High Fat Diet May Not Be
Enough To Overcome Early Fat Deficit
Alaska
seal pup study may hold key to decline of harbor seal population
SAN FRANCISCO (April 3, 2006) – Female harbor seal pups
whose blubber falls below average levels may be at higher risk of delayed
sexual maturation or death, even if they get enough fat in their diets later
on, according to a new study sponsored by The American Physiological
Society and presented at Experimental Biology 2006.
The study found that harbor seals pups that were
heavier when captured from the wild continued to gain weight and grow
regardless of whether the researchers placed them on a high fat or low fat
diet. But smaller seals, regardless of which diet group they were put in,
continued to lag behind in weight gain and even lost blubber depth.
“Our longitudinal study is leading to a better
understanding of the role that fat consumption plays in the life of the
harbor seal and could tie the declining population of seals and other marine
mammals with the change in availability of seal prey, including herring,”
lead author Lori Polasek said.
*Paper presentation: “Growth and body
condition of harbor seals on high and low fat diets,” 12:45 p.m.-3 p.m.
Monday April 3, Physiological Ecology and Evolutionary Physiology,
492.3/board #C768. On view 7:30 a.m. - 6 p.m. in the Convention Center
Exhibit Hall. Research was by Lori Polasek and Shannon Atkinson of the
University of Alaska, Fairbanks and Alaska Sealife Center, Seward, and Anne
Hoover-Miller of the Alaska Sealife Center.
The research is part of a much larger effort to find
out why harbor seals and other marine mammals are disappearing from the
chilly waters off the Alaskan coast. The population decline, which has
become a focus of research in the past 10-15 years, is potentially due to a
variety of factors, Polasek said, including:
Herring, capelin and eulachon are good sources of
much-needed fat in the diet of harbor seals, but they have decreased in
numbers in some areas where seals live, Polasek explained. “We are examining
the importance of fatty fish in the diet of young, growing seals to see if
our experimental seals reflect changes observed in wild populations and
could explain the rapid population declines seen since the 1970s.”
Low fat or high fat diet
The study began in 2004 with four one-month-old newly
weaned female harbor seals, the animals most susceptible to changes in prey
availability. In 2005, the researchers added four more newly weaned
one-month-old females to the study.
For marine mammals, most energy is provided by fat and
protein, Polasek said. And fat is the easiest to convert to energy and
blubber. Herring is one potential source of fat for harbor seals, she said.
After weaning, pups must learn to effectively fish and accumulate sufficient
fat stores for the winter, but changes observed in the distribution,
composition or abundance of fish may make it harder to ingest and store
enough fat. The study aimed to find out what effect the fat content of the
harbor seals’ diets has on their ability to survive and reproduce.
At the beginning of the study, the pups weighed between
21 and 33 kilograms (9.5 to 15 pounds). The researchers placed four of the
seals in the high fat diet group and four in the low fat group. Both groups
received an identical base diet, including pollock, squid, and capelin, all
of which contain less than 4% fat. The remainder of the diet was herring.
The low fat group received herring that contained less
than 6% lipids, while the high fat group received herring that was 16%
lipids. (Herring have a higher fat content in winter before laying their
eggs, but lose about one-third of their fat after releasing their eggs.)
Except for the fat content of the herring, the nutrient
content was the same for both groups, Polasek explained. The researchers
monitored the animals in the low fat group to ensure fat intake was not
falling below minimal levels, she said. “We don’t want to put them in
nutritionally stressed situation that would harm them,” Polasek said. “We
don’t want to drop the fat too low, but we want to know how seals respond to
a restricted fat diet.”
Starting weight most
important factor
The results so far have been surprising: growth rates,
seasonal fluctuations in body mass, and percent body fat have been generally
independent of the seal’s diet. Instead, the starting weight has been the
critical predictor of whether the seal would add body mass and body fat,
Polasek explained.
Seals weighing less than 26 kilos (about 12 pounds)
showed little progress in gaining weight and even lost blubber, regardless
of which diet they were on. On the other hand, the study’s largest seal,
which weighed 33kg when she began in 2004, has shown good growth despite
being on the low fat diet, Polasek said. A seal that weighed 26kg at the
start of the study, on the high-fat diet, has also continued to grow in a
similar manner.
These results fit with a recent study by a team from
Göteborg University that found wild seal pups smaller than 26 kg when they
were weaned were less likely to survive the winter. It seems that mother
seals, which produce high fat milk, must impart enough fat to get pups over
the 26kg mark, Polasek noted. And once weaned, they must ingest sufficient
food to get through the winter when food is less abundant.
Seals rely on blubber both for insulation and as a
source of energy. Nutritionally stressed adolescent females may take longer
to reach sexual maturation and mature females that are nutritionally
stressed may not reproduce, offering clues to why some harbor seal
populations are falling, Polasek said.
Next steps
The seals will continue their low fat and high fat
diets as the longitudinal research continues. “We’re curious to find if it
will take longer for the low fat diet animals to reach sexual maturation,”
Polasek said. Human females who consume an extremely low fat diet reach
sexual maturity later than those who consume a high fat diet, she noted.
In addition, the researchers are:
-
Examining fecal matter to see how efficiently the animals in
the two groups digest their food. Animals on the low fat diet may be
processing their food less efficiently and eliminating needed nutrients.
-
Measuring bone growth to see if one group grows more
quickly.
-
Monitoring the concentrations of cortisol, in response to a
stressor that dilates the pupils and increases heart beat. Animals in the
low fat group may take a longer time to recover.
-
Comparing their data with observations and tests on seals in
the wild done by the Alaska Department of Fish and Game.
Funding: The National Oceanic and Atmospheric
Administration supported the research.
Editor’s Note: For
further information or to schedule an interview with a member of
the research team, please contact Christine Guilfoy at the APS newsroom @
415.905.1024 (March 31-April 5); or 978.290-2400 (cell) or 301.634.7253
(office),
cguilfoy@the-aps.org; or Donna Krupa at (703) 967-2751 (cell) or
(301) 634-7209 (office).
* * *
The
American Physiological Society was founded in 1887 to foster basic and
applied bioscience. The Bethesda, Maryland-based society has more than
10,000 members and publishes 14 peer-reviewed journals containing almost
4,000 articles annually.
APS
provides a wide range of research, educational and career support and
programming to further the contributions of physiology to understanding the
mechanisms of diseased and healthy states. In May 2004, APS received
the Presidential Award for Excellence in Science,
Mathematics and Engineering Mentoring (PAESMEM).
Experimental Biology is an annual
scientific meeting convened by the Federation of American Societies of
Experimental Biology, including the American Physiological Society (APS)
and other biomedical societies. The meeting features “nominated” lectures,
symposia, research presentations, awards, a job placement center, and an
exhibit of scientific equipment, supplies, and publications. This year’s
participating Societies are APS, American Association of
Anatomists, American Society for Biochemistry and Molecular Biology,
American Society for Investigative Pathology, American Society for
Nutritional Sciences, and the American Society for Pharmacology and
Experimental Therapeutics.
