Subtle pedipalp dimorphism: a reliable method for sexing juvenile spiders

Volume: 36Start Page: 513End Page: 51

Gaping displays reveal and amplify a mechanically based index of weapon performance

Physical prowess, a key determinant of fight outcomes, is contingent on whole‐organism performance traits. The advertisement of performance, via display, is poorly understood because it is unclear how information about performance is encoded into display characteristics. Previous studies have shown that weapon performance (i.e., bite force) predicts dominance and reproductive success in male lizards. We tested the hypothesis that gaping displays by adult male collared lizards (Crotaphytus) can provide an index of weapon performance by exposing the major jaw‐adductor muscle complex and that white patches at the mouth corners amplify this index. For territorial adult males, the breadth of the muscle complex, which is not correlated with body size, was a strong predictor of bite force. For nonterritorial yearling males and females, however, measures of body and head size predicted bite force. The patches are highly conspicuous, exhibit UV‐reflecting properties within the visual range of lizards, and provide size‐independent information about bite force only in adult males. We conclude that exposure of the muscle complex during gaping displays can provide rival males with a reliable, body‐size independent, biomechanically based index of weapon performance, an index that the mouth‐corner patches amplify. Indexes that transmit information through mechanistic links to performance are expected to be widespread among animals.14 page(s

Conspicuousness of Dickerson’s collared lizard (Crotaphytus dickersonae) through the eyes of conspecifics and predators

Selection should favour coloration in organisms that is more conspicuous to their own visual system than to those\ud of their predators or prey. We tested this prediction in Dickerson’s collared lizard (Crotaphytus dickersonae), a\ud sexually dichromatic desert reptile that preys on insects and smaller lizard species, and which in turn is prey for birds and snakes. We modelled the spectral sensitivities of the lizards and their avian and snake predators, and compared the conspicuousness of the lizards’ entire colour patterns with each class of viewers. Almost all comparisons involving females strongly supported our prediction for greater chromatic and brightness conspicuousness against local terrestrial visual backgrounds to their own modelled visual system than to those of avian and snake predators. Males, in contrast, exhibited far fewer cases of greater conspicuousness to their own visual system than to those of their predators. Our own perception of spectral similarity between blue C. dickersonae males and a local nonterrestrial\ud visual background (i.e. the Sea of Cortéz) prompted a further investigation.We compared sea (and sky) radiance with dorsum radiance of C. dickersonae males and with males from two distantly-related Crotaphytus collaris populations\ud in which males possess blue bodies. In all three visual models, C. dickersonae males exhibited significantly lower\ud chromatic contrast with the sea (and sky) than did their noncoastal, blue-bodied congeners. Among potential\ud explanations, the blue body coloration that is unique to male C. dickersonae may offset, if only slightly, the cost of visibility to predators (and to prey) through reduced contrast against the extensive, local, nonterrestrial blue\ud backgrounds of the sea and sk