How does prey abundance influence sexual size dimorphism in tropical snakes?

Gregory P. Brown, Thomas R. L. Madsen, Rick Shine

In most animals males and females differ in body size; sometimes males are the larger sex and sometimes females are. The difference in size between males and females (called the degree of Sexual Size Dimorphism or SSD) is thought to reflect a disparity in the costs and benefits of large body size between the sexes. Large body size might be a benefit if it lets you produce more eggs, or win more fights against rivals. But it may also be a cost if it makes you more visible to predators or requires more food to maintain.

We studied two species of tropical snakes to determine if the ability of males and females to grow rapidly during periods of high food abundance reflected the benefits that large size held for them. Both snake species preyed on frogs. In Slatey-grey Snakes, males and females increased growth rate to the same extent when frogs were abundant. In this species, both males and females have higher reproductive success if they are big. Large males father more offspring than smaller ones and large females produce more eggs and larger eggs than small females do. In Keelback snakes, females grow faster when frogs are abundant but males do not. In this species larger females also produce more and larger eggs than small females, but large males are unlikely to have higher reproductive success than small males. This sex-specific growth plasticity in Keelbacks could contribute to temporal or spatial variation in SSD.

We found that the extent to which snakes utilize prey resources for growth depends on how much their reproductive success is enhanced by being large. Thus previous levels of prey abundance can partly shape the extent to which SSD is expressed in wild populations.

Image caption: Two species of frog-eating snake show different patterns of sex-specific growth plasticity in response to prey abundance. In Keelback snakes (left) females grow faster when frogs are abundant but males do not. In Slatey-grey snakes (right) both sexes increase growth rate when frogs are abundant. Photo credit G. P. Brown.
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