Elephant Seal (Mirounga angustirostris)
Northern elephant seals have characteristically long noses giving them their common name – although these noses aren’t quite as long as an elephant’s! They are slow moving, lethargic (on land), and are the largest and the most unusual of the pinnipeds. This species is sexually dimorphic with adult males weighing 2–7 times more than adult females. Adult males measure up to 4.5 m (20 ft) long and weigh up to 2300 kg (8000 lb); adult females measure up to 3.6 m (11 ft) long and weigh up to 750 kg (2000 lb). The Northern elephant seal coat is dark gray to brown. Males can live up to 17 years, females up to 22 years. They are a highly migratory species and are well known for their incredible diving abilities. Like their cousins, the southern elephant seals, these seals are able to dive to extreme depths for longer periods of time compared to other seals. Males dive to 350–800m (2640 ft) on average and females to 300–600m (1980 ft). The dives average 13–17 minutes, longer for males, followed by a brief, surface interval of less than three minutes. The deepest dives for Northern elephant seals were recorded at 1500 m (5000 ft) deep for 1.5–2 hours. These seals remain submerged 80–95% of time spent at sea. Researchers have identified five different dive patterns associated with travel, sleep and foraging. Northern elephant seals are also in the habit of holding their breaths for as long as 25 minutes while hauled out.
Range and habitat
Northern elephant seals can be found on the coast and offshore islands of California and Baja California. They migrate twice a year from California to Mexico and to the male feeding areas in the North Pacific Ocean in the Gulf of Alaska and near the eastern Aleutian Islands. Females tend to feed further south between 40–45°N. The Northern elephant seal is the only mammal with a biannual migratory pattern. The first migration occurs following the winter breeding season, and the second after the summer molt. Males spend about 250 days at sea each year and travel at least 21,000 km (13,100 mi); the females spend about 300 days at sea and travel at least 18,000 km (11,250 mi). Population estimates for California is about 84,000 Northern elephant seals in California, 32,000 in Mexico based on data from 1991. Seals congregate onshore during breeding season, molt season, and the juvenile haul out period in the autumn.
Cephalopods (squids, octopus, etc.) are an important component of the Northern elephant seal diet. Other prey includes Pacific whiting, skates, rays, sharks, halibut, dogfish, cod, octopus and pelagic red crabs.
Pups are born about 1.5 m (4.9 ft) in length and 30 kg (66 lb) in weight. Females reach sexual maturity at 3–5 years of age, males at 4–6 years although males reach their reproductive peak at 9–12 years. Large adult males arrive at the breeding grounds in December to compete for access to females who usually give birth to the previous year’s pup in January, 2–7 days after her arrival. The pups are born with black hair, which is replaced by a silvery coat after they are weaned, about 3–4 weeks later. Toward the end of the nursing period, the mother mates then leaves to feed. Her pup lives off its blubber reserves for 1–2 months before hunting for its own food. Adults do not feed while they are ashore during breeding season, some males for up to three months, losing approximately 36% of their weight during this period. Northern elephant seals undergo an annual molt on shore during the warmer months between March and July for about 2–3 weeks. They undergo a “radical molt” because the fur comes off in sheets. Each sex and age class molts at a different time, beginning with immature seals early in the season and ending with the bulls, which molt in July. In the autumn, just prior to the breeding season, juvenile seals congregate at the colony sites for around two months, beginning in late September and extending through to November.
Northern elephant seals were almost hunted to extinction in the 19th century for their blubber. Only about 100–1,000 remained on the Mexican Isla de Guadalupe. In the early 20th century, the species received protection under Mexican and U.S. law, which allowed the Northern elephant seal to successfully recover. In California the population grows at an annual rate of 20–30% and the seals are establishing new rookeries. The recovery has been so successful that numbers may now be approaching peak levels and, in some colonies such as the Farallon Islands, haul out space is limited. Northern elephant seals are threatened by El Niño events such as the 1997–98 event when Northern elephant seal pup mortality rose to 80%. El Niño events cause severe winter storms, elevated sea levels, heavy rains and high tides that can submerge colonies and wash away pups. It is known that the Farallon Islands colony decreased by nearly half due to habitat changes caused by winter storms during the event. In California, entanglement in gillnets causes an average of 100 deaths per year. Northern elephant seals are preyed on by great white sharks, a significant cause of mortality in juvenile seals, and sometimes also by orca (killer whales).