Proximate and ultimate drivers of variation in bite force in the insular lizardsPodarcis melisellensisandPodarcis sicula

Abstract

Bite force is a key performance trait in lizards because biting is involved in many ecologically relevant tasks, including foraging, fighting and mating. Several factors have been suggested to impact bite force in lizards, such as head morphology (proximate factors), or diet, intraspecific competition and habitat characteristics (ultimate factors). However, these have been generally investigated separately and mostly at the interspecific level. Here we tested which factors drive variation in bite force at the population level and to what extent. Our study includes 20 populations of two closely related lacertid species, Podarcis melisellensis and Podarcis sicula, which inhabit islands in the Adriatic. We found that lizards with more forceful bites have relatively wider and taller heads, and consume more hard prey and plant material. Island isolation correlates with bite force, probably by driving resource availability. Bite force is only poorly explained by proxies of intraspecific competition. The linear distance from a large island and the proportion of difficult-to-reduce food items consumed are the ultimate factors that explain most of the variation in bite force. Our findings suggest that the way in which morphological variation affects bite force is species-specific, probably reflecting the different selective pressures operating on the two species.

From micro to macroevolution: drivers of shape variation in an island radiation ofPodarcislizards*

“…To do so, we measured head dimensions of every specimen, log 10 -transformed them, and calculated the mean distance between males and females along the first axes of a PCA. This measure of sexual dimorphism in head dimensions was previously demonstrated to be a good indicator for the level of sexual competition in these populations (Taverne et al 2020). In vivo bite force was measured for all individuals as described in Taverne et al (2020).…”

“…Bite force and muscle data were log 10 -transformed, proportions of the type of food consumed (e.g., plants, hard arthropods) were arcsine-transformed, and the homogeneity of variances and normality of the distribution of the residuals were verified using Bartlett and Shapiro tests, respectively. For anal-yses including phylogeny we used a previously published tree describing the relationships between the populations in this study system (see Taverne et al 2020). Preliminary genomic analyses (Sabolić et al in preparation) indicated that there is effectively no gene flow between populations, and thus treating them as independent evolving lineages for phylogenetic comparative analyses is justified.…”

“…These populations range from insectivorous to omnivorous, with lizards relying on difficult to chew food items (i.e., plant material and hard prey) to face the food scarcity observed in the smallest and most depauperate environments. Additionally, a recent study demonstrated that the proportion of these mechanically resistant items in the diet as well as the level of sexual competition are important drivers of variation in bite force in these lizards (Taverne et al 2020). Variation in bite force is partly driven by variation in head shape (Herrel et al 2001Verwaijen et al 2002;Lappin et al 2006;Huyghe et al 2009;Wittorski et al 2016), as taller and wider heads provide more space for muscles (Herrel et al 2007).…”

Comparison of head size and bite force in two sympatric munia species

“…In addition, the way in which morphological variation may affect bite force is species-specific, probably reflecting the different selective pressures operating on two closely species. 35 Nevertheless, further research is needed to identify the exact reasons for the similarity in bite force between scaly-breasted munias and white-rumped munias.…”

The Terrific Skink bite force suggests insularity as a likely driver to exceptional resource use

“…Different prey communities, their abundance and seasonal variation on islands impose changes in a lizard’s dietary niche and often lead to the utilization of novel resources 22 , a powerful driver of morphological evolution. Head size (width and height) is known to be associated with bite force, which in turn is related to foraging, mating, and fighting 13 , 23 . Discerning between these factors is difficult but essential to disentangle the evolutionary processes driving morphological variation in different ecological contexts 24 .…”