SEXUAL SIZE DIMORPHISM AND AGE IN THE MIDWIFE TOADS ALYTES OBSTETRICANS AND A-CISTERNASII

We studied sexual size dimorphism is in two breeding populations of mi dwife toads (Alytes obstetricans and Alytes cisternasii) from which sk eletochronological data provide information on individual age. The pop ulation values show that females are larger than males in both species (A. obstetricans, SVLmale = 41.74 mm, SVLfemale = 49.69 mm; A. cister nasii, SVLmale = 35.79, SVLfemale = 38.58). The values obtained for po pulation size dimorphism (Ln((x) over bar(male))-Ln((x) over bar(femal e))-0.174 for A, obstetricans and -0.074 for A. cisternasii) are are h igher than the age-specific values (range -0.126 to -0.055 for A. obst etricans; -0.101 to -0.043 for A. cisternasii). Differences between ag e-specific and population dimorphism values are particularly marked in ih. obstetricans, where the age distribution of the sample of males a nd females differ the most. This result suggests that caution should b e used when considering population data can size dimorphism to test ev olutionary hypotheses about the evolution of this phenomenon. The dimo rphism 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 investment h ypothesis (Williams, 1966; Trivers, 1972) that male parental care may act as a limitation on sexual selection on male size.