A Comparative Test of Adaptive Hypotheses for Sexual Size Dimorphism in Lizards

Abstract

It is commonly argued that sexual size dimorphism (SSD) in lizards has evolved in response to two primary, nonexclusive processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition (intrasexual selection hypothesis), and (2) natural selection for large female size, which confers a fecundity advantage (fecundity advantage hypothesis). However, outside of several well-studied lizard genera, the empirical support for these hypotheses has not been examined with appropriate phylogenetic control. We conducted a comparative phylogenetic analysis to test these hypotheses using literature data from 497 lizard populations representing 302 species and 18 families. As predicted by the intrasexual selection hypothesis, male aggression and territoriality are correlated with SSD, but evolutionary shifts in these categorical variables each explain less than 2% of the inferred evolutionary change in SSD. We found stronger correlations between SSD and continuous estimates of intrasexual selection such as male to female home range ratio and female home range size. These results are consistent with the criticism that categorical variables may obscure much of the actual variation in intrasexual selection intensity needed to explain patterns in SSD. In accordance with the fecundity advantage hypothesis, SSD is correlated with clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, reduced major axis estimator of fecundity slope, length of reproductive season, or latitude). However, evolutionary shifts in clutch size explain less than 8% of the associated change in SSD, which also varies significantly in the absence of evolutionary shifts in reproductive frequency and mode. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD is explained using these variables. Intrasexual selection for large male size and fecundity selection for large female size have undoubtedly helped to shape patterns of SSD across lizards, but the comparative data at present provide only weak support for these hypotheses as general explanations for SSD in this group. Future work would benefit from the consideration of alternatives to these traditional evolutionary hypotheses, and the elucidation of proximate mechanisms influencing growth and SSD within populations.

Growth patterns, sexual dimorphism, and maturation modeled in Pachypleurosauria from Middle Triassic of central Europe (Diapsida: Sauropterygia)

“…In extant lizards, males are often larger than females and grow for a longer period than females, but the opposite pattern also exists in this group (Cox et al, 2003). In many taxa (e.g., Anolis lizards) members of the larger sex also mature at older ages than members of the smaller sex (Stamps et al, 1994;Stamps and Krishnan, 1997).…”

“…For example, selection for larger male size and later onset of reproduction could be driven by male-male competition, i.e., by competition for mating opportunities, larger males have more reproductive success than smaller. This selection scenario seems to be applicable to a number of extant lizards (reviewed in Cox et al, 2003), in which males are larger than females. It would indicate that the larger pachypleurosaur specimens with the delayed maturity are indeed males.…”

“…Males can become double the size of the smallest pregnant female (Nicole Klein, personal observation, 2006, on Keichousaurus specimens in Yichang collection). However, extant lizard species in which females are larger than males also exist (reviewed in Cox et al, 2003) and thus both situations could have also existed in pachypleurosaurs. Fecundity selection favors large size and an earlier onset of maturation in females than in males, i.e., larger and earlier reproducing females can have more offspring during their life.…”

Multivariate and geometric morphometrics in the analysis of sexual dimorphism variation in Podarcis lizards

“…However, most of them can be summarized by three major forces differentially acting on males and females of a population: sexual, fecundity, and natural selection. In those species where males engage in fights and copulation is forced, sexual selection may act via male-male combats or/and female choice and thus favor bigger male body size, whereas fecundity selection may favor bigger female body size (Olsson et al, 2002;Cox et al, 2003). Moreover, natural selection acting on both sexes might pose constraints to the evolution of certain characters, for example via habitat use.…”

Sex size and shape differences in the lacertid community (Podarcisspp. andArchaeolacertasp.) from the Lake Skadar region (Montenegro)

“…It is well known that Rensch's rule (male -larger SSD increases with body size) has been repeatedly found in larger species of many vertebrates (reviewed by Reiss 1989), including lizards (e.g. Fitch 1981Cox et al 2003). Previous studies on lacertids found no relation between size and the SSD level (Brañ a 1996;Fairbairn 1997).…”