they are weaned, the pups have grown to about . Just before the pups are weaned, a new mating cycle takes place. Females ovulate at the end of their lactation period, but remain close to their pups, ready to defend them if necessary. During the mating season, male seals will "sing," emitting a long-drawn-out warbling note that ends in a sort of moan or sigh. This sound may attract females, or may be used by the males to proclaim their territory or their readiness for breeding. Males occupy the same areas from one year to the next. Like many Arctic mammals, bearded seals employ a reproductive strategy known as delayed implantation. This means that the blastocyst is not implanted for two months after fertilization, most often becoming implanted in July. Thus, the seal's total gestation period is around eleven months, though its active gestation period is nine months. Natural predators of the bearded seal include polar bears, who rely on these seals as a major food source. Killer whales also prey on these seals, sometimes overturning ice floes to reach them. Walruses also eat these seals, mainly pups, but such predation is rare. Bearded seals are believed to live up to 31 years. Vocalization The vocalizations produced by the bearded seal are very unique, possibly because their trachea is different from that of other Northern Pacific phocids. A majority of the rings in the trachea are incomplete with only a membrane attaching the two ends. The sounds of the bearded seal usually consist of a long oscillating trill lasting for a minute or more followed by a short, deep moan. This "song" is often repeated frequently. The number of call types within a population can vary geographically, with four types found in Svalbard and eleven in the Western Canadian Arctic. The most frequent sounds are trills, moans, and sweeps. A sweep can be compared to a short trill. Bearded seals produce distinct trills from late March to late June, with a decline in rhythmicity in late May and June. This timeline coincides with their breeding and pupping season, which is from April to May. The repetitive and transmittable nature of bearded seal trills leads researchers to believe that they are utilized for communication, likely during courtship and breeding. Males use these sounds to establish mating territories and communicate their fitness, but it is likely that females produce these sounds as