DOI: 10.5061/dryad.1jwstqjrv
title: Data from: Proximate and ultimate drivers of variation in bite force in the insular lizards Podarcis melisellensis and Podarcis sicula

description: Bite force is a key performance trait in lizards since biting is involved
 in many ecologically relevant tasks, including foraging, fighting, and
 mating. Several factors have been previously 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. 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, likely by driving the 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, likely reflecting the different selective pressures
 operating on the two species.
description: <i>In-vivo</i> bite force was measured using an
 isometric Kistler force transducer (type 9203) connected to a Kistler
 charge amplifier (type 5995, Kistler Inc., Winterthur, Switzerland).
 Lizards were made to bite the plates of the transducer, left bare, at
 least five times, and the greatest bite force across the five trials was
 retained as an estimate of an individual’s maximum bite force. Gape angle
 was standardized across trials by adjusting the distance between the bite
 plates for each individual to maintain gape angle constant at around 30°.
 Bite position was standardized by assuring that the tips of the jaws of
 each lizard were up against the metal stop mounted on the device. Bite
 force was log<sub>10</sub>-transformed before statistical
 analyses. The maximum hardness of each food item was
 estimated using the regression equations proposed by Herrel &
 O’Reilly (2006) that relate prey length and hardness, according to the
 item hardness category. We measured all individuals
 using digital calipers (Mitutoyo absolute digimatic; ± 0.01mm; see
 Supplementary Fig. 1) and recorded snout-vent length (SVL) and linear head
 dimensions including head length (HL), head width (HW), head height (HH),
 lower jaw length (LJL), quadrate to jaw tip length (QT), and coronoid to
 jaw tip length (CT). Two other functionally relevant variables were
 calculated: the in-lever for jaw opening (open = LJL – QT) and the
 in-lever for jaw closing (close= QT – CT). For both
 species separately, a principal component analysis (PCA) was run on head
 dimensions. The contribution of each specimen along the three first
 principal components (PC) was extracted and used to calculate the mean
 contribution of each sex of each population on these axes. The sexual
 dimorphism in head dimensions (SDh) for each site was determined as
 follows: where m<sub>i</sub> and f<sub>i</sub> refer respectively to the mean contribution of the males and the females of the population of interest along with the PC<sub>i</sub>. Two other proxies for intraspecific competition were estimated: the proportion of individuals which missed the longest toe on one of the hind feet and the proportion of individuals with a regenerated tail.
description: Data has been log-10 transformed, expect for the proportions
 which have been arcsin transformed.