Why Penguin Eggs Are Tough to Crack

SEATTLE, Washington, May 11, 2004 (ENS) - Penguin eggs come with extra-thick shells to withstand being laid on hard surfaces and survive being kicked around in penguin fights. It takes a lot of extra calcium to produce these thick shells and researchers now believe they know the source of that calcium.

New research led by a University of Washington biologist shows that during the period when eggs are being laid, female penguins have more mollusk shells, mainly clams and mussels, in their stomachs than males do.

The mollusk shells gradually leach the calcium used to form eggshells into the female penguins' systems.

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 more mollusk shells, which apparently supplements calcium taken from their bones for eggshell formation.

"Basically, they are getting a slow-calcium release for weeks before they lay their eggs," said Dee Boersma, a University of Washington biology professor.

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.

Penguin 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 Boersma, who has studied penguins in South America, Antarctica and various South Seas islands.

The thicker shells are important for penguins, which often nest in large colonies. Boersma has documented that, particularly in more densely populated colonies, periodic 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.

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 accidents caused by humans, 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.

"There is a point where you can not build strong enough eggshells to survive fights," Boersma said. "If you could, the chicks probably could not get out. Remarkably, they do not lose that many."