Because greater crested penguins lay two eggs but hatch only one

For decades i penguins greater crestatesan endangered subantarctic species, show strange behavior: during each reproductive season they lay two eggs, but usually hatch only the second, abandoning the first. Now, a study published in the journal PLOS One, re-analyzing old data from 1998 seems to shed some light on this singular reproductive strategy: these penguins would be forced to “eliminate” the first egg because they cannot find the same amount of food as their ancestors.

Bizarre behavior

Greater crested penguins (Eudyptes sclateri) I’m there least studied penguin species in the world, which reproduces on the islands of the Antipodes and the Bounty Islands, uninhabited island groups in the south-east of New Zealand. These penguins are not only poorly studied (probably because of the habitat where they live), but they are also in danger: in the last 50 years, in fact, there has been a drastic decline in their population, placing the species at risk of extinction. For this reason, studying their reproductive biology is essential to elaborate strategies for the preservation of the species: so, in 1998, an expedition of New Zealand researchers landed on the islands of the Antipodes to study them closely.

In particular, 270 penguins have been marked with a unique code (so that they can be observed from a distance), of which behaviors relating tocouplingto the fighting and to the incubation of the 113 nests present in the colony itself; in addition, the researchers recorded the dates of spawning, observing their fate after a month. Between more bizarre behaviors observed there is certainly what involves the latter: female penguins, in fact, during each breeding season lay two egg after about five days. Nevertheless, it is only the second egg laid to be hatched; the first, of considerably smaller dimensions, most of the time rolls down from the nest. Not only that: researchers have shown that, even when not accidentally lost, the first egg is often abandoned, or broken.

Of the 113 nests studied, in fact, about 80% of the eggs laid first were lost before the second egg, while the rest were lost within a week of the second laying. To investigate the nature of this phenomenon, in a nearby colony, the researchers placed a ring of stones around 14 nests to keep the eggs from rolling away, and then compared them to 28 other nests left intact. Surprisingly, the result between manipulated and intact nests was the same: none of the eggs laid first had survived. In the human intervention group, 65% of the eggs were found broken.

Adapt to the times

Since, even after twenty-four years, these observations still remain the most recent and extensive data on crested penguins, the researchers analyzed them again, finding a plausible explanation to this bizarre phenomenon. First of all, the size difference between laid eggs is quite unusual for most birds, while, in other species in which there is the deposition of several eggs of different sizes, it is the first egg that tends to be larger. Reasoning by size, one can guess why it is the first egg to be abandoned: a larger egg indicates more sustenance and greater chance of survival. But why is there this difference?