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introduction to elephant seal

Adaptations | Life on Land | Life at Sea | Mortality | HIstory of the Colony | FAQs

 

elephant seal bull and pupNorthern Elephant Seal (Mirounga Angustirostris)

The northern elephant seal is the largest seal in the northern hemisphere and the second largest seal in the world (after the southern elephant seal). Adult males are 14 to 16 feet (4 to 5 m) in length and 4,000 to 5,000 pounds (1,400 to 2,300 kg) in weight. The females are much smaller at about 9 to 12 feet (2.5 to 4 m) in length and weigh 900 to 1,800 pounds (400 to 800 kg). Pups are 3 to 4 feet (1 m) long at birth and weigh about 70 pounds (32 kg).


Elephant seals derive their name from their great size and from the male's large nose, which serves to intimidate other males both through its size and its effect on their loud challenge call in the competition for females. Males begin developing this enlarged nose, or proboscis, when they reach puberty at about five years, and it is fully developed by eight to nine years.


In the open ocean eight to ten months of the year, they come ashore twice a year – in the winter for the pupping and mating season, and in the late spring and early summer to molt and grow new fur. Juveniles, not participating in the pupping and mating season, come ashore for a month during the September to December period.

 

Along with the whales, elephant seals were heavily hunted from the 18th century to the early 20th century, primarily for their blubber which was processed into oil for lamps and lubrication. The last surviving colony was on Guadalupe Island, unusual among islands off the Pacific coast of North America in its distance from the mainland (170 mi, 270 km). It is estimated that the population of that colony dropped to as low as 50 seals, putting the species at the very edge of extinction. The development of kerosene and protection by Mexico in the early 20th century greatly reduced their harvesting. Since that time their population has grown to 225,000 and it continues to increase. There is, unfortunately, no information on their population before they were hunted.


Northern elephant seals are found in the eastern and central North Pacific Ocean from Mexico's Baja California to the Gulf of Alaska. They birth, breed and molt on Islands and the mainland from the middle of California to the middle of Baja California. Using GPS signals, elephant seals have been tracked from rookeries at Año Nuevo, north of Santa Cruz, California, along with a few from Isla Cedros off the coast of Baja California. Those tracks provide us with a picture of the range of these animals. They travel as far as 3,000 mi, (4,800 km) from the rookery and make two round trips each year.

FES

 

GPS tracks of seals

 

ELEPHANT SEAL LIFE ON LAND

 

Elephant seal life on land is better understood than life at sea because it is more easily observed. When in the rookery, whatever the time of year, the seals, with the obvious exception of nursing pups, do not eat or drink water. They rarely urinate or defecate and, when resting, do not breathe much of the time, conserving both water and energy. While most visits to the rookery are for periods of approximately one month, the older males are in the rookery for about three months during the birthing/breeding period and fast for that entire period.

 

While there are always some seals in the rookery, their visits follow a clear annual pattern as shown in the chart of population and activity below:

 

Population - time chart

The birthing and breeding season includes the months December through March; males older than 5 years begin to arrive in late November, most arriving in December. As they arrive they lay claim to a section of the beach, a claim that is frequently challenged, resulting in a dominant alpha male typically with 2 to 3 sub-dominant beta males controlling each section. While very dramatic battles may arise in the determination of these dominance groups, the males settle most of the dominance face-offs with intimidation. Significant battles are involved in a very small percent of these confrontations, an accomodation to the energy conservation required for their long fast. Pregnant females begin arriving in December with most coming in January. Births occur a few days after arrival.

 

The birth occurs on the beach, often at night. The pup - twins have never been documented - nurses from its mother for four weeks, quadrupling its birth weight to around 300 lb (135 kg). Mother's milk is very rich, reaching around 60% fat by weaning time. During the last week of nursing, the mother goes into estrous and copulates primarily with the males in the dominance group and shortly thereafter goes to sea, leaving her pup behind. There is no evidence that that mother-child relationship plays any future role in the life of either. The pup remains in the rookery, fasting, for an additional 8 to 10 weeks. As the beach becomes less populated due to the departure of females, the pups begin going into the shallow water just off shore. Clumsy with their rapidly acquired weight, it takes some time before they are able to swim well. Most of their time in the water is spent at night, preparing for dives at sea where it will be dark, day and night. Most depart by late March or early April.

 

The males depart the rookery as the last of the females leave.

FES

 

molting adult maleBeginning in late March and extending into September, each of the seals, with the exception of the new-of-the-year, returns to the rookery for a month to grow new skin and hair. Because they do not circulate blood next to the skin while in the ocean, they cannot grow the new skin and hair cells continuously as we do. Instead, they come on land where they are surrounded by air rather than cold water, renew blood circulation next to the skin and grow new skin and hair, shedding the old in the process. They can look very ratty during this period but they are doing fine.

 

The number of seals on the beach is largest in April and May, with adult females and juveniles of both sexes arriving to molt. The beach may not seem as full as it did during birthing, since they lie so close together. Each seal will take about a month to molt, and most will be gone before the sub-adult and adult males arrive to molt sometime between June and September.

 

Overlapping with the older males, the juveniles, seals too young to participate in the birthing and breeding season, come on the beach to rest. Their numbers peak in October and November but some can be found on the beach through December. The purpose of this haul-out is less clear than for the molt or birthing-breeding. It does, however, set the twice a year in the rookery pattern, provides experience with fasting and exercises and strengthens the muscles and bones important for their future visits to land. It is also an opportunity for the young males to spar with each other, important to their future life as adults and entertaining for their human guests.

