January 10, 2000
High-tech tracking devices reveal new details of elephant seal migrations and
By Tim Stephens
Thousands of elephant seals are now returning from their fall migrations for the
winter birthing and breeding season at Año Nuevo, a major rookery for seals
and sea lions 55 miles south of San Francisco. How the elephant seals spent their
months at sea, a question once shrouded in mystery, is becoming much clearer thanks
to some sophisticated new tracking technologies deployed by UCSC scientists. Among
other things, the researchers have found striking differences between male and female
elephant seals in their migration patterns and feeding strategies.
The research team, led by professors of biology Burney Le Boeuf and Daniel Costa, has
been studying the diving behavior and diving physiology of northern elephant seals
since 1983. Other researchers--Dan Crocker, Susanna Blackwell, Paul Webb, and Dorian
Houser--joined the team as graduate students and are now at other institutions.
|UCSC biologists mount a time-depth recorder on the back of a female elephant seal
at Año Nuevo. The device is glued onto the fur and the epoxy mount falls off
during the seal's annual molt. The researchers are (l-r) Susanna Blackwell, Jeannine
Williams, Paul Webb, and Dan Crocker (additional
Photo: Burney Le Boeuf
The group's most recent findings come from tracking the daily movements of 47 elephant
seals during the two foraging trips the animals make each year, spending months at
a time at sea. Satellite tracking devices and data recorders attached to the 27 adult
males and 20 adult females traced their migration routes and diving patterns with
unprecedented accuracy. The results revealed several interesting features of elephant
seal migrations, including the following:
- Male and female elephant seals travel to different areas and feed on very different
kinds of prey.
- Male elephant seals show remarkable consistency in their migrations, following
the same routes to the same places year after year.
- Females follow more variable routes, apparently pursuing prey that move from
place to place in the open ocean.
UCSC researchers deployed the first time-depth recorder on an elephant seal in
1983 and documented the seals' phenomenal diving abilities. Adult females, for example,
can dive to depths of nearly 5,000 feet and can remain submerged for more than an
"We were amazed at the results, which gave us our first insights into the animals'
behavior at sea, but these recorders provided only information on what the animals
did, not where they went or how they got there," Costa said. "Tracking
the animals' movements by satellite allows us to put their diving behavior in the
context of where it occurs in the ocean."
The group's most recent results have not been published yet, but the researchers
have presented their findings at various scientific meetings. In December, Le Boeuf
and Crocker presented papers at the biennial meeting of the Marine Mammal Society
Scientists have long known the general migration patterns of male and female elephant
seals. Both sexes leave the rookery after the breeding season ends in March. The
females depart first and return to molt in May, after which they head back out to
sea on an eight-month foraging trip. The males are at sea for about four months in
the spring, return to molt in mid-summer, then embark on another four-month foraging
trip. The food the seals gather on these trips is crucial, because it must sustain
them through long periods of fasting while they are on land.
"Fifteen years ago, we knew woefully little about the diving and foraging behavior
of marine mammals," Le Boeuf said. "We knew the seals entered the sea to
feed and returned at predictable times, but we knew nothing about what they did and
how they did it. Technological advances in instrumentation, microcomputers, and satellite
technology have changed all of that."
In their recent tracking studies, the researchers found that the males headed north
and traveled directly to certain areas along the continental margin, ranging from
coastal Oregon to the Aleutian Islands in western Alaska, almost 3,000 miles from
Año Nuevo. The largest males tended to travel the greatest distances to their
feeding grounds. Once a male reached its destination, it stayed there to feed before
heading back to the rookery. Females, in contrast, ranged across a wide area of the
northeastern Pacific, feeding in the deep waters of the open ocean and moving from
place to place.
"The sexes are foraging for prey quite differently, and they have to because
of the discrepancy in size between males and females," Le Boeuf said.
Male and female elephant seals are markedly different in size and appearance, a phenomenon
known as sexual dimorphism. The adult males, with their extraordinary trunk-like
noses, can weigh almost 3 tons and are three to seven times larger than mature females.
In several cases, the researchers were able to track the same animal during more
than one migration and found that they tended to follow the same route every time.
The males, in particular, were remarkably consistent, traveling identical routes
to and from the same feeding grounds year after year. Females tended to start out
in the same direction each time, but ended up following different routes on successive
"The males seem to be going to a fixed place to feed, whereas the females are
feeding on patchily distributed prey that shift around from place to place,"
Le Boeuf said.
Diving profiles also indicated different feeding behaviors in the two sexes. The
predominantly flat-bottomed dives of the males suggest a descent to the seafloor
to feed on bottom-dwelling prey, such as skates, rays, ratfish, small sharks, and
hagfish. The females' jagged-bottomed dives suggest they were pursuing prey that
moved up and down in the water column with available light, primarily several species
Male elephant seals gain substantially more weight during these foraging trips than
the females do, despite spending less time at sea feeding. They may be finding more
food, eating higher-quality food, or both, Le Boeuf said. Putting on weight is of
critical importance to male elephant seals, because a male's size largely determines
his ability to win fights with other males, and his dominance status is directly
related to success in mating with females. A dominant male is able to control a large
harem of females, mating with them exclusively and keeping all other males away.
This reproductive system, known as polygyny, is the evolutionary force driving the
sexual dimorphism and divergent foraging strategies in elephant seals, Le Boeuf said.
Because the largest males father most of the pups, natural selection has favored
larger and larger males with correspondingly high-energy requirements.
The males expose themselves to additional risks, however, in foraging along the continental
margin. Killer whales and white sharks are known to prey on elephant seals along
the coast and around the Aleutian Islands, while the females are less likely to encounter
such predators in the open ocean. Thus, the males migrate longer distances and take
more risks than the females, but it pays off for the survivors in terms of foraging
success and, ultimately, reproductive success.
The females' foraging strategy is not without its vulnerabilities, however, as was
demonstrated during the 1997-98 El Niño season. El Niño brings unusually
warm waters into the northern Pacific and significantly alters the distribution of
fish species, including squid. In 1998, females tracked by the researchers spent
more time looking for patches of food and less time feeding in any one location than
in normal years, said Crocker, now at Sonoma State University. The result was that
the females did not gain as much weight as usual, which could significantly affect
their breeding success.
UCSC researchers are using similar tracking technologies to study a variety of marine
mammals. "We are able to monitor continuously the diving pattern of a seal,
sea lion, or whale in minute detail for up to a year at sea, and we are learning
unexpected things," Le Boeuf said. "Ironically, we now know more about
behavior at sea than about some behaviors on land."
During the winter breeding season, Año Nuevo State Reserve offers guided
walks for the public to view the elephant seals. For information, contact the reserve
at (650) 879-2025 or (650) 879-0227 for recorded information, or visit the Año Nuevo State Reserve Web site.
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