Penguins Do Not Mate for Life, and the Divorce Rate Depends on the Nest

Penguins turn up constantly as a symbol of lifelong devotion. It makes for a good wedding reading. It is also, for the species people usually have in mind, close to the opposite of true.

The reality is more interesting than the myth, because fidelity varies enormously between penguin species, and what predicts it is not how devoted the birds are. It is whether they have an address.

The Numbers Are Not Close

Emperor penguins are the ones from the documentaries, huddling through the Antarctic winter. They are also among the least faithful birds you could pick. Studies of emperor colonies have found roughly 15 percent of birds pair with the same partner in consecutive seasons. Around 85 percent do not.

Move along the family and the figure moves with it. Adélie penguins land somewhere in the middle, with a substantial share re-pairing each year but plenty not. Gentoo penguins are at the other end, with fidelity rates that can exceed 90 percent in some colonies.

Same family, same broad biology, wildly different outcomes. So the question is what separates a gentoo from an emperor.

It Comes Down to Real Estate

Gentoos build a nest. It is not much, a scrape of ground and a heap of pebbles, but it is in a fixed spot and the same pair tends to return to roughly the same place. That nest is a rendezvous point. Two birds arriving weeks apart can still find each other, because they both know where to go.

Emperors have no nest at all. They breed on sea ice in the Antarctic winter, and the male incubates the single egg balanced on his feet under a fold of skin, shuffling around the colony to share warmth. There is no fixed location, and the ice itself moves and breaks up. A returning emperor has nothing to navigate back to except a crowd of thousands of near-identical birds on a featureless surface, in the dark.

They do find each other, sometimes. Emperors have a distinctive two-voiced call that partners learn and can pick out of colony noise. But it only works if both birds are present at the same time, and that is where the second pressure comes in.

Waiting Is Expensive

The Antarctic breeding window is brutally short. Everything has to be timed so chicks fledge when food is available and conditions are survivable. Arrive late or start late and the chick does not make it.

That turns waiting for last year's partner into a gamble with the entire season as the stake. A bird that holds out for a specific mate who is delayed, or who died at sea over the winter, may miss the window completely and raise nothing that year. A bird that pairs with whoever is available and ready starts on time.

Given that trade, low fidelity is not a failure of devotion. It is the correct answer to the problem. The birds under the most severe timing pressure are precisely the ones who re-pair least.

What Is Actually Impressive

Stripping out the romance leaves the genuinely remarkable part intact.

A male emperor incubates through the Antarctic winter, standing on the ice in temperatures that drop below minus 40 Celsius with winds over 100 miles per hour, without eating, for around 65 days. He can lose close to half his body weight doing it. The huddle the colony forms is a real cooperative structure, with birds rotating slowly from the cold outer edge to the warm centre and back, so no individual takes the exposed position permanently.

Meanwhile the female has walked to open sea, sometimes tens of kilometres away, fed, and walked back, timing her return to when the egg hatches so she can feed the chick while the male finally goes to eat.

That is a genuinely extraordinary piece of coordination. It just is not a love story, and it does not need to be one.

Sources

• Williams, T. D. (1995). The Penguins. Oxford University Press.
• Bried, J., Jiguet, F. and Jouventin, P. (1999). Why do Aptenodytes penguins have high divorce rates? The Auk.
• Ancel, A., Beaulieu, M. and Gilbert, C. (2013). The different breeding strategies of penguins: a review. Comptes Rendus Biologies.
• Jouventin, P., Aubin, T. and Lengagne, T. (1999). Finding a parent in a king penguin colony: the acoustic system of individual recognition. Animal Behaviour.