Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world
Species: Porphyrio porphyrio melanotus
Habitat: living in m?nages a cinq in Australasia and New Zealand, with other subspecies on mainland Asia, Africa and southern Europe
Some people are very forbearing. Take Georgiana Cavendish, duchess of Devonshire (1757-1806). Her best friend, Elizabeth Foster, had a long-standing affair with Georgiana's husband. Yet for many years Georgiana shared a house with them. Elizabeth even had two illegitimate children with the duke: Georgiana raised them with her own.
Of course, Georgiana was constrained by the society she lived in. Nowadays most people in her position would disown their friend and divorce their spouse. It's the same in the animal kingdom. Females often get cross if their male mates with a rival, particularly if they get landed with the resulting offspring.
But one bird is just as tolerant of adultery and illegitimate offspring as Georgiana was. Female p?kekos cheerfully let other females lay eggs in their nest, and share the task of caring for them, despite the cost to themselves.
M?nage ? trois ou cinq
P?kekos are a subspecies of purple gallinule. They are an iconic animal in New Zealand, where thanks to their pugnacious personalities and intelligence they're often used in TV adverts (here's an example). If you're not sure how to pronounce their name, the stress is on the first syllable.
On New Zealand's South Island, they live in groups up to eight strong. Each group will have between one and three breeding males and one or two breeding females, and perhaps a few non-breeding adults as well. The breeding adults are polygamous: every male mates with every female, regardless of how dominant they are.
Within each group, the females often lay their eggs in the same nest. All the adults then share the work of caring for them. This marks the p?kekos as oddities in the bird kingdom.
"Only 3 per cent of birds have a system where adult birds care for young not their own," says James Quinn of McMaster University in Hamilton, Ontario, Canada. Of those the vast majority are older siblings that haven't yet managed to breed for themselves. Caring for unrelated young, as the p?kekos do, is virtually unheard of.
Caring sharing birds
Quinn has found that sharing a nest in this way actively harms a female p?keko's ability to reproduce. Joint clutches can contain 10 eggs, compared to just six in solitary clutches. The total number of eggs that hatch from a joint clutch is higher, on average, but eggs are also more likely to be accidentally broken in a crowded nest ? so the chances of any individual egg hatching goes down. Weighing the two factors against each other, a female is better off going solo.
Given that, why do the female p?kekos put up with other females laying eggs in their nests? They could throw the offending eggs out, or just attack the other female when she tries to lay. But they don't.
Quinn wondered if the females were worried about retaliation. After all, if a female threw out a rival's eggs, the rival might then attack her eggs in return. To find out if this went on, he monitored 19 joint nests. From each he removed all the eggs laid by one of the two females.
Six nests were abandoned entirely, and another five fell victim to predators. But at the remaining eight, the parents continued caring for the eggs, seemingly unaffected. There was no sign of increased aggression between the females. "Robbed females do not retaliate," Quinn says.
We need a boy
Quinn suspects that the males might be responsible for the females' bizarre cooperativeness. Male p?kekos play a big role in childcare, including incubating the eggs at night. So if they abandon the nest, the offspring are doomed.
From a male's point of view, it's better to incubate a joint clutch because more eggs ? all of which carry his genes ? will hatch. So if one of the females was destroying eggs and the clutch got smaller, the male might abandon it and start again in the hope of getting a joint clutch.
For now, that's just speculation. But Georgiana might have appreciated the irony. For female p?kekos, tolerating adultery might be the only way to guarantee that their males help out with the kids.
Journal reference: Animal Behaviour, DOI: 10.1016/j.anbehav.2012.01.027
Read previous Zoologger columns: Itsy bitsy teeny weeny chameleons , Don't bite ? how the zebra got its stripes, The only males with more brain than females , How a blurry-eyed spider pounces on target
, Gecko's amputated tail has life of its own, Unique life form is half plant, half animal, Transgender fish perform reverse sex flip, My brain's so big it spills into my legs, Dozy hamsters reverse the ageing process, To kill a mockingbird? No, parasitise it, Chill out with the world's coldest insect, 'Werewolf birds' hook up by the full moon, Cannibal shrimp shows its romantic side, The only cross-dressing bird of prey.
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