SCIENCE OBSERVER
South Beach Diet
Frank Diller
If you want to make a penguin, you've got to break a few eggs. But in
Punta Tombo, Argentina, the largest Magellanic penguin
(Spheniscus magellanicus) breeding ground in the world,
it's important to keep the eggshells from prematurely cracking under
external pressures.
At 0.81 millimeter in thickness, without the egg membranes,
Magellanic penguin shells are at least 56 percent thicker than bird
eggs of similar mass. This extra protection is important, as the
eggs incubate for an average of 42 days in a burrow or underneath a
bush with little or no nesting material. Calcium makes up slightly
more than one-third of the average penguin eggshell. Where does the
calcium come from? According to a study published in the January
issue of The Auk, a quarterly journal of the American
Ornithologists' Union, family-minded penguins may ingest and retain
in their stomach the tough shells of mollusks as a supplemental
calcium source.
A typical penguin following the anchovy migration south through the
Atlantic Ocean to Punta Tombo weighs between 3.5 and 4 kilograms.
When the seabirds take to the land for three to five weeks at a time
to molt or lay eggs, the fish feast ends and the penguins fast,
losing up to a kilogram of body weight. "It's the Weight
Watchers of birds," says Dee Boersma, a professor of biology at
the University of Washington and director of the Magellanic Penguin
Project in Punta Tombo.
Female penguins don't eat for at least a week before laying an egg
or two. But Boersma and her colleagues, Ginger Rebstock of UW and
David Stokes of Sonoma State University in California, noted that
both sexes of the landlocked waterfowl fill up on mollusk shells
during the breeding season. As they are gradually dissolved by
stomach acid, the shells might alleviate penguin hunger
pangs—and they last longer in the stomach during the time
before eggs are hatched, since adults aren't regurgitating the
contents of their stomachs to feed the chicks.
This habit of ingesting calcium-rich items, such as shells, grit,
ash or bones, is considered to be rare among seabirds since their
regular diets are high in calcium from fish and shellfish. But the
supplement gives Magellanic penguins one of the thickest seabird
eggshells around—an important insurance policy for eggs laid
on the hard Patagonian ground.
A storm can flood a burrow, causing it to collapse on the eggs, but
a penguin's temper might be the biggest threat to its offspring.
Fights are common in, around and over nests—and when penguins
go at it, says Boersma, "they're kind of doing karate chops to
one another, and they stomp on the eggs." But a surprisingly
large number of eggs escape the threats of rain and
penguin-on-penguin violence.
Boersma, her colleagues and a large number of students and
volunteers from the United States and Argentina examined 10,023 eggs
between 1984 and 2001. After ruling out those broken by predators
(birds, skunks, foxes and the occasional armadillo), they tallied
only 257 eggs that broke before hatching—2.6 percent of the
total yield.
Despite the strength and viability of the eggs themselves,
reproductive success of the Magellanic penguins seems to require
more than tough shells. "Penguins can lay two eggs and raise
two young. Most don't," Boersma says. "The best result
that we've seen in twenty-one years is that they raise one young."
Boersma continues to investigate the reasons behind the limited
penguin population growth as part of the Magellanic Penguin Project.
Begun in 1982 by the Wildlife Conservation Society after some
Japanese companies expressed an interest in harvesting the waterfowl
for meat, oil and golf gloves, the project full of scientists,
students and volunteers has banded more than 50,000 penguins over
the past two decades.
"I'm just crazy about penguins, and I believe that they can
tell us about the environment and the long-term health of the
planet," says Boersma. She notes that funding for such
long-term projects is difficult to come by, but the resulting yield
of valuable data can be quite large.
Over the years the project has published articles on a variety of
topics, including the impact of oil pollution, fisheries and tourism
on penguin habitat. And the nature of the longitudinal effort
promises more information about penguins, she says. "This is
the kind of data base where they'll be writing papers long after I'm gone."