All phocids have an annual reproductive cycle in which females give birth during a fairly distinct breeding period. They are particularly noted for short lactation periods, lasting from as little as four days in the hooded seal to as long as two to three months in a couple of species. Females become receptive near the end of lactation or shortly thereafter in all species. Females of all species exhibit delayed implantation or embryonic diapause during which the fertilized ovum suspends development and remains in the uterus without implanting. This is thought to help synchronize parturition among females and produce the highly synchronous breeding seasons found in these species.
The short lactation periods of phocids are associated with fasting or feeding little during this time. Also associated with this is the buildup of extensive blubber layers by females to provide the major nutrients to produce milk for the young. Milk fat content is highest in those species with the shortest lactations, as is pup mass gain. Females may lose over 40% of their mass at the beginning of the breeding season and between 60% and 80% of that loss is in the milk transferred to the pup. Males do not participate in the care of offspring in any species.
Males of all phocids fast or feed little during the period when receptive females are available. Consequently, they too build up extensive blubber layers prior to the mating season
to fuel the energy-intense competition for access females. Mass loss as a percentage of initial body mass in males, however, is not as great as it is in females. Among several species for which there are such data, the average percentage loss is between 15% and 35% compared to the values given above for females.
Mating patterns in phocids is best known among the three species that mate on land: gray seals and the northern and southern elephant seals. In these cases, the primary mating tactic is one of defending females directly. Recent studies, which use genetic paternity analyses, show that there may be alternative tactics that are more opportunistic but are also
A harp seal (Pagophilus groenlandicus) mother and pup. (Photo by Tom Brakefield. Bruce Coleman, Inc. Reproduced by permission.)
successful, even if to a lesser degree. For example, gray seal males that defend and mate with females before they depart only fertilize about 22% of them and males that capture and mate with departing females, fertilize about 9% of the females they capture. A few studies of a couple of species that mate at sea suggest that males may be more likely to engage in efforts to attract females by displaying than guarding them directly; such a mating system is known as a lek-type system. Females may actually choose their mates in this system, although the evidence for female choice is not strong for any of these species yet.
Females begin producing a single young each year from four to seven years of age, whereas males become sexually mature a couple of years later than females in many species. Furthermore, males do not become socially competitive for several additional years; they may be 10 years old or more before they succeed in mating. Research seems to indicate that both males and females are potentially reproductively active until they die. There is some evidence in a few species to suggest older females may not perform as well as middle-aged females in rearing fat, healthy pups. The youngest and inexperienced mothers may also perform poorly.
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