As PhD research goes, Brian Wisenden was enviable, watching baby fish swimming swiftly through the clear waters in the Costa Rican tropical dry forest. By recording their growth and numbers, he hoped to look at their risks of being eaten. Instead, he witnessed something odd. Many groups were increasing in numbers. In these groups, some were smaller than others, suggesting they weren't siblings (兄弟姐妹). Wisenden had accidentally discovered that the fish, called convict cichlids, adopt each other's babies. Why would they do that, he wondered?
In the human world, we think of adoption as a selfless act. But in nature, its presence is puzzling. Taking on the burden of bringing up babies with no genetic link would seem to reduce an animal's chances of survival or at least provide no gain. Yet, adoption is surprisingly common in the world.
Take the eastern grey kangaroo. Between 2008 and 2013, Wisenden followed the fates of 326 baby kangaroos in the National Park in Victoria and recorded 11 cases of pouch swapping. The circumstances behind some of these adoptions aren't known, but four were straight swaps and another four occurred after a mother had lost her own baby.
How come? Before independence, baby kangaroos go through a period inside and outside their mother's pouch. Following out-of-pouch forays, mothers normally sniff their young before allowing them back in, but Wisenden's team suspect that during an emergency they may skip the sniff test, allowing a vulnerable baby to quickly climb in before fleeing from danger. Once inside the wrong pouch, the young may fake the mother's odor, making them smell confusingly like her own progeny. So, poor baby recognition is the prime cause of "accidental" adoption.
Some of nature's adoptions are, actually, driven by young looking for better prospects. In burrower bugs, for example, females lay a nest of eggs close to those of unrelated bugs. Mother bugs tend their developing eggs before they hatch, then feed their babies nuts from weedy mint plants. Finding nuts is a competitive business, so not every mother bug gets her fair share. And if the delivery rate isn't up to scratch, clever young may abandon their mothers to join a better-fed group. That's similar to behavior in several species of gull whose babies, if poorly fed, may leave home in search of better parents.
The consequences of adoption following mistaken identity can be dire. The true babies of adopting mothers were abandoned. But it can have remarkable benefits, not just for adoptees but also for adoptive parents.
