Sexually transmitted disease in birds: occurrence and evolutionary significance.

Sexually transmitted diseases (STDS) span two current areas of sexual selection theory, namely the roles of multiple mating in determining individual reproductive success, and of parasites in mate choice, yet have been relatively neglected in the ecological literature. I reviewed the occurrence of STDS in populations of commercially kept birds and found widespread evidence for the existence of pathogenic STDS in such populations. STDs may have important consequences for the evolution of behaviour, reproductive physiology and some secondary sexual characteristics. Where STDS are costly they are hypothesized to affect the evolution of mating systems, and, via selection for hostility in the female reproductive tract, to explain high levels of sperm mortality after insemination. The potential for coevolutionary cycling is large, as some STDS may coevolve with female and male reproductive physiology, which may themselves coevolve. Although little information currently exists concerning the occurrence of STDS in wild birds, techniques for their identification are well established. This study raises a number of testable predictions about the consequences of STDS for avian reproductive biology, and I suggest that STDS should be considered as a potentially powerful factor in future studies of mate choice and sperm competition.