Spatiotemporal patterns of sperm storage and last-male sperm precedence in birds

1. In birds, females store sperm in specialized sperm storage tubules (SSTs) located at the junction of the uterus and vagina. To determine the proximate mechanism by which the last male to copulate sires a disproportionate share of the subsequent offspring, the patterns of sperm storage over the breeding season and across the uterovaginal junction (UVJ) in the Yellow-headed Blackbird Xanthocephalus xanthocephalus, were examined. 2. SSTs at the vaginal end of the UVJ matured and began storing sperm earlier than SSTs in the uterine region. Vaginal-end SSTs also stored a greater number of sperm (per SST and overall) and were more likely to store sperm in multiple layers than uterine-end SSTs. 3. Depletion of sperm from SSTs during the laying period was rapid, but uterine-end SSTs appeared to lose more sperm overall than did SSTs at the vaginal end of the UVJ. 4. The spatial and temporal pattern of sperm storage in Blackbirds suggests a new proximate mechanism for last-male sperm precedence in birds. Unlike previous hypotheses, the SST maturation hypothesis requires neither sperm stratification nor sperm displacement to account for last male precedence. Instead, it is suggested that precedence arises through the storage of last-male sperm in the uterine-end SSTs. As uterine-end SSTs lie closest to the infundibulum (site of fertilization), sperm stored in this region obtain an advantage over sperm from vaginal-end SSTs in the race to fertilize an egg. 5. This new mechanism suggests that females may be able to control the paternity of their offspring through the sequential maturation of their SSTs. By following the simple rule copulate last with the male you want as a sire - each female could ensure that sperm from this male would be placed in the best position to successfully fertilize her eggs.