Performance capacity, fighting tactics and the evolution of life–stage male morphs in the green anole lizard (Anolis carolinensis)

Abstract

The evolution of alternative male phenotypes is probably driven by male-male competition for access to reproductive females, but few studies have examined whether whole-organism performance capacities differ between male morphs, and if so whether any such differences affect fighting ability. We show how ontogenetic changes in performance and morphology have given rise to two distinct life-stage male morphs exhibiting different fighting tactics within the green anole lizard (Anolis carolinensis). Field studies show a bimodal distribution of adult males within a single population: larger 'heavyweight' males have relatively large heads and high bite forces for their size, whereas smaller 'lightweight' males have smaller heads and lower bite forces. In staged fights between size-matched heavyweight males, males with greater biting ability won more frequently, whereas in lightweight fights, males with greater jumping velocity and acceleration won more often. Because growth in reptiles is indeterminate, and the anole males examined are sexually mature, we propose that the heavyweight morph arose through selection against males with small heads and poor bite forces at the lightweight-heavyweight size transition. Our findings imply that one may not be able to predict male fighting success (and hence potential mating success) by examining aspects of male 'quality' at only one life stage.

A performance-based cost to honest signalling in male green anole lizards (Anolis carolinensis)

“…[21]). The costs of such escalated combat can be substantial in A. carolinensis, resulting in severe injury or even death [15,69], and hence may be an important factor limiting the evolution of dishonesty in this system. This scenario is consistent with recent theoretical models suggesting that it is the cost of cheating that maintains signal honesty, rather than any inherent cost to the signal itself (reviewed in [8]).…”

“…Lizards were kept in the laboratory under these conditions and feeding regimes for three months, which is sufficient time for juvenile green anoles to attain sexual maturity. The relationship between dewlap size and bite force is age-dependent in A. carolinensis males such that the dewlaps of much older and larger males (specifically 'heavyweight' males .64 mm SVL; see [15]) do not honestly signal bite force [42]. We therefore only considered young 'lightweight' adult males in the current study, both because of the time required for lizards to attain heavyweight status (approx.…”

“…We therefore only considered young 'lightweight' adult males in the current study, both because of the time required for lizards to attain heavyweight status (approx. 2-3 years in nature; S. P. Lailvaux 2009, unpublished data), and because heavyweight males are less common than lightweights in natural A. carolinensis populations [15,46].…”

“…To test for differences in growth curves between the restricted and unrestricted diet treatments for SVL and mass, we used a general linear mixed-model with diet treatment as a fixed factor and individual as a random factor (to account for repeated measures) fit by restricted estimated maximum likelihood (REML) and implemented using the lme function of the nlme package (S. P. Lailvaux 2010, unpublished data) for R (R Core Development Team 2010). Because functional traits such as bite force typically scale positively with body size in a variety of animal taxa, including A. carolinensis [15,53], it is necessary to control for size effects in studies of this type. To compare the growth curves for dewlap size and bite force independent of body size, we included SVL as a covariate in these mixed models.…”

“…Specifically, the overall area of the extended dewlap is significantly positively correlated (independent of body size) with maximum bite force in young adult males [42], suggesting that rivals may glean information on an individual's bite force from the size of his dewlap, most probably in the initial stages of interactions over long ranges [37,43]. Indeed, bite force has previously been shown to predict male combat outcomes in those sexually dimorphic anole species that appear to use dewlap size to signal bite force (such as A. carolinensis; [15]) but not in less-dimorphic species where no link between dewlap size and bite force exists [41]. Importantly, there is no direct mechanistic relationship between dewlap display and bite force, as different muscles are responsible for each (i.e.…”