The Evolution of Performance‐Based Male Fighting Ability in CaribbeanAnolisLizards

Abstract

Despite the empirical and theoretical attention paid to the role of sexual signals in resolving agonistic interactions between conspecific males, few studies have applied a comparative perspective, particularly across species that vary in combat intensity. We investigated the relative roles of a male sexual signal (dewlap size) and whole-organism performance capacity (bite force) on male combat outcomes in nine species of Caribbean Anolis lizards that differ markedly in territoriality, as indicated by sexual size dimorphism. We found that (1) dewlap size was generally an honest signal of bite force in dimorphic but not less dimorphic species; (2) maximum bite force consistently predicted male combat success in dimorphic but not less dimorphic species; (3) in contrast to a priori predictions, dewlap size significantly predicted male combat success in less dimorphic but not dimorphic species; and (4) the incidence of biting but not dewlapping increases as species become more dimorphic. These findings suggest that more dimorphic (and hence more territorial) species escalate to biting during fights more readily compared with less territorial species. The ecological and behavioral qualities of species may therefore modify both the shape and the size of sexually selected traits as well as the nature of the information those traits convey.

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

“…The striking consequence of this differential growth of bite force and dewlap size in restricted individuals is that the significant and positive correlation between bite force and dewlap size that is seen in young adult males of this species in nature [42], and that is also seen in the unrestricted diet treatment in the current study (figure 2a), is absent in the restricted treatment group (figure 2a). Dewlaps that are typically an honest signal of bite force in A. carolinensis and other territorial anole species [41,57] are thus rendered unreliable under limiting resource conditions owing to differential allocation of resources in bite force and dewlap development. This result suggests that the costly nature of performance, and of other traits like performance that are linked to signal expression, might have important implications for the maintenance of reliable signalling in nature.…”

“…We photographed extended dewlaps against a 1 Â 1 cm grid using a Canon A610 Powershot digital camera mounted on a tripod, and digitized the resulting images using tpsDig v. 1.3.1. This method has previously been shown to yield repeatable results in a variety of Anolis lizard species, including A. carolinensis [41,42,47]. We measured in vivo bite force using an isometric Kistler force transducer (type 9023, Kistler, Winterthur, Switzerland) connected to a type 5058a Kistler charge amplifier (see [48,49] for a detailed description) using standard methods.…”

“…Moreover, the male dewlap is an honest signal of a whole-organism performance capacity (i.e. bite force) in young adult male A. carolinensis, as well as in a variety of other territorial anole species [40,41]. 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].…”

“…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.…”

Comparative tests of the role of dewlap size in Anolis lizard speciation

“…Confirming whether relative dewlap size evolution can be explained by divergent sexual selection will require demonstration of sufficient genetic variation (as has been found for dewlap colour [56]) and measurements of selection in populations that are either adapting to distinct light environments or that have come into secondary contact [50]. The observed mix of positive, neutral, and negative relationships between male dewlap size and measures of fitness or performance [57,58] hints at variation in the form of selection, and studies of the form of selection at different stages of speciation could prove illuminating. Overall, our results show evidence that dewlap size evolution is linked to speciation, but not through any effect on the speciation rate of anoles.…”