Boback, Scott M., and D. Mark Carpenter. Body Size and Head Shape of Island Boa constrictor in Belize: Environmental Versus Genetic Contributions. In Biology of the Boas and Pythons, edited by Robert W. Henderson and Robert Powell, 102-117. Eagle Mountain, UT: Eagle Mountain Publishing, 2007.

Marked differences in body size and head shape have been documented between island and mainland populations of Boa Constrictors (Boa constrictor) in Belize. Island B. constrictor average one-half the length and one-fifth the mass of snakes from the mainland, and also have longer heads and larger eyes. However, the genetic and environmental contributions to these differences are unknown. Herein, we describe patterns of offspring size and reproductive investment in island and mainland B. constrictor, present the results of common-garden experiments designed to elucidate whether body size and head shape differences between these populations are due to phenotypic or genetic responses to local conditions, and test whether prey size affects patterns of head shape. The results demonstrate that, despite a five-fold difference in mass, island and mainland B. constrictor contribute similar proportions of their mass to litters (relative clutch masses, RCM), but that neonate island B. constrictor are born at a shorter length and lighter mass than those on the mainland. However, litter size was significantly smaller in island B. constrictor and their relative expenditure per progeny (EPP) was greater than mainland snakes. These differences in investment are consistent with the idea that offspring size is optimized. After rearing in the common garden, island B. constrictor initially grew faster and made up the difference in birth mass (in 50 days) and birth length (in 100 days). Ultimately, however, island B. constrictor did not achieve the sizes of snakes on the mainland, supporting the idea that differences in free-ranging B. constrictor are due to a genetic difference in growth rates. Additionally, island B. constrictor were born with cranial elements of different sizes, which is consistent with external measures from free-ranging snakes. After adjusting for differences in body size, island B. constrictor had longer mandibles and narrower braincases than snakes from the mainland. When fed prey of different sizes (large vs. small rodents) for 22 month, island and mainland B. constrictor showed no differences in size-adjusted cranial elements. Thus, the results of this study demonstrate that cranial elements and body size (via growth rates) are determined at least in part by genetics, and thus support the possibility that head shape and body size in free-ranging B. constrictor might be locally adapted.

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