Reptiles differ vastly in how they communicate. Some species are predominantly visually-oriented, whereas other species rely almost exclusively on chemical signals for communication. Despite such marked differences in communication modalities, there is surprisingly little known about how communication modalities translate into differences in neuranatomy among species. Chris Robinson, an undergraduate working with Dr. Michele Johnson at Trinity University, presented a study examining the relationship between sense perception and neural density in six species of lizards.
Chris predicted that visually oriented lizards should have larger and more densely packed neurons in two visual centers – the lateral geniculate nucleus (LGN) and the optic tectum (OT) – whereas lizards that employ chemical modalities should have a similar pattern in the nucleus sphericus of the amygdala (NS). He included three iguanid species in this study, the green anole Anolis carolinensis, the curly tail Leiocephalus carinatus, the Texas spiny lizard Sceloporus olivaceous, as well as the whiptail Aspidoscelis gularis, the skink Scincella lateralis, and the Mediterranean house gecko Hemidactylus turcicus. To determine which sensory modalities best characterized each species he performed focal behavioral observations. During these observations, he quantified the number of chemosensory behaviors (rubbing the cloaca on a substrate, licking the air or substrate) and visual behaviors (head bobbing, dewlapping, and tail curling). Chris amassed over 120 hours of behavioral observations, and 10-33 hours per species, which is no small feat.
The results from the behavioral trials indicate that Anolis carolinensis, Leiocephalus carinatus, and the Texas spiny lizard (Sceloporus olivaceous) are visually-oriented lizards, whereas Aspidoscelis gularis and Scincella lateralis use chemical modalities. The curly-tail and green anole both engaged in headbobbing displays. Finally, he found that the house gecko, Hemidactylus turcicus, uses chemical and visual modalities with about equal frequency.
Consistent with his hypotheses, Chris found a significant positive relationship between soma size in the LGN (a center associated with color vision) and the number of displays, although phylogenetic correction eroded this relationship. Chris finds a modest, though non-significant, correlation between LGN soma density and number of visual displays. He found that Anolis carolinensis, in particular, had extraordinarily high display levels and soma density. He found no relationship between chemosensory behaviors and soma size or density in the NS. Although the experiments are still currently underway, Chris reports that there is a clear trend between chemosensory behaviors and soma size and density in the nucleus sphericus. He posits that because all lizards use vision to some extent, but fewer use chemical modalities, differences among species in the NS may be expected to be stronger than in the LGN and OT.