not the other. Food limitation also can generate spatial and temporal variation in SSD. Under restricted prey abundance, curtailed growth may mask SSD even if the optimal size is greater for one sex than the other. Because an increase in food availability elicits increased feeding and growth by the sex that benefits more from increased body size, variation in prey abundance can generate variation in SSD. We used mark-recapture methods to study growth rates relative to prey (frog) abundance in two species of sexually dimorphic colubrid snake species in tropical Australia. In slatey-grey snakes (Stegonotus cucullatus), a species in which larger body size enhances reproductive output in both sexes (because larger males win combat bouts, and larger females produce more/heavier eggs), increased abundance of frogs caused equivalent increases in growth rates in both sexes and hence did not affect SSD. In keelbacks (Tropidonophis mairii), a species in which larger size enhances reproductive output in females more than males (reflecting a lack of male-male combat), increased abundance of frogs elicited higher growth rates of females only. Thus, SSD in keelbacks was modified by prey abundance. Our results show that the magnitude of sex differences in adult body size can be influenced by proximate environmental factors and support the hypothesis of sex-specific targets for maximum feeding rates. A lay summary is available for this article.