Many male lizards use a variety of head and jaw movements in elaborate displays when courting potential mates and/or confronting conspecific rivals. In male collared lizards (Crotaphytus), for example, conspicuous gaping displays are an important component of the behavioral repertoire of territorial encounters. During these displays, males open their jaws widely, providing potential rivals with both a head-on and lateral view of their gaping mouths. Kris Lappin and his colleagues were curious about the functional role of such displays and suspect they may be involved in relaying information about the lizard's fighting prowess.

Lappin and his research group proposed that a head-on look into a gaping lizard's mouth provides an excellent viewpoint from which to size up the animal's primary jaw-closing musculature, its adductor mandibulae complex (AMC). Moreover, both head-on and lateral views of a gaping mouth reveal bright patches overlaying the AMC that highlight its extent. Biting is the key mode of attack among territorial adult male lizards, so their jaws and associated closing musculature are hence their major weapon. So Lappin and coworkers tested the notion that this gaping behavior does indeed encode information about the size of a collared lizard's jaw muscles and, in turn, its potential fighting/biting ability.

The team caught wild lizards in the Wichita Mountains of Oklahoma, USA, and took various morphometric body measurements, including the head and AMC size. Next they measured the animals' bite force by encouraging them to chomp down on a specially designed force transducer, recording each individual's maximum bite force. Among adult males, the AMC size, as seen from a frontal view, was the best predictor of the animal's maximum bite force (i.e. better than any measurement of head or body size), and was not correlated with other morphological measures. Importantly, AMC breadth was not a good predictor of bite force among adult females and juvenile males, which generally are not territorial and do not fight.

In a parallel set of studies, Lappin and coauthors addressed the size and spectral properties of the bright patches found at the corners of the lizards' mouths, which cover the AMC when viewed from the side. As was the case with the frontal size measures, the size of the lateral patches was also important in explaining variation in bite performance in adult males, but not in juvenile males or adult females. Finally, reflectance spectra of the lateral patches revealed that they are the brightest parts of the skin, are in the iguanian lizard's visual range and contrast strongly with the adjacent integument and oral epithelium. It would appear that the patches are highly visible and can help to amplify or improve the visibility of the AMC, possibly giving rival males ample warning that they could be in for a good mauling.

In summary, gaping displays in adult male collared lizards may well be critical signals communicating jaw-adductor muscle size and, more practically, fighting ability, so animals don't have to find out the hard way. I look forward to manipulative experiments in which lateral patch size is artificially augmented to see how conspecific rivals react to the little lizard with the (apparently) big bite.