African wild dogs (Lycaon pictus) live in cooperative packs with a clear-cut dominance hierarchy in each sex. Reproduction is largely monopolized by the dominant male and female. Alpha females produced 76% of all litters in the Selous Game Reserve and 81% in Kruger National Park. Only 6-17% of subordinate females gave birth each year, compared to 82% of dominant females. In nonmating periods, subordinate females had higher estrogen levels and higher estrogen/progestin ratios than alpha females, apparently preventing ovulation. During mating periods, subordinate females had lower estrogen levels than dominants, mated less often, and were less aggressive. Subordinate males mated at low rates, wore less aggressive than dominants, and had lower testosterone levels. Beta males were similar to alpha males behavioraDy and hormonaUy, suggesting that alpha males may share paternity with beta males. If paternity is more evenly shared than maternity, then subordinate males have a larger incentive than subordinate females to remain in the pack. Following this expectation, dispersal in Selous was female biased (49% versus 24% dispersing annually). Perhaps as a result of mortality associated with dispersal, the adult sex ratio was male biased, although the pup sex ratio was unbiased. In Kruger, neither dispersal nor the adult sex-ratio was biased. Reproductive suppression is widely thought to be caused by social stress in subordinates, but basal cortkosterone levels were higher in dominants than in subordinates