Ecological and evolutionary sources of geographic variation in individual growth rates of the eastern fence lizard Sceloporus undulatus (Iguanidae)

Peter Henryk Niewiarowski, University of Pennsylvania


My dissertation examined the relative importance of proximate ecological versus ultimate evolutionary sources of phenotypic variation in hatchling growth rates of lizards from Nebraska and New Jersey. Individuals from these two populations span the range of variation in growth rate in this widely ranging species. Variation in individual growth rates is associated with variation in life history traits such as age and size at maturity, adult body size, fecundity, and longevity. It has been hypothesized that slow growth in eastern populations like New Jersey is a result of either relatively low food availability or a strong thermal constraint on activity. However, supplemental feeding of free-ranging lizards in New Jersey did not increase growth rates. Furthermore, hatchlings from New Jersey reared in thermally unconstrained laboratory conditions did not grow faster than their free-ranging counterparts. I used a reciprocal transplant experiment to examine the relative importance of growth environment and source population in determining hatchling growth rates. Hatchlings from Nebraska grew twice as fast in Nebraska compared to those transplanted to New Jersey. In contrast, hatchlings from New Jersey grew at the same rate in both New Jersey and Nebraska, roughly half the rate of Nebraska hatchlings in Nebraska. This suggested that S. undulatus in Nebraska and New Jersey have diverged genetically with respect to growth rate. However, the reduced growth rates of Nebraska hatchlings transplanted to New Jersey also suggested that the growth environment could induce a phenotypic response similar in magnitude and direction to the apparent genetic differences. A comparison of food resource availability suggested that food was more abundant in New Jersey, the habitat where growth was slowest. In contrast, a thermal biophysical comparison of the Nebraska and New Jersey habitats suggested that there was more time available for activity in Nebraska than in New Jersey. Finally, differences in growth rate among transplanted and resident lizards from both populations was associated with differences in the relative allocation of energy made to storage lipids.

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Recommended Citation

Niewiarowski, Peter Henryk, "Ecological and evolutionary sources of geographic variation in individual growth rates of the eastern fence lizard Sceloporus undulatus (Iguanidae)" (1992). Dissertations available from ProQuest. AAI9235181.