Northern Elephant Seals are the second largest species in the order Pinnipedia and they display significant sexual dimorphism in size and secondary sexual characteristics (Deutsch et al. 1994). Males are three to four times heavier, and nearly 1.5 times the length, of adult females. Adult males reach 4.2 m in length and a maximum weight of 2,500 kg. Mean length for males is 3.85 m and mean weight is 1,844 kg. Maximum weight is seen when the bulls are newly arrived for the breeding season after a summer and fall largely spent feeding (Deutsch et al. 1990). The mean length of adult females is 2.65 m, and they can reach a length of 2.82 m. Mean mass is 488 kg, with a maximum recorded mass of 710 kg, both values are from shortly after females give birth. Newborn pups are about 1.25 m long and weigh 30-40 kg. Pups are born in a long woolly black lanugo coat that is shed without the epidermis starting at about 4-5 weeks and usually lasting several weeks until after the pup is weaned (Reiter et al. 1981). After molting, the newly weaned juvenile’s coat is made up of short hairs like those found on adults and they are counter shaded dark grey above and silver grey below.

During the time on land for breeding or molting, Elephant Seals undergo a complete fast (no food or water) and their body weight declines dramatically due to the demands of lactation (females) and defending territories (males) in the breeding season, and growth of new hair and skin during the molt (Champagne et al. 2012). Adult females can lose almost half their mass during lactation, when they take their single pup from birth to weaning in approximately 25 days. The situation is similar for breeding males, which also lose about half their mass while ashore for periods that can exceed three months.

Females reach sexual maturity at the age of 3-4 years and males at 7-9 years. Gestation lasts 11.3 months, including a delay of implantation. The annual pregnancy rate of mature females is 95%. Longevity is more than 14 years for males and 21 years for females (Condit et al. 2013).

Northern Elephant Seals are highly polygynous, but not strictly territorial. Males compete for access to females by ranking themselves in a hierarchy. There is much male-male fighting, vocalizing, and displaying during the breeding season, when bulls may be ashore for months at a time (LeBeouf 1972). One of the most impressive displays occurs when a male rears up on his hindquarters, thrusts one half to two-thirds of his body upward and produces a distinctive clap threat vocalization as a challenge to other bulls. The sound is a rolling, resonant, metallic-sounding series of backfire-like sounds punctuated with pauses (Sanvito and Galimberti 2003).

Females give birth within a few days of coming ashore, from late December to March. They have a throaty sputtering growl, made with the mouth wide-open, which is used as threat signal. Females and pups have a warbling scream that they use to call to each other, and in the case of the pup, to notify its mother when it is disturbed (LeBeouf 1972). Mother and pup form a strong bond immediately after birth, and females aggressively bite other pups that approach them, occasionally killing them with bites to the head (LeBeouf 1972). However, bulls are a greater source of pup mortality as they regularly crush pups when they charge through aggregations of mothers and pups to chase off or fight encroaching males, or when approaching females for mating. Occasionally, bulls suffocate pups by stopping on top of them and not moving again soon enough (Spraker et al. 2014).

Northern Elephant Seals are one of the deepest and longest diving marine mammals. Time-depth recording devices have documented dives deeper than 1,700 m and as long as 109 minutes by adult females. Rest intervals at the surface are usually short, lasting only several minutes between dives that have an average duration of 23 minutes and depth of 520 m (Robinson et al. 2012). After leaving the rookeries, most Elephant Seals spend 80-90% of their time underwater, helping explain why they are infrequently seen at sea.

The diet of Northern Elephant Seals is largely unknown due to difficulties associated with identifying prey for such a wide-ranging species. Observations based on scat and stomach content analysis of animals on land have identified squid and mesopelagic fish as the main items consumed. New evidence from animals sampled at the Año Nuevo colony (northern California) on diet composition from biochemical techniques indicates that adult females are eating approximately 51% squid, 23% myctophids, and other mesopelagic fish (C. Goetsch pers. comm.).

Great White Sharks and Killer Whales are predators on Northern Elephant Seals (Jefferson et al. 1981). Recent work at the rookeries has revealed that large Great White Sharks aggregate around these sites when Elephant Seals return to haul out (Klimley et al. 2001). Seals that swim at or near the surface as they are approaching or departing the islands are particularly vulnerable to ambush attacks by fast-rising sharks that patrol near the bottom in waters 7-10 meters deep.

Northern Elephant Seals are a wide ranging pelagic species that occurs throughout most of the eastern Pacific Ocean north of the 27°N latitude, from the coast of Baja California to the Gulf of Alaska and the Aleutian Islands, westward to about the International Dateline (180° longitude) (Le Boeuf et al. 2000, Robinson et al. 2012). Breeding takes place on offshore islands and at a few mainland localities from central Baja California, Mexico, to Northern California, USA. The species breeds at over 15 discrete locations, of which a dozen are well-known and occur mostly on islands of southern California and Baja California (Lowry et al. 2014). A few pups are born in Oregon, Washington, and southern British Columbia.

Adult Northern Elephant Seals migrate to and from their rookeries twice a year, returning once to breed during December-March and again later in the year (April-August) for several weeks to molt, at different times depending on sex and age (LeBeouf et al. 2000). They also show up at additional coastal sites as far north as southern Oregon for molting. Their post-breeding and post-molt migrations take most females north and west to oceanic areas of the North Pacific and Gulf of Alaska twice a year (LeBeouf et al. 2000, Robinson et al. 2012). Adult males, on the other hand, tend to occupy the shelf break along the Aleutian Islands. Few movement data are available for younger age classes and consequently their at-sea distribution is poorly known. Vagrants have been documented in Japan and at the Midway Islands.