The gray whale makes the longest migration of any mammal known, each autumn and spring they pass between their Arctic summer feeding grounds and the warm lagoons near the equator where females give birth (5). This yearly round-trip may entail individuals travelling up to 20,400 kilometres (2). Sexual activity can occur at any time of the year, but tends to be concentrated on the migration south (5). Little is known about the mating strategies of this species, but various numbers of individuals can be involved (5). The breeding cycle last two years: gestation takes about 13 months and the single calf is then suckled for a further seven months (5). At birth the calf is smooth compared to the encrusted adults and lacks sufficient blubber that would allow it to survive in Arctic waters (2). The mother may have to hold the calf near the surface to help it to breathe during the first few hours after birth (2). This species is the only cetacean to feed by straining the sediment on the sea floor (5). Individuals roll onto their sides after diving to the bottom and take large amounts of sediment into their mouth. As the whale rises to the surface it strains the contents of the mouth through the baleen, leaving a trail of mud and sand behind it. The invertebrate prey consisting of bottom-dwelling crustaceans, worms and molluscs is isolated in this way and swallowed (5). A number of seabirds are attracted to feeding gray whales, and take advantage of invertebrates that escape the filtering process (2). Sufficient fat reserves are stored in the feeding grounds to allow individuals to go without food during the breeding season; on return to the feeding grounds about a third of the body weight may have been lost (5). Killer whales are the only non-human predator of the gray whale. Attacks directed towards calves have been observed; adult gray whales often try to position themselves between the killer whales and the calf in order to protect it, and they may also head for shallow waters and kelp beds to take refuge from the attackers (2).
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