EVOLUTION OF HERBIVORY IN LACERTID LIZARDS : EFFECTS OF INSULARITY AND BODY SIZE

-I tested the putative correlation between insularity and herbivory in lacertid lizards. Analysis of literature data on 97 populations of 52 species shows that lizard populations on islands more often include plant material in their diet than do mainland populations. To investigate whether this finding reflects adaptation due to recent selection or is merely a product of the phylogenetic history of the populations, I reconstructed the ancestral states for diets and insularity and incorporated them in the analysis. Changes in habitat (island-mainland or mainland-island) often went with changes in diet (herbivore-insectivore or insectivore-omnivore). Insectivorous lizards that find themselves on islands more often turn towards herbivory than do lizards living in mainland situations. Lizards that already have plants in their diet when living on the mainland seem more successful in colonizing islands. Herbivorous populations of lacertids tend to be larger than insectivorous populations, but there is considerable overlap No difference in mean snout-vent length was found between island and mainland populations. etolog , Vol. 33, No. 4, pp. 6 3-674, 19 ciet for the Study of Amphibians and Reptiles i or in Lacertid Lizards: Ef ects of Insularity and As expected from their great taxonomic diversity, lizards have evolved a great range of diets; insects and other arthropods, mammals and birds, vertebrate eggs, snails, fish, nectar, and various mixtures of leaves, flowers, and fruits. Although the dietary range of lizards is great, the distribution of foods eaten reveals that complete herbivory is rare compared to a diet of arthropods. Only about 3% of all extant lizard species are known to eat significant quantities of plant food (King, 1996). On the other hand, many species occasionally shift away from a diet only of arthropods, toward an omnivorous or herbivorous diet (e.g., Pough, 1973; Schluter, 1984; Perez-Mellado and Corti, 1993; King, 1996). Several authors have speculated on the ecological circumstances that could induce insectivorous lizards to expand their diet to include plant material (Pough, 1973; Schluter, 1984; Perez-Mellado and Corti, 1993; King, 1996). Because plant material is less digestible and may produce substantially slower growth (Pough, 1973; Schluter, 1984), herbivory is often considered a 'forced move', that lizards will not make unless arthropods are insufficient to meet their energy requirements (but see Johnson and Lillywhite, 1979; Sadek, 1981; Troyer, 1984). Lizards living in habitats where insect abundance is (periodically) low may benefit from (partial) herbivory. Large body size is also considered an incentive for herbivory, because larger lizards putatively have greater energetic needs and are les adept at catching small prey because they must move a greater mass in pursuit (Sokol, 1967; Pough, 1973; Sadek, 1981; but see Greene, 1982). The bulk of exclusively herbivorous lizard species is indeed larger than 300 g (Sokol, 1967; Pough, 1973; but see Greene, 1982). If the above presumptions are true, insular lizard populations can be expected to feed more often on plant material than mainland populations. Indeed, islands (especially small ones) often have poor arthropod faunas (Janzen, 1973a, b), and at least for some families and genera of lizards, there is a tendency for larger body size in island races (Case, 1978). Although many au663 This content downloaded from 157.55. 39.231 on Thu, 06 Oct 2016 04:43:17 UTC All use subject to http://about.jstor. org/terms