Why do mother penguins "abandon" the first egg they lay?

Produced by: Science Popularization China

Author: EVEE (School of Life Sciences, Peking University)

Producer: China Science Expo

Speaking of penguins, I believe everyone must be familiar with them. We can always see penguins moving their bodies "swaggeringly" in various TV programs and when surfing the Internet. However, have you ever seen a "punk penguin"?

In the penguin family, there is a type of penguin belonging to the genus Crested Penguin that has an impressive appearance. Their appearance has a distinct "punk" style, and most species have a brush-like tuft of yellow hair above their eyes, which looks very conspicuous.

Macaroni penguin

(Image source: Wikipedia)

There are a total of 7 species in the genus Crested Penguin. In addition to their eye-catching appearance, these species have a very special feature - female penguins will lay a clutch of two eggs with a large difference in size during the breeding season. And strangely, the first egg laid (egg A) will be much smaller than the egg laid later (egg B).

Why are two eggs laid by the same penguin so different?

Same parents, different fates

The study showed that females of species within the genus Crested Penguin lay two eggs of different sizes, and the difference in size between the two eggs varies between species, ranging from 25% to 70%. Corresponding to these differences, the fate of the two eggs also varies between species.

In species where the difference between the two eggs is relatively small, such as yellow-browed penguins (Eudyptes pachyrhynchus) and Steller's penguins (Eudyptes robustus), female penguins incubate both eggs at the same time, and in approximately 31% to 60% of nests, both chicks will hatch successfully.

In contrast, penguins with the most extreme egg dimorphism, such as the erect crested penguin (Eudyptes sclateri) and the macaroni penguin (Eudyptes chrysolophus), will only incubate the larger egg. Some observations suggest that the first egg may be removed from the nest by the female or even broken by the mother, leaving only the larger second egg to hatch.

Two eggs of the vertical crested penguin, the first one is much smaller than the second one

(Photo credit: Lloyd Davis Photography, lloyddavis.com,

CC-BY 4.0)

As the saying goes, "Even a tiger won't eat its own cubs," so why would a mother penguin peck her own eggs to pieces?

You know, erect-crested penguins often live on the steep rocky coasts of New Zealand's Bounty Islands and Antipodes Islands. Due to their remote location, scientists' research on them is extremely limited, so it is not impossible that some misunderstandings exist.

In order to gain a deeper understanding of the specific details of the breeding behavior of erect crested penguins, scientists conducted relevant research in 1998 on Antipodes Island, the main island of the Antipodes Islands, and recorded the breeding of erect crested penguins on the island.

To facilitate observation and research, the researchers captured 270 vertical crested penguins and marked them on their backs with yellow paint. From September 29 to October 22, 1998, the scientists observed the penguins every day and recorded the laying dates of the first and second eggs, as well as the specific reasons for the loss of the first egg.

Crested Penguin

(Photo credit: Lloyd Davis Photography, lloyddavis.com,

CC-BY 4.0)

According to scientists' observations, female penguins laid a total of more than 100 smaller eggs (A eggs). However, none of them "survived" until a week after the larger egg (B egg) was laid. The vast majority (80.1%) of A eggs were destroyed before or on the day the B eggs were laid.

Specifically, about one-tenth of the A eggs were destroyed in the nest, about 30% of the A eggs were kicked out of the nest, and some eggs disappeared. Scientists speculate that these missing eggs may have rolled down the steep slope and were taken away by skuas.

The situation of the smaller egg

(Image source: Reference 1)

These results fully demonstrate that the erect-crested penguins will indeed "ignore" their first egg. Even if they are born in the same family and have the same parents, the two eggs have completely different fates.

Why was the first egg abandoned by the mother?

This behavior is not common among birds. Although most birds lay more than one egg, they will devote most of their energy to the first egg they lay, and only incubate the second or third egg if the first egg encounters an accident. Scientists have proposed several possible explanations for why vertical-crested penguins and macaroni penguins lay the first egg but ignore it.

Some believe that penguin mothers lay smaller first eggs to reduce losses from fighting during courtship.

