Convergent evolution of plant chemical discrimination by omnivorous and herbivorous scleroglossan lizards

Abstract

Because sensory capacities and responsiveness must correspond to adaptively important environmental cues, they may require evolutionary modification to match shifts in the adaptive milieu. Many lizards identify prey by tongue-flicking, especially actively foraging species of Scleroglossa, the speciose lizard taxon that includes snakes. Correspondence between diet and chemosensory responsiveness, although known in two genera of colubrid snakes, has not been established in lizards because most species are generalist predators. Omnivory and herbivory are uncommon among scleroglossans, but have evolved frequently enough to permit tests of the hypothesis that shifts to plant consumption have been accompanied by acquisition of responsiveness to plant food chemicals. Based on literature surveys I traced the evolution of scleroglossan lizard diet and food chemical discrimination. Three tests taking phylogenetic relationships into account (concentrated changes tests, Pagel's tests, and a modification of Ridley's test) showed that correlated evolution has occurred, with strong responses to plant chemicals having evolved convergently in at least five taxa that underwent shifts from insectivory to omnivory or herbivory. In all four origins of omnivory and herbivory in taxa derived from ancestral active foragers, lizards retained prey chemical discrimination. In the single origin of omnivory in an ancestrally ambush foraging taxon, both plant and prey chemical discrimination evolved de novo. These findings show that one aspect of chemosensory identification of food has been evolutionarily adjusted to diet in a major group of vertebrates.