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

Males of the Old World quail genus Coturnix are unique among birds in posse ssing a well-developed proctodeal gland. The gland and associated cloacal m usculature are sexually dimorphic, androgen dependent, and produce a foamy substance that is introduced into the female along with semen during copula tion, suggesting that the foam plays some role in increasing male reproduct ive success. I experimentally tested three hypotheses about the function of this foam in Japanese Quail (Coturnix japonica): (1) foam functions in spe rm competition, (2) foam reduces the female's receptivity to a second male, and (3) foam increases the probability of fertilization when a hard-shelle d egg is present in the uterus (hypothesis 3 was originally proposed by Che ng 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.