SCOTT SHALAWAY Why are penguin eggshells thick? Try nesting on rocks
Imagine you're a Magellanic penguin, one of just 16 species, and you live along the coast of southern Argentina.
Like all penguins, you're flightless and quite awkward on land where you waddle or hop from point A to point B. But that's OK because you spend most of your life in the ocean where you literally fly through the water courtesy of flipper-like wings. Emperor penguins can dive to depths of almost 800 feet and stay submerged for up to nine minutes
Unlike most birds, you insulate yourself from the ever-cold environment with a thick layer of blubber and extremely dense plumage.
Yes, you're a penguin and proud of the many adaptations that allow you to live in such an environmentally hostile territory.
It's the time you spend on land while nesting that poses the greatest challenges. And it's not because of your awkward gait or limited mobility. It's the challenge of nesting.
Magellanic penguins incubate two eggs for 42 days on hard rocky surfaces with little or no nesting material. Such a "nest" would seem hazardous for something as fragile as an egg. Nesting colonies are large, there's lots of fighting among adults, and eggs invariably get jostled about. Despite these circumstances, a study just published in the Auk, the journal of the American Ornithologists' Union, reports that eggs seldom break.
From 1984 to 2001, only 2.6 percent of 10,023 eggs examined at a nesting colony at Punta Tombo, Argentina were broken or cracked.
Why do so few eggs break despite being laid in such an unprotected setting? The simple answer is that Magellanic penguin eggshells are very thick. Eggshell thickness averaged 0.81 mm. Turns out this is 56 percent thicker than expected when compared to birds that lay eggs of comparable size.
OK, that makes sense. Thicker shells make stronger eggs. The next and more perplexing question is, what's the source of the calcium used to build the eggshells? Magellanic penguins eat anchovy, an abundant schooling fish, almost exclusively. To answer this question, the investigators, Dee Boersma, Ginger Rebstock and David Stokes, investigated the penguin's diet and nutrition.
They found that a female Magellanic penguin needs about 10.5 grams of calcium to produce two eggs. They can mobilize about 15 percent of their skeletal weight for egg production, but that translates to only seven to eight grams of calcium, leaving a deficit of two to three grams of calcium.
Compounding this conundrum is that fact the female Magellanic penguins fast prior to and during egg laying. The missing calcium has to come from somewhere. Careful observations and examination of stomach contents provided the answer.
During the egg period (pre-egg laying, egg laying and early incubation), females were observed to ingest mollusk shells. I use the word ingest rather than eat because stomach content analysis revealed that in many cases just half of a clam shell was present. And often these shells had a drill hole caused by invertebrate predators. This suggests that the clam's innards were gone before the penguin swallowed the shell. A reasonable conclusion is that hen penguins "eat" bivalve shells during the egg period for the calcium necessary for eggshell formation.
The final question is, is this physiologically possible? Mollusk shells contain about 36 percent calcium. When ingested by birds, 50 percent to 70 percent is assimilated during digestion. Therefore 42 to 58 grams of mollusk shells would be required to make up the calcium deficit incurred producing two eggs. That's less than 2 ounces of mollusk shells. As anyone who has ever visited the shore knows, beaches are strewn with shells. If the female eats just a few more ounces of shells, she restores the calcium she mobilized from her own skeleton in the first place.
And yes, male Magellanic penguins eat shells, too, but they don't show a peak during the egg period. They need calcium to simply maintain their skeletal system.
Ah, field biology. Isn't it grand?