In the Antipodes Islands and the Bounty Islands in the South Pacific Ocean, which belong to New Zealand, live straight-crested penguins (Eudyptes sclateri). Due to the difficulty of reaching these distant islands, there is little research about them. Recently, researchers from New Zealand decided to reanalyze data they collected in a study from 1998. The researchers documented a unique phenomenon: female penguins lay two eggs a few days apart, but neglect the first egg and only care for the younger, preferred sister. Eventually only the second egg will hatch.
Straight-crested penguins have two characteristic stripes of yellow feathers perpendicular to their eyes. They live along the coasts of Australia and New Zealand and on some islands in the Southern Ocean and the South Pacific. Their breeding grounds are in the Antipodes and Bounty Islands. About 24 years ago, in September 1998, researchers followed the behavior of 270 penguins at 113 nesting sites in the Antipodes Islands. In the Southern Hemisphere it is the spring season, which is also the breeding season for the penguins, and the researchers stayed to study their breeding habits for two months after spawning.
Their reproductive habits have hardly been studied. Straight-crested penguin parent and chick. Photo: COULANGES, Shutterstock
The rejected egg
Female penguins lay their eggs in two stages – first one small egg, and five days later a second egg, more developed and about 80 percent larger than the previous one. The staggered spawning phenomenon is also common in other species of crested penguins, but the size differences are much less extreme.
40 percent of the pairs of straight-crested penguins abandoned the first egg and almost never hatched it. In most nests she rolled out of the nest or the laying site and disappeared. In some cases the parents were seen pushing her out of the nest themselves, and more than once she deteriorated like this on the slope and broke.
The neglect also continued inside the nest. When the researchers fenced off 14 penguin nests with rocks to make sure the egg would stay in them, it wasn’t to protect the rejected eggs. In practice, none of the eggs of the first spawn survived. The penguins did not nest on them and sometimes even broke them themselves.
In an attempt to understand the physiological mechanism underlying this abandonment behavior, the researchers tested the hormone levels in the blood of the penguins. It was found that during the first spawning period the levels of the male sex hormone testosterone are relatively low in the males and high in the females. During the incubation period and the second spawning they increase in the males, while in the females they decrease. Therefore, they hypothesize that the increase in testosterone in the females indicates their lack of readiness for reproduction at the time of the first spawning. In males, the increase in testosterone after the second spawn may help them protect the incubating females. But even if the hypothesis is correct, it does not explain why the females waste one litter on an egg that will never hatch, and why the priority is given to the second egg.
Inspired by the laying and nesting style of other crested penguins, it has also been hypothesized that the first egg is used as insurance in case the female fails to lay a second egg. But the hypothesis was ruled out because most of the eggs of the first spawn were lost even before the second spawn or on the day of the spawn itself.
According to another hypothesis, the ancestors of the penguins of this species raised two offspring, but a lack of food caused them during evolution to develop into a species that raises only one offspring. For some unknown reason the chances of survival of the second egg are higher than those of the first, and it is possible that this is how the straight-crested penguins got used to abandoning the first egg. “There are so many hypotheses because we simply don’t know much about these penguins,” concluded the leader of the research expedition, Lloyd Davis, in an interview with the New York Times.
Like many other animals that live in cold regions, climate change is now also affecting the size of the penguin population, and their habitats are shrinking. The straight-crested penguins are currently in danger of extinction and the unique phenomenon of neglecting the eggs from the first laying makes it difficult to increase the number of offspring that reach maturity. The researchers want to raise awareness.
For more articles on the website of the Davidson Institute, the educational arm of the Weizmann Institute of Science