Sexual Dimorphism, Growth, and Longevity of Two Toad Species (Anura, Bufonidae) in a Neotropical Savanna
In most amphibians, females are the larger sex presumably due to selection for larger reproductive output. Sexual selection may, however, produce larger males and also lead to the evolution of secondary sexual characteristics. The proximate causes of sexual dimorphism are related to differences in growth rates and longevity, but these data are scanty for most amphibian species and often depend on long-term demographic studies. Here we use skeletochronology and mark–recapture data obtained on a weekly basis over five years to investigate the sexual dimorphism in body size and shape, growth, and longevity of two toad species, Rhinella rubescens and R. schneideri, in the Cerrado of central Brazil. In adult individuals of both species, males have more robust forelimbs than females, but females attain larger asymptotic body sizes. In Rhinella rubescens, females have wider heads, whereas in R. schneideri, females bear larger parotoid glands and higher heads than males. Females of R. rubescens grow faster and reach sexual maturity earlier than males (♀: 105 days, ♂: 170 days); in contrast, males of R. schneideri grow faster and attain sexual maturity earlier than females (♀: 450 days, ♂: 340 days). The estimated age of the oldest males and females, respectively, were three and 3.5 years for R. rubescens and five and four years for R. schneideri. Males of R. rubescens were frequently captured a few months after reaching sexual maturity, between six months to one year of age, whereas in R. schneideri, males were frequently captured when sexual maturity was reached from one to 1.5 years of age. Our data indicate that sexual dimorphism results from different growth trajectories before and after sexual maturity. Differences between the two species in morphology, growth, and longevity likely reflect their separation into two morphologically and phylogenetically distinct sister clades.
Individuals of (A) Rhinella rubescens and (B) R. schneideri captured at Estação Ecológica de Águas Emendadas (ESECAE). Photos: Davi L. Pantoja.
Study site, Estação Ecológica de Águas Emendadas (ESECAE) and Lagoa Bonita, a protected area in Distrito Federal, Brazil.
Regression between isometric body size and size-adjusted variables, (A) forearm diameter and (B) head width, for males and females of Rhinella rubescens. (C) Frequency distribution of males (gray) and females (crosshatched) of R. rubescens according to isometric body size.
Regression between isometric body size and size-adjusted variables, (A) snout-vent length, (B) forearm diameter, (C) width of parotoid glands, (D) length of parotoid glands, and (E) head height, for males and females of Rhinella schneideri. (F) Frequency distribution of males (gray) and females (crosshatched) of R. schneideri according to isometric body size.
(A) Relationship between growth rate and snout-vent length (mm), according to sex, in Rhinella rubescens. (B) Growth curves of males and females of R. rubescens according to the Von Bertalanffy's model. Lines indicate age and size at sexual maturity for males and females.
(A) Relationship between growth rate and snout-vent length (mm), according to sex, in Rhinella schneideri. (B) Growth curves of males and females of R. schneideri according to the Von Bertalanffy's model. Lines indicate the age of sexual maturity for males and females.
Photomicrography of histological sections from the toes of Rhinella rubescens and R. schneideri. (A) Compact bone surrounding spongy bone, with hyaline cartilage in bony trabeculae, in R. rubescens. (B) Compact bone of R. rubescens. (C) Compact bone of R. schneideri. (D) Compact bone of R. schneideri with two lines of arrested growth (LAG) indicated by arrows.
Frequency distribution of males (gray bars) and females (hatched bars), accord to age (years), in (A) Rhinella rubescens and (B) R. schneideri.
Contributor Notes
Associate Editor: B. Stuart.