Nesting behavior in the eastern fence lizard (Sceloporus undulatus) at its range limits
The natal environment has have important effects on the quality, survival, and reproduction of offspring. For egg-laying species, selection favors females who are able to choose nesting sites that maximize the quality of their offspring. In the oviparous eastern fence lizard, these nesting requirements limit the northern distribution of the species in the United States. It has been demonstrated that females dig nests that provide optimal moisture and temperature environments for incubation. These studies all worked with populations that occur in regions of sandy soil. Since the soil substrate varies considerably across the latitudinal range of this species, it is likely that lizards at the northern range limit may choose nests at different depths than females in southern populations. Sandy, southern soils will heat and desiccate more quickly; however, northern soils have greater clay content and deep nests may not warm sufficiently for successful incubation. Additionally, many northern populations occur on steep, rocky slopes that may prevent the digging of nests. We tested the oviposition behavior of female fence lizards in the laboratory from three different populations: Alabama, Arkansas, and Pennsylvania.
The results of this study suggest that females from northern populations may nest differently than those from southern populations. In the laboratory, fence lizards typically dig nests to bury their eggs or oviposit directly on the substrate surface. All 29 females from the Arkansas (n = 17) and Alabama (n = 12) populations exhibited typical nesting behavior, with the majority digging nests in the sandy substrate provided. By contrast, only 3 Pennsylvania females (n = 10) laid their eggs on the surface and none of them dug nests. The remaining females laid eggs beneath the basking structure and filled the cavity with relatively dry sand. This unique nesting behavior using existing cavities may be an advantage for northern populations that encounter rocky soils. These nests tend to be shallower. Since these northern populations experience lower temperatures but relatively frequent rainfall, a shallow nest may be required for successful incubation. These results have important implications for range predictions in this species, as current models assume that all populations dig nests. Further studies should address the characteristics of these nests and potential substrate preferences among gravid female fence lizards.