The phalloid organ, orgasm and sperm competition in a polygynandrous bird:the red-billed buffalo weaver (Bubalornis niger)

The buffalo weavers, Bubalornis spp., are unique amongst birds in possessin g a phalloid organ, a phallus-like structure anterior to the cloaca. We stu died the red-billed buffalo weaver Bubalornis niger, to determine whether t he phalloid organ has evolved in response to sperm competition. The phalloi d organ was significantly longer in males that were resident at nests than in non-resident males, and among resident males was significantly longer in those males with a harem than in those without. Red-billed buffalo weavers bred colonially and had either a cooperatively polygynandrous (usually two unrelated males and several females) or a polygynous (one male and several females) mating system. Cooperative polygynandry provided females with the opportunity to copulate with more than one male and paternity analyses usi ng DNA fingerprinting revealed that 63% of 16 multiple-offspring broods, co mprising 43 offspring, had multiple sires, which included both nest-owning males and extra-group males. Sperm competition was therefore intense. Obser vations and experiments with buffalo weavers in captivity revealed that the phalloid organ was not intromittent during copulation, but functioned as a stimulatory organ which necessitated protracted copulation in order to ind uce male 'orgasm' and ejaculation, a feature apparently unique to this spec ies.