Adaptive divergence in diets between the sexes in a tropical snake (Stegonotus australis, Colubridae)

Males and females within a population may differ in dietary composition either as a non-adaptive consequence of sexual dimorphism, or because specific food types enhance fitness more in one sex than the other. To test between those two explanations, we can ask whether the consumption of a food type (a) is constrained by sexually dimorphic traits such as body size, or (b) differentially benefits the sex that consumes that food more frequently. A 23-year field study of Slatey-Grey Snakes (Stegonotus australis) in tropical Australia provided data on 663 meals, of which 130 were reptile eggs (primarily from Keelback Snakes (Tropidonophis mairii)). Over the same range of snake SVLs, eggs were consumed more often by female than by male Slatey-Grey Snakes (25.8 versus 15.2% of records), but consumption of reptile eggs was independent of snake body size. Female Slatey-Grey Snakes were not more common or more active than males during Keelback nesting periods, but they were more likely to be captured in the vicinity of Keelback oviposition sites than were males. In years with higher availability of Keelback eggs, female Slatey-Grey Snakes had higher clutch sizes and clutch masses (plausibly reflecting the nutritional benefits of eating reptile eggs to provision reptile eggs). In combination, our results suggest an adaptive basis to the sex-based divergence in dietary composition in Slatey-Grey Snakes.