Otariids are strict annual breeders except for the Australian sea lion which, for reasons yet unknown, follows an unusual 17.5 month cycle. Females of all species bear a single, large pup per season. Twins are rare (1 in 10,000 births) in some species. The uterus is bicornuate (has two sections), which allows females to undergo a postpartum estrus. All otariids, including the Australian sea lion, undergo an embryonic diapause (delayed implantation) for about four months before the embryo implants and begins active growth. Diapause seems to function as a timing mechanism, insuring that births occur near particular dates. The timing can be quite precise; in northern and Antarctic fur seals, individual females give birth within three to four days of the same date (specific to each female) in successive years.
All otariids are polygynous; adult sex ratios of 60 females per adult male have been observed in populations subjected to sealing, but undisturbed sex ratios are typically between 2:1 and 10:1. Polygyny is an apparent result of several factors, including breeding at a few large colonies instead of many small ones, gathering at a predictable time and place, higher age-specific mortality in males than in females, length of the
breeding season, male competition rate, and a postpartum es-trus. Males may mate 100 or more times during a breeding season, depending on their location relative to females.
Females have a single estrus that occurs less than 10 days postpartum (except in the California sea lion where it is 30 days). Most females mate only once per estrus, rarely twice. If they fail to mate, estrus may last less than two days (36 hours in the northern fur seal). If they mate, estrus may be terminated by coitus (the Whitten effect). Females of most species mate indiscriminately with whichever male is nearest when they enter estrus. Females usually do not mate with juvenile or peripheral males because adult males exclude them from the breeding sites. However, in colonization situations, when males intercept females on their way to feeding (South American and Hooker's sea lions), or in captivity, females of some species readily mate with juvenile or peripheral males. Pregnancy rates may exceed 90% in some age classes of females.
The duration of the breeding season varies with latitude. The interval in which 90% of the females enter estrus varies from 21 days in the Antarctic fur seal to 70 days in the Galápagos fur seal. Many aspects of the social system change when the breeding season is long lasting.
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