 

 

ELEPHANT SEAL LIFE AT SEA

 

The northern elephant seals come to the rookery twice a year – once in the late spring and summer for approximately one month to molt and once in the fall or winter to rest if they are young, or to give birth and breed if they are mature. The rest of the year, ten months of the year for most of the animals, they are at sea. While at sea they are solitary and spend 90% of their time deep under water. They dive routinely 1,000 to 3,000 feet (300 to 900 meters) staying down an average of twenty-five minutes before returning to the surface to breathe for two to three minutes. Individual seals have been known to dive to 5800 feet and to stay down as long as 2 hours.This behavior continues 24 hours a day for periods of several months uninterrupted by visits to land or longer periods on the surface.

 

The male seals typically forage for food along the edge of the continental shelf from Oregon to as far west as the Aleutian Islands, as much as 3,000 miles (5,000 km) from the Central Coast of California. The shelf drops off into deeper water, an area called the continental slope. It is over this slope that the seals forage for a variety of prey. The seals employ a grab and swallow technique of feeding, taking such prey as hake, dogfish, rays, octopus and crabs, generally from the ocean bottom. The danger from orcas is greater than in the mid-ocean depths frequented by the females but the density and quality of prey allow the adult males to gain the weight needed to survive their fasts in the rookery and the roughly five months a year travel time between the rookery and feeding ground.. Their two foraging trips are approximately equal in length at four months for older males and five months for juveniles.

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The female seals, who do not grow as large as the adult males and who each year spend two more months at sea, forage primarily in deep mid-ocean water. There they feed at depths well above the bottom. Again there is uncertainty about the diet but we know that even during the day it is very dark at foraging depths and they forage 24 hours a day. Their eyes are particularly sensitive to the bioluminescent colors of prey animals that abound at their foraging depths - lanternfish and the squid that hunt them. Unlike the males, the two foraging trips of adult females are of quite different lengths – approximately two and one-half months between weaning their pup and returning to molt and seven and one-half months between the end of the molt and birthing. Only during this longer second period is the female nourishing a fetus as well as herself.

 

One female was equipped with an accelerometer on her jaw so that eating actions could be recorded. The image below is a three-dimensional plot for that seal using actual GPS, time-depth and jaw-motion recordings. Jaw motions are represented by the red dots. The number of these motions indicate that the seal was ingesting numerous small prey. Note the intervals when the dive does not go to the foraging depth - called drift dives. They can serve two functions - allowing time for digestion and sleeping. Studies recording the orientation of the seal's body during a dive give strong indication that they sleep on some of the dives. Rather than the head down dive that occurs most often, some of the dives show the seal falling like a leaf after reaching a depth of around 120 ft (40 m).

 

FES

 

In its foraging dive, the northern elephant seal first exhales, emptying its lungs of almost all air. Per pound of mass, at the start of the dive an elephant seal will have less than 40% the amount of oxygen in its lungs that a human diver would have but it has almost seven times as much oxygen in its blood and over eight times as much in its muscles. Within a short time into the dive, the lungs are collapsed expelling any remaining air. This both reduces buoyancy and protects the seal from the bends – an affliction caused by the absorption of gases by tissue at high pressures and the resulting damage on ascent when that pressure is rapidly decreased. All of the oxygen used to provide the energy needed during the remainder of the dive is stored in the red blood cells and the muscles. Only at the beginning of the dive does the seal swim with continuous use of its tail fins. During the rest of the descent, almost 90% of the time the seal simply glides. Intermittent stroking dominates the ascent.

 

The Tagging of Pelagic Predators - TOPP - web site has maps showing the current, day by day paths of individual seals. To see an animation of the foraging trips of some female elephant seals during the two months between breeding and molting and the eight months between molting and return for birthing, click HERE.

 

To enable the long period without breathing and the rapid recharge at the surface, the northern elephant seal has very different surface and underwater metabolism. At the surface the seals take between 15 and 19 breaths per minute and have a heart rate of between 80 and 110 beats per minute. On diving, the heart rate drops typically to one-third the surface rate and occasionally as low as 3 beats per minute. Further, circulation is limited almost completely to the heart and the brain so that oxygen consumption is minimized.

 

Dive profiles are approximately V-shaped with vertical speeds of 2 to 5 mph (4 to 8 kph). As that is a rather continuous rate of speed for the time at sea, a seal will swim approximately 15,000 to 20,000 miles (25,000 to 32,000 km) each year. They are capable of swimming at speeds up to 10 mph (16 kph). Seals away from the continental shelf dive somewhat deeper during daylight hours than at night. This probably reflects the behavior of the prey rather than the effects of reduced light on the seals.FES


MORTALITY

 

As with most wild animals, life is fraught with danger.  As visitors during the birthing season see, a number of seal pups do not survive.  High seas can carry a pup out into the ocean; until after a few weeks of nursing, they cannot survive.  On the beach they can be unintentionally crushed by a bull, or starve because they have been separated from their mother and failed to find a surrogate.  In a year with mild weather, the loss of pups at Piedras Blancas is around 5-7%, the rest survive to go on their first voyage.  In a bad year, with major storms from the south, the loss can be as high as 20%.

 

Very few seals, older than the pups, ever die on the beach.  Loss in the ocean, however, is significant.  Little is known in any detail about what the seals die from.  Known predators are the great white shark and the orca or killer whale. Some seals are caught in fishing nets and drown.  Some likely die from failure to forage successfully, disease, or parasites.  What is known is that many do not return.  A study at Año Nuevo showed that only about 2 of 3 males survived each year and that, for most of their life span, 6 of 7 females survived each year.  The more than double mortality rate of the male may be the result of their foraging preference for the continental shelf area along the Bay of Alaska which is an area rich in killer whales.


 

Adaptations | Life on Land | Life at Sea | Mortality | HIstory of the Colony | FAQs

 

 

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Photo contributions by Phil Adams, Joan Crowder, Peter Hemming, Brandt Kehoe, Tim Postiff & Nancy McKarney