Bite-force performance and head shape in a sexually dimorphic crevice-dwelling lizard, the common chuckwalla [Sauromalus ater (= obesus)]

Abstract

The Common Chuckwalla [ Sauromalus ater ( = obesus )] is a large, sexually dimorphic lizard with a flattened head that takes refuge from predators in rock crevices. Males use their relatively large heads to bite competing males during territorial fights and to restrain females during copulation. Flattened heads with an antipredator function (i.e. seeking refuge in crevices) and enlarged heads with intrasexual competition and reproductive functions suggest possible antagonism between selective pressures on head morphology in males. To examine this hypothesis, we performed a morphometric analysis and measured the bite-force performance of 49 adult chuckwallas. Males had disproportionately wider heads than females, but did not have deeper heads. Males bit with nearly four times the force of females, consistent with the notion of sexual selection for high bite force in males. Although constrained by crevice-wedging behaviour, head depth was a good predictor of bite force in both sexes. In males, however, osteological head width also was a good predictor of bite force. These results are consistent with the hypothesis that head shape in males is under antagonistic selective pressures, which may partly explain the pattern of head shape dimorphism. The disproportionately wide head of males may reflect anatomical modifications to enhance bite force in response to sexual selection in spite of presumed constraints on head shape for crevice-wedging behaviour

Geometric morphometric analysis of cranial variation in the Egernia depressa (Reptilia: Squamata: Scincidae) species complex

“…Given that these lizards are strongly tied to their shelter site, the lateral aspect of the cranium exhibits variation that may functionally refl ect this. Rocky habitats impose selective pressures on specifi c morphological traits such as fl attened heads and bodies (Ingram and Covacevich 1989;Pianka and Vitt 2003;Lappin et al 2006;Revell et al 2007). A fl attened head/body may enhance the ability to retreat deeply in to crevices because rock crevices are constrained in primarily one dimension (i.e.…”

Bite‐force performance of the last rhynchocephalian (Lepidosauria:Sphenodon)

“…Males of a species of Jamaican anole, which are larger and bite harder than females, also eat larger and harder arthropods . In other cases, strong sexual differences in size and bite force occur when there are no obvious dietary differences (e.g., Sauromalus, Lappin et al 2006a). Sphenodon, at least in one significant respect, exhibits a positive relationship between sexual size dimorphism and prey size, in that only large males regularly tackle large vertebrates such as sea bird nestlings (Walls 1981;Cree et al 1995Cree et al , 1999Markwell 1998;Blair et al 2000;Gaze 2001).…”

Allometry of head size and shape dimorphism in the grass snake (Natrix natrix L.)

“…Thus, a strong bite, which improves the snake's ability to hold actively struggling prey, usually anurans with powerful hind limbs (Filippi et al, 1996;Shylakhtin et al, 2005;Najbar and Borczyk, 2012), may be of greater importance for bigger snakes, which as a consequence of their size prey on larger and thus stronger animals. Head width is positively correlated with bite strength in lizards (Herrel et al, 2001;McBrayer, 2004;Lappin et al, 2006); however, as far as I know, there is no bite-force data for colubrid snakes.…”