Lek organization and mating strategies in the bullfrog

Abstract

1. The mating behavior of a dense population of bullfrogs (Rana catesbaiana) in central Michigan was studied intensively for two summers. Frogs were measured to allow age estimation and marked with waistbands for individual identification. 2. Bullfrog breeding was typified by a long mating season in which males arrived in late May or early June (preceding the females) and remained sexually active well into July. In the evenings, the males congregated into choruses. The enhanced (pooled) acoustical stimulus, and the easy localizability of these aggregations caused them to function as centers of attraction for sexually active bullfrogs of both sexes. 3. Within a chorus, the males defended areas using stereotyped challenges, threats, and physical struggles. This led to a regular spacing of males, each positioned on a discrete territory or calling station. The spatial organization of a chorus reflected the social dominance of the males in the population: centrally located territories were occupied by older (larger) individuals, while younger frogs were stationed on the peripheral territories or in isolated locations completely removed from the choruses. 4. Specific calling aggregations were short-lived, and appeared to go through a cycle of formation, peak activity, and then dissipation in a period of only three to five nights. During five weeks of 1966, eight different choruses formed and dissolved in different locations throughout the breeding pond. Individual males were highly mobile, and moved from one aggregation to another during the breeding season. 5. Female bullfrogs showed great asynchrony in the timing of periods of ovulation and sexual receptivity. The duration of sexual activity for any individual female was extremely short, generally lasting only one night. During this period, the female actively moved to the choruses and spent many hours moving from male to male and/or territory to territory. Mate selection was based on female choice and amplexus did not occur until a female made physical contact with the male. 6. The bullfrog mating system is characterized by (1) an extended breeding season of continuous male sexual activity, and (2) extremely brief and asynchronous periods of individual female receptivity. This produces a strong male bias in the operational sex ratio (defined as the ratio of potentially receptive males to receptive females at any time). The result is intense sexual selection and strong male-male competition for the few available females. 7. Three potential mating systems that might result from the combination of intense sexual selection plus female choice of mate are discussed: harems; polygyny based on differential territory quality; and leks. I hypothesize that neither females themselves nor mating requisites of importance to females (such as oviposition sites) constitute resources that can be controlled or defended economically by males. Competition thus takes the form of direct interactions between males, leading to the spacing out of individuals within the chorus in accordance with physical and/or social dominance. The result is a communal display ground or lek. Males aggregate for the purpose of attracting females; females visit the concentration for the purpose of mate selection. 8. An individual male should increase its reproductive fitness by maximizing its accessibility and attractiveness to receptive females. The components of a successful male strategy should include: early arrival at the breeding pond; prolonged duration of continous sexual activity; establishment of a territory within a chorus (and, optimally, establishment within the central area of a chorus); and efficient movement between the different choruses as they form and disband. Results showed that older (larger) males outperformed younger individuals in each of these points. 9. An alternative or satellite male strategy, involving a prolonged adoption of a submissive or female posture is also discussed. 10. The mating assemblages described for bullfrogs appear to be basically similar to the lek mating systems reported for birds and mammals. These similarities apply to (1) the general structure and function of the lek, (2) the male strategies for maximizing reproductive success within the lek, and (3) the ecological determinants that shape the evolution of lek mating systems.