Anderson, Alexandra E.
Gienger, C.M.
Department of Biology
Center of Excellence for Field Biology
Austin Peay State University
Clarksville, Tennessee, USA
The Gila Monster (Heloderma suspectum) is a large-bodied venomous lizard that inhabits deserts across the Southwestern United States and Northern Mexico. Previous studies have observed sexual dimorphism in this species; males have larger heads than females after accounting for body size. This trait is thought to be under sexual selection due to the intense male-male combat observed in their mating system. Sexual dimorphism that could facilitate higher fecundity in females, such as larger abdomen (trunk) size, has not been previously tested. Female Gila Monsters lay a clutch every 2-4 years, with each clutch being relatively large, suggesting they could benefit from having larger trunk sizes (relative to males) to accommodate large clutches and more offspring. Using morphometric data from preserved zoological museum specimens, we tested whether trunk size could be a target of selection and if a difference in trunk size exists between males and females. Our results indicate that after accounting for body size, there is no significant difference between trunk sizes of males and females; adult males and females are also similar in size on a multivariate basis. Juvenile females are significantly larger in a multivariate way than males, potentially due to rapid growth to attain large body size for breeding purposes. Trunk size for both sexes had a significant effect on the amount of reproductive tissue (testes or eggs) that could be contained in the body, indicating that a larger abdomen could bring about higher fecundity.