Elevational Covariation in Environmental Constraints and Life Histories of the Desert Lizard Sceloporus Merriami

Abstract

We examine environmental constraints on life history characters among three elevationally distinct populations of the desert lizard Sceloporus merriami in west Texas. We show how environmental gradients in temperature and food abundance interact to constrain body temperatures, daily activity times, growth rates, and age—specific body size. We suggest that these differences resulted in opposite responses from the males and females with respect to their size and age at first reproduction. The highest elevation site, Maple Canyon (1609 m), has more rainfall, higher food availability, and cooler temperatures compared with the lower elevation sites, Grapevine (1036 m) and Boquillas (560 m). Lizards were active throughout the day at Maple Canyon, but were restricted to midday inactivity at the lower sites. Body temperatures (Tb) were higher during inactivity and late—afternoon activity at the lower sites. We suggest that low food availability, fewer hours to forage, and higher Tb constrained the energy budgets of lizards at Boquillas. Consequently, these lizards had low activity rates and low daily growth rates. Paradoxically, lizards at Maple Canyon, the most food—rich site, also exhibited low daily growth rates. This may have resulted from higher daily activity expenditures (longer activity period) and lower food processing rate (low inactive (Tb). Grapevine lizards had the highest individual activity and growth rates. On a seasonal basis, the pattern of activity time was reversed, and Maple Canyon lizards had a shorter annual activity by °1 mo compared with Grapevine and 2 mo compared with Boquillas. Despite low growth rates at Boquillas, a 2—mo longer activity season proximally resulted in the largest yearlings by May–June. Maple Canyon yearlings grew at a similar rate, but over a shorter time period and were the smallest. At Grapevine, high summer growth rates resulted in large yearlings by late summer and largest adults, whereas low summer growth rates at Boquillas and Maple Canyon led to smaller adults. The resulting difference in body size between yearling and older age classes was great at Maple Canyon and at Grapevine, which forced yearling males to delay reproduction until they had grown to a size to compete successfully for breeding territories. However, Boquillas yearlings and age >1 yr lizards were much more similar in size, and many yearlings were able to acquire breeding territories. This suggest that male age of first reproduction was lowest at the low evaluation site. In contrast, more yearling females reproduced earlier at the highest elevation site than at other sites. At Grapevine and Boquillas, females similar in size to breeding females at Maple Canyon did not breed in May–June and instead grew and were reproductively active in July–August. We hypothesize that the brief growing season at Maple Canyon may have constrained females to breed early to allow sufficient time for themselves, as well as for their offspring, to amass winter stores. Also, this would increase the likelihood that their offspring were large enough to breed as yearlings. Results suggest an interaction between resource levels and biophysical constraints that may greatly influence differences among populations in important life history characteristics. Although these responses are hypothesized to be proximally induced by environmental constraints, the resulting life history differences in age—specific resource allocation to growth, storage, and reproduction may significantly affect fitness.