On the evolution of sexually transmitted diseases in birds

Sexually transmitted diseases (STDs) in animals are caused by pathogens that are transmitted during copulation. Birds have played an important role in the development of STD-centered theories of mating behavior. However, it is not known whether STDs exist in wild bird populations. While the avian cloaca with its dual functions of gamete transfer and excretion seemingly predisposes birds for the evolution of STDs, the life history patterns of most birds (i.e., seasonal breeders with relatively brief annual periods of sexual activity) suggest otherwise. The importance of STDs as selective forces that shape host biology depends on whether host life history patterns provide the necessary conditions for the evolution and spread of virulent pathogens that rely on host copulation for dispersal. Infrequent dispersal opportunities for microbes that rely on host sexual contact for dispersal should favor the evolution of low pathogen virulence, persistent infection, and lengthened infectious periods. I examine the disease characteristics of documented STDs that are relevant to birds in order to evaluate the (1) importance of STDs as selective forces in birds and (2) ecological conditions in which avian STDs are likely to be discovered. I conclude that socially monogamous short-lived birds with short breeding cycles and altricial nestlings (e.g., temperate zone songbirds) are not likely to have coevolved with highly virulent STDs which would be important selective forces. However, virulent STDs may have evolved in birds with one or more of the following life history characteristics: long life spans, long breeding cycles, multiple matings, and precocial young because these conditions produce increased opportunities for pathogen transmission and can favor the evolution of virulence. These life history characteristics are found and exaggerated in the birds for which STDs have been documented, the domesticated fowl and waterfowl.