Prey-handling and the evolutionary ecology of sand-swimming lizards (Lerista?: Scincidae)

Abstract

Fossorial lizards differ in morphology from their surface-dwelling relatives. The Australian sphenomorphine skink genus Ctenotus consists of surface-dwelling species, and is closely related to the genus Lerista, which includes both surface-dwelling and fossorial species. Sand-swimming represents the derived condition and has evolved independently in several lineages of Lerista. The heads of lizards in the two genera differ in shape (blunt snout for Ctenotus versus wedge-shaped for Lerista) and in length relative to the body (approximately 20% of snout-vent length for Ctenotus versus 12% for sand-swimming Lerista). Do these specializations affect the sizes or types of prey that can be consumed by Lerista? We compared prey-handling by Ctenotus and Lerista to correlate morphological differences with differences in prey-handling ability, and to distinguish the effects of snout shape and head length. Feeding trials included three categories of insect prey that the lizards normally eat: soft-bodied larvae (Lepidoptera), hard-bodied larvae (Coleoptera), and roaches (Blatoidea). In comparisons based on the mass of a prey item relative to the mass of a lizard, Lerista had longer handling times for all prey categories and were limited to smaller prey than were Ctenotus. However, when comparisons were based on the length of prey relative to the length of a lizard's head, Lerista ate some elongate prey as fast or faster than did Ctenotus, and both genera successfully swallowed prey more than twice the length of their own head. Thus, the differences in prey-handling performance of Ctenotus and Lerista probably result from the fact that Lerista have a relatively shorter head than Ctenotus. All Lerista species, surface-dwelling and fossorial, have short heads compared to primitive sphenomorphine lizards. Fossorial species of Lerista have elongate trunks, and consequently their heads are shorter in proportion to trunk length than those of surface-dwelling Lerista. However, most fossorial species of Lerista are longer and heavier than any of their surface-dwelling congeners, and the heads of these fossorial species are large relative to the prey they encounter. As a consequence, the diets of large fossorial species of Lerista do not appear to be limited by their morphological specialization for sand-swimming.