Size-dependent male reproductive success and size-assortative mating in the midwife toad Alytes obstetricans

In a field population of the midwife toad, Alytes obstetricans, a temperate zone species with extensive paternal care, size-assortative mating was observed. During the more favourable mating season of 1987, larger males carried bigger single clutches, as expected from assortative matings. Furthermore, larger males experienced higher mating success over the whole season as well as during single egg carrying intervals. However, during the extraordinarily dry breeding period of 1988, no size-related male mating success was found. Males were in both years highly successful caretakers as far as hatching success of the tadpoles was concerned. Brood care was associated with conditional costs for egg-carrying males in 1988, but not in 1987. Neither hatching success nor hatching size of the tadpoles were correlated with male size. Possible causes, leading to the observed mating advantage of larger-sized males in some years, but not in others, are discussed.