Sexual size monomorphism of a Pygmy Rattlesnake (Sistrurus miliarius) population in southwestern Missouri, USA

What is it about?

Along with their venom, North American pitvipers are known for their sexual dimorphism, with males being larger in body size than females, and for their ritualistic male to male combat displayed during their mating season. During these combat bouts, each male attempts to pin the other’s head to the ground. Thus, the larger individual usually has the advantage. This creates a drive for body size to increase that is absent in females. Pygmy Rattlesnakes are one of the smallest species of rattlesnakes in North America and are widely distributed throughout the southeastern parts of the United States. However, most of the current knowledge about the species was gathered from populations in Florida. From those populations, we have learned that males are not that much bigger than the females. Our study aimed to see if this was the case in another population of Pygmy Rattlesnakes in the most northwestern end of their distribution, southwestern Missouri. We found similar results to the Florida populations; the males and females were very similar in their body sizes. We propose that the sexes in this species might have a more balanced selective pressure for body sizes, possibly due to females also needing to achieve body sizes similar to males in order to increase their reproductive success, by giving birth more often, to offset this species’ high mortality rates found in other studies.

Featured Image