Reproductive Behavior and Male Mating Success in a Neotropical Toad, Bufo typhonius

Abstract

Bufo typhonius on Barro Colorado Island, Canal Zone, had an explosive breeding system, with most choruses lasting less than one day and all oviposition in a chorus occurring in less than 10 hours. Relatively permanent streams and pools were used for breeding. Several choruses occurred at each site during the four-month breeding season. Some females laid twoegg clutches in six weeks, and some males participated in more than one chorus. Male mating success was determined by direct male-male competition for individual females. Most males less than 38 mm long did not obtain mates. For males over 38 mm, the probability of mating did not increase significantly with increasing size. There was no correlation between male and female size in amplexed pairs. ANIMAL MATING SYSTEMS are molded by ecological factors, particularly temporal patterns of reproduction and the spatial and temporal distribution of limited resources (Trivers 1972, Emlen and Oring 1977). In anuran amphibians the temporal availability of receptive females is an important determinant of the form and intensity of male-male competition and the relative mating success of individual males (Emlen 1976, Wells 1977b). Females of species with prolonged breeding seasons are available on an asynchronous schedule, so each male competes for many females. Females can choose mates from among the available males, and males may influence their choice by defending oviposition sites or other resources needed by females (Wells 1977a, 1977b; Howard 1978). In species with very short breeding periods, many females arrive at the breeding site simultaneously. Males actively search for mates and attempt to displace males already in amplexus. Opportunities for mate selection by females are reduced, so there is little advantage to males in defending resources attractive to females. Because female availability is restricted in time, and because so many are in amplexus simultaneously, it should be difficult for a small number of males to monopolize a large proportion of the females. Hence the variance in male mating success should be lower in explosive breeders than in prolonged breeders (Trivers 1972, Emlen 1976, Wells 1977b, Emlen and Oring 1977). Data now available for two temperate zone ranid frogs, Rana clamitans and R. catesbejana, show that matings are heavily skewed in favor of large males, with a small percentage of males monopolizing a major proportion of the females (Wells 1977a; Howard 1978). Comparable data for explosive breeders are not available, although several recent papers discussed factors that might influence male mating success in these species (Licht 1976, Davies and Halliday 1977, Wells 1977b). In this paper I report relative mating success of individual males in a population of Bufo typhonius, a neotropical toad with an explosive breeding system. I also describe the general reproductive behavior of this species and speculate on ecological factors that favor explosive breeding.