Large male mating advantage in natterjack toads, Bufo calamita: sexual selection or energetic constraints?

Abstract

Variation in the effect of male body size on mating success was studied in natterjack toads during two breeding seasons with different temporal patterns of reproductive activity. Larger males were more likely to mate than small males in the season with a single peak and continuous breeding activity, whereas random mating by size was found when reproduction occurred during several peaks separated by periods of inactivity. Playback tests did not reveal any consistent preference by females for call variables related to male body size other than call intensity. Moreover, direct observation of pair formation resulting from capture of females by non-calling males (searchers and satellites) suggested that most females did not have the opportunity to make any choice. Direct male-male competition for females already in amplexus was negligible as suggested by the low rate of take-over success. Body size advantage occurred indirectly through a positive correlation with chorus attendance. One hypothesis is that smaller males may attend the breeding site less frequently because of energy limitations. Energetic constraints may arise in the long term, affecting the energy that should be allocated to growth, which is higher in small males, or in the short term, affecting the capacity for sustained chorus attendance. Growth rate was not correlated with chorus attendance. However, smaller males lost weight more rapidly in both seasons. This might explain why they had a lower attendance rate than larger males, giving rise to a higher mating success in the latter.