The evolution of sexual size dimorphism in reptiles

Abstract

Most studies interpret reptilian sexual size dimorphism (SSD) as a means to reducing resource competition by way of sexual selection, fecundity selection, and natural selection. This chapter assesses the importance of these processes using data on 832 species of snakes, lizards, and turtles. The data reveal allometry consistent with Rensch's rule in most, but not all reptilian taxa, and support the hypothesis that sexual selection for large male size has influenced the evolution of reptile SSD. However, more data on male combat and territoriality are needed to test more fully this hypothesis. Although fecundity increases with female body size in many reptiles, comparative data provide only weak support for the fecundity advantage of large female size. The chapter concludes that further progress in assessing the relative importance of different selective processes in reptiles will require studies that more fully integrate evolutionary hypotheses with knowledge of proximate physiological and developmental mechanisms.

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

“…Niche portioning has also been suggested to drive size differences in sexes (Shine, 1989;Cox et al, 2007). Sex differences in age-specific mortality can lead to bimodal distributions of age at maturation within a population (Monnet and Cherry, 2002;Kupfer, 2007).…”

“…In this case, Wijk 09-472 would correspond to a female, because it has the earliest onset of maturation seen within our sample on this taxon. Finally, environmental conditions could also select for a sexual size dimorphism in order to reduce intraspecific competition between sexes for food (prey, dietary partitioning) or more generally lead to niche divergence of sexes (reviewed in Shine, 1989;Cox et al, 2007).…”

“…In some species of Anolis, Tropidurus, Amblyrhynchus, and Varanus males are on average 50 % larger than females. By contrast, females exceed only males by as much as 20 % in Polychrus, Mabuya, and Aprasia (reviewed in Cox et al, 2007). In snakes males are also more often larger than females.…”

“…This situation is seen in 50-60 % of all turtle species. Size differences in turtles can be very impressive, e.g., in Kachuga and Graptemys females average two up to three times the length of males (reviewed in Cox et al, 2007). In large Alligator, Caiman, and Crocodylus species, males exceed females in length by 20 up to 40 % (reviewed in Cox et al, 2007).…”

“…Size differences in turtles can be very impressive, e.g., in Kachuga and Graptemys females average two up to three times the length of males (reviewed in Cox et al, 2007). In large Alligator, Caiman, and Crocodylus species, males exceed females in length by 20 up to 40 % (reviewed in Cox et al, 2007). However, unfortunately the majority of comparative studies on sexual dimorphism in extant reptiles have only focused on size differences between sexes, whereas much less is known in reptiles about differences in the onset of maturation between sexes.…”