The ecological cost of morphological specialization: feeding by a fossorial lizard

Summary

Head size and shape of reptiles may reflect selection for multiple uses. For example, sexual selection for large head size may enhance feeding efficiency. In contrast, morphological characteristics of the heads of fossorial reptiles suggests that fossoriality may have evolved at the expense of reduced effectiveness in feeding. Our research focused on the question: Does a fossorial lizard feed less effectively than a non-fossorial lizard? To answer this question, we measured the time, number of bites, and oxygen consumption by sand-swimming (Chalcides ocellatus) and epigeal (Eumeces inexpectatus) skinks feeding on crickets. These lizard species were similar in mass, but different in body form: Chalcides had longer bodies and smaller heads than Eumeces. For lizards of the same mass, Chalcides were unable to eat prey as large as those eaten by Eumeces, Chalcides took longer to eat prey of the same size than did Eumeces, and the aerobic energy cost of eating crickets of the same relative size (cricket mass/lizard mass) tended to be greater for Chalcides than for Eumeces. The ecologically relevant costs of feeding appear to be the upper limit to the size of prey and the time of feeding. Both costs would restrict the energy intake per unit time of Chalcides. Moreover, given the same energy requirements and prey community, Chalcides would have to feed more often and would take longer to feed than would Eumeces. Both factors would increase the exposure of Chalcides to predators relative to that of Eumeces. To reduce the risk of predation, Chalcides would have to reduce energy intake or fulfill its energy requirements with relatively small prey, or both. These conclusions are potentially confounded in two ways. The first is that male Eumeces have relatively large heads as a result of sexual selection. Thus, the differences we observed between Chalcides and Eumeces (most of our specimens were males) could have been the result of reduced costs of feeding for Eumeces due to sexual selection and not the result of enhanced costs of feeding for Chalcides. A more likely explanation is that differences in the costs of feeding observed between these species reflect adaptations for fossoriality by Chalcides and sexual selection on Eumeces. Our results may also be confounded because we compared laboratory reared Chalcides with field captured Eumeces. Any deterimental effects of captivity on the vigor of Chalcides would increase their costs of feeding relative to those of Eumeces. Although short-term captivity is not associated with changes in the metabolic capcity of lizards, effects of long-term captivitiy are unknown.