But it takes a lot of extra calcium to produce thicker shells, and a penguin can't just run to the corner drugstore to pick up some calcium-rich antacid tablets. New research led by a University of Washington biologist shows that during the period when eggs are being laid, female penguins have significantly more mollusk shells, mainly clams and mussels, in their stomachs than males do. The mollusk shells gradually leach calcium used to form eggshells.
Penguins typically nest on hard surfaces near coastlines and, if soil conditions are right, they might build burrows. Because they can't fly and are made to be agile swimmers rather than to walk gracefully on land, it is rare that they can collect much soft nesting material.
Still, their eggs are rarely broken because the shells are more than 50 percent thicker than expected for their size, which is about twice that of a chicken egg, said Dee Boersma. The University of Washington biology professor has studied penguins in South America, Antarctica and various South Seas islands, and said the thick-shell pattern follows for some other birds too.
"The birds that lay eggs on rocks or those that tend to lay them from great heights, like ostriches and rheas, also tend of have thicker eggshells," she said.
Both female and male penguins typically fast for a week or more before eggs are laid. Both genders ingest mollusk shells before the fast begins, which could alleviate hunger during the fast. But females lacking other sources of calcium ingest significantly more mollusk shells, which apparently supplements calcium taken from their bones for eggshell formation.
The thicker shells are important for penguins, which often nest in large colonies. Boersma has documented that, particularly in more densely populated colonies, periodic nasty fights break out and eggs or newly hatched chicks are destroyed as the adults scurry back into their nests or burrows, either during or after a fight.
"There's a point where you can't build strong-enough eggshells to survive fights. If you could, the chicks probably couldn't get out," she said. "Remarkably, they don't lose that many."
Eggs also are lost when heavy rains saturate the ground and cause burrows to collapse, but even then the thicker eggshells prove to be advantageous, since adults are able to retrieve some of those eggs.
Boersma is the lead author of a paper documenting the penguin eggshell research published recently in The Auk, a quarterly journal of the American Ornithologists' Union. Her co-authors are Ginger Rebstock, a UW research associate in biology, and David Stokes, an assistant professor of environmental studies at California's Sonoma State University.
From 1984 through 2001, Boersma and her colleagues gathered data on 10,023 eggs at a Magellanic penguin reserve at Punta Tombo, Argentina. Just 257 of the eggs, or 2.6 percent, were broken by anything other than predators or human-caused accidents, similar to the rate for birds that do not nest on hard surfaces or fight in the vicinity of the nest. The researchers determined that 43 percent of the broken eggs were destroyed because of fights.
The researchers also performed necropsies and examined stomach contents of penguin carcasses collected at different times of the year. In the post-egg-laying period, few penguin stomachs contained mollusk shells and the findings were the same for male and female. But during the period around egg-laying, females were far more likely to have shells in their stomachs than males.
"Basically, they're getting a slow-calcium release for weeks before they lay their eggs," Boersma said.
The research was supported by the Wildlife Conservation Society, the Exxon/Mobil Foundation, the MKCG Foundation, the W.K. Kellogg Foundation and Friends of the Penguins.