But the researchers found that during mating, there were not many fights between male penguins, and the eggs laid by female penguins were unlikely to be destroyed due to fighting (only about 2%). On the contrary, more often than not, it was the female penguins who were the "culprits" who destroyed the smaller eggs.

Group of vertical-crested penguins

(Image source: Wikipedia)

Others believe that the mother penguin may have known that the smaller egg would not hatch, so she gave up her first baby.

To explore the possibility of this explanation, in the 1990s, scientists collected macaroni penguin eggs for artificial incubation. They found that both the first egg (egg A) and the second egg (egg B) could hatch normally, but the weight of the chicks hatched from egg A was only half of that of the chicks hatched from egg B. However, after a year of feeding, there was no difference in their weight.

Macaroni Penguin

(Image source: Wikipedia)

Based on the second explanation, some people have suggested that the mother penguin may know that the chicks hatched from the first smaller egg are more difficult to survive, and she does not have enough energy to take care of two children at the same time, so she simply incubates the larger egg.

Overall, this explanation is more acceptable. Both the crested penguins and the macaroni penguins forage offshore and may not be able to bring enough food back to the nest to raise two chicks, so they choose to invest in the larger, more promising egg.

But this brings up another question: since the mother penguin knows that she does not have enough energy to hatch and feed two little penguins, why doesn't she just lay one egg?

Penguin mothers are "bitter": they inherit the habits of their ancestors, but can only bow to reality

As mentioned earlier, all species of the crested penguin genus lay two eggs in a clutch, which means that these species originated from an "ancestor" that laid two eggs. The habit of laying two eggs has been engraved in the genes of the vertical crested penguin, so they will lay two eggs no matter what during the breeding season. It can be said that laying two eggs is actually a helpless act for them.

Before the breeding season arrives, both the crested penguins and the macaroni penguins will start their journey back to their spawning grounds. This migration distance usually reaches several thousand kilometers, which is very long for penguins. Studies have shown that after arriving at the spawning grounds, the macaroni penguins will quickly occupy the previous nests and lay the first egg after 3-15 days. Their egg-laying cycle is 16-20 days, which means that the yolk development of female penguins must be started during the return migration. But even so, they are not fully prepared when they lay the first egg, so the first egg they lay is relatively small, and the second egg is usually laid a few days later, so it will be larger.

The limited energy of these penguins means that they can only focus on incubating the most promising egg, and they will move the smaller eggs directly out of the nest. If the A egg is forced to stay in the nest, the mother penguin will even take more radical actions.

Crested penguin nest

(Image source: Wikipedia)

When scientists placed a circle of stones outside the penguin nest to prevent the female penguin from kicking the eggs out of the nest, the fate of the smaller egg did not change, because there is no way to "out of sight, out of mind", and these eggs will even be pecked by the penguin parents.

Perhaps the mother penguin was also sad when she pecked the egg to pieces.

Editor: Ying Yike

References:

【1】Davis, LS, Renner, M., Houston, D., Zhu, L., Finkler, W., & Mattern, T. (2022). The breeding biology of erect-crested penguins, Eudyptes sclateri: Hormones, behavior, obligate brood reduction and conservation. Plos one, 17(10), e0275106.

【2】Williams, TD (1990). Growth and survival in macaroni penguin, Eudyptes chrysolophus, A-and B-chicks: do females maximize investment in the large B-egg?. Oikos, 349-354.

【3】Hiscock, JA, & Chilvers, BL (2014). Declining eastern rockhopper (Eudyptes filholi) and erect-crested (E. sclateri) penguins on the Antipodes Islands, New Zealand. New Zealand Journal of Ecology, 124-131.

【4】Crossin, GT, Trathan, PN, Phillips, RA, Dawson, A., Le Bouard, F., & Williams, TD (2010). A carryover effect of migration underlies individual variation in reproductive readiness and extreme egg size dimorphism in macaroni penguins. The American Naturalist, 176(3), 357-366.

(Note: Latin text should be italicized.)