Evolution of the third eye: a phylogenetic comparative study of parietal-eye size as an ecophysiological adaptation in Liolaemus lizards

Abstract

The parietal, or third, eye is a photosensory organ situated in the middle of the skull of many lizards. Despite many hypotheses, its exact ecological functions are still unclear. Studies have compared the presence and absence of a functioning parietal eye, although there are no quantitative studies of parietal-eye traits in relation to ecology, physiology or behaviour. In the present study, we report the first comparative study of relative parietal-eye size in relation to climatic and thermophysiological variables. We studied thirty species of Liolaemus, a genus of South-American lizards inhabiting a range of climatic conditions, but found little evidence for adaptation to thermal environment, in that parietal-eye size did not vary meaningfully with latitude, altitude or any measures of environmental temperature. Neither did it relate to thermophysiology; there was a weak relation to thermal tolerance, although this was partially confounded with body size, which explained 23% of the among-species variance after controlling for within-species variation. The negative results obtained could not be explained by phylogenetic constraints because we found no evidence of phylogenetic inertia. We also observed high intraspecific variation indicating that parietal-eye size may not be under strong selection for accuracy.