TAYLOR, E.N.; DENARDO, D.F.; Arizona State University; Arizona State University: Why are male rattlesnakes larger than females?

In most rattlesnake species, adult males are heavier and longer (snout-vent length) than females. The predominant hypothesis for this sexual size dimorphism (SSD) is sexual selection for large males, since male rattlesnakes fight for access to females. Another complimentary hypothesis is that it is beneficial for females to be smaller than males because small females have low metabolic costs of maintenance, allowing them to accumulate energy stores rapidly and reproduce frequently. Alternatively, SSD may be the result of phenotypic plasticity rather than natural selection, with females growing more slowly than males because they have higher energetic costs of reproduction, leaving less energy available for growth. Identification of the proximate, physiological mechanism responsible for SSD would help elucidate the evolutionary and ecological significance of SSD in rattlesnakes. I have conducted a series of experiments to understand how male Western Diamond-backed Rattlesnakes (Crotalus atrox) become larger than females. Male neonates are slightly larger than females, but this difference does not account for the extreme difference in size observed in adult snakes. Testosterone does not appear to stimulate growth in male C. atrox, as it does in many other vertebrates, as castrated free-ranging males grow at the same rate as sham-operated males. Males and females raised on controlled diets in the laboratory grow at the same rates and never diverge in size, with both sexes growing to sizes much in excess of free-ranging snakes. Similarly, supplementation of the diets of free-ranging females leads to dramatic increases in growth. It therefore appears that SSD in C. atrox is the result of phenotypic plasticity, since females are capable of growing to male-like sizes. If this is true, then the natural selection hypotheses for SSD in rattlesnakes may not be supported.