Sexual Size Dimorphism and Age in the Midwife Toads Alytes obstetricans and A. cisternasii

Abstract

We studied sexual size dimorphism is in two breeding populations of midwife toads (Alytes obstetricans and Alytes cisterasii) from which skeletochronological data provide information on individual age. The population values show that females are larger than males in both species (A. obstet- ricans, SVLmale = 41.74 mm, SVLfemale = 49.69 mm; A. cisternasii, SVLmale = 35.79, SVLfeale = 38.58). The values obtained for population size dimorphism (Ln(xmale)-Ln(xfemale) -0.174 for A. obstetricans and -0.074 for A. cisternasii) are are higher than the age-specific values (range -0.126 to -0.055 for A. obstet- ricans; -0.101 to -0.043 for A. cisterasii). Differences between age-specific and population dimorphism values are particularly marked in A. obstetricans, where the age distribution of the sample of males and females differ the most. This result suggests that caution should be used when considering population data on size dimorphism to test evolutionary hypotheses about the evolution of this phenomenon. The dimorphism values obtained for the two species are not markedly different from those obtained from other temperate anurans without male parental care. This result does not support the prediction of the invest- ment hypothesis (Williams, 1966; Trivers, 1972) that male parental care may act as a limitation on sexual selection on male size.