Sperm Competition and the Reproductive Anatomy of Male Superb Fairy-Wrens

Abstract

In Superb Fairy-Wrens (Malurus cyaneus), groups of males cooperate with a single female to rear young, yet offspring are usually sired by males from outside the group. In this unusual mating system there is potential for intense sperm competition. During the breeding season, males develop a sperm storage structure (cloacal protuberance) and testes that proportionally are among the largest found in passerines. We compared the development pattern and size of cloacal protuberances of males differing in age and social status. Protu3;2;­ berance size increased with body mass. Age, intragroup dominance, and pairing status did not influence the overall size of the protuberance, but old males had a larger tip on their protuberance. This prominent tip has not been reported in other species, and we speculate that it serves as an intromittent organ. Other birds with large testes and cloacal protuberances have high copulation rates, but copulation in Superb Fairy-Wrens is only very rarely observed. We propose that the cloacal protuberance and large testes of Superb Fairy-Wrens provide large sperm reserves primarily for extrapair copulations. These may occur frequently, or involve the transfer of large ejaculates.