Foam produced by male Coturnix quail:: What is its function?

Males of the Old World quail genus Coturnix are unique among birds in possessing a well-developed proctodeal gland. The gland and associated cloacal musculature are sexually dimorphic, androgen dependent, and produce a foamy substance that is introduced into the female along with semen during copulation, suggesting that the foam plays some role in increasing male reproductive success. I experimentally tested three hypotheses about the function of this foam in Japanese Quail (Coturnix japonica): (1) foam functions in sperm competition, (2) foam reduces the female's receptivity to a second male, and (3) foam increases the probability of fertilization when a hard-shelled egg is present in the uterus (hypothesis 3 was originally proposed by Cheng et al. 1989a). Insemination shortly before oviposition fertilized fewer eggs than inseminations earlier in the day, but only if males had a reduced foam complement, supporting the third hypothesis. The other two hypotheses were not supported. Copulation reduced female receptivity, but this effect was not due to the male's foam.