(Image: Philip Harris)
AS PhD research projects go, Brian Wisenden’s was enviable: wading knee-deep in shallow streams in the Costa Rican tropical dry forest, watching baby fish darting through the crystal clear waters. By recording their growth and numbers, he hoped to get an insight into their risks of being eaten.
Advertisement
Instead, he witnessed something odd. Many broods were increasing in numbers, not decreasing, as they were picked off by predators. In these groups, some of the fry were smaller than others, suggesting they weren’t siblings. Wisenden had accidentally discovered that the fish, called convict cichlids, adopt each other’s offspring. Why would they do that, he wondered?
Puzzling parenting
In the human realm, we think of adoption as a good and selfless act. But in nature, its presence is puzzling. Taking on the burden of rearing offspring to which they have no genetic link would seem to reduce an animal’s chances of survival or at least provide no gain, meaning it couldn’t have evolved via natural selection. Yet adoption is surprisingly common in the wild. As Wisenden and others start to understand why, they are finding that animals adopt for all sorts of reasons. In some cases, the practice is far from benign, with disturbingly dark underpinnings. But in others it can have remarkable benefits, not just for adoptees but also for foster parents and even their natural offspring.
“Adoption can have disturbingly dark underpinnings”
Nuclear families are a rarity in nature: many parents simply abandon their offspring to their fate. Where parenting does occur, it takes myriad forms. In some cases childcare falls on individuals who aren’t the parents – a juvenile chimp might mind its younger siblings, for example, or animals that live in groups might share babysitting duties. But the individuals involved are usually kin, so the cost of caring is offset by the benefit of helping to pass on the genes they share. There can be other advantages too, such as safety in numbers. Adoption is another matter.
The classic example is the most easily understood. As any parent knows, raising kids is expensive and energetically exhausting – meaning there’s a lot to be gained if you can get someone else to do the job for you. That’s the strategy famously taken by brood parasites such as cuckoos and cowbirds. Here, adoption is by coercion and potential adopters evolve ways to try to avoid their fate. But if the host mounts careful surveillance, the parasite becomes ever more sneaky to outwit it. They are locked in an evolutionary arms race, fuelled by the fact that adoption is often very costly for those left holding the baby.
In the case of the common cuckoo, it means certain death for the host’s offspring. Usually the “adoptee” hatches before its nest mates and tosses out the remaining eggs. Any resident chicks will suffer the same fate. “It’s programmed to kill both,” says Mark Hauber at Hunter College at the City University of New York.
By eliminating the competition, young cuckoos monopolise the entire food bonanza, wheedling host parents into servitude with relentless begging calls. Nestlings of the Horsfield’s hawk cuckoo even . They flash these to foster parents, ramping up the food supply by making it look like there are three mouths to feed, not one. Another species, the North American brown-headed cowbird, typically doesn’t kill its nest mates but uses them to its advantage, brought by the harried parents.
Until recently, this adoption by deception was assumed to always come at a cost to the host, but it seems that in rare cases it may have benefits. What appears to matter is size. Unlike most brood parasite chicks, the young of the great spotted cuckoo are smaller than the offspring of its adoptive parent, the Eurasian carrion crow. it emerged that the crow chicks cash in on the enthusiastic begging of their adopted sibling to gain more food for less effort. The cuckoo chicks also expel a very stinky secretion that seems to deter predators, says Daniela Canestrari at the University of Oviedo in Spain, who was involved in the study. Whether the benefits outweigh the costs for crows over the long term is still uncertain, but ongoing work by Canestrari and colleagues suggests that they do, at least some of the time.
In brood parasites it’s the parents that push their young into adoption, with the youngsters taking over the scam when they hatch. But adoption in the wild can also result from genuine cases of mistaken identity.
For kangaroos adoption is a case of mistaken identity, but for convict cichlids it’s more like kidnapping (Image: Wendy J. King)
(Image: courtesy of Brian Wisenden)
Take the eastern grey kangaroo. Between 2008 and 2013, Wendy King at the University of Queensland, Australia, and her colleagues followed the fates of 326 joeys in Wilson’s Promontory National Park in Victoria and . The circumstances underlying some of these adoptions aren’t known, but four were straight swaps and another four occurred after a mother had lost her own joey. All of the adoptive mothers were already producing milk, and they usually adopted young of the same age and sex as their own, caring for them until they were weaned.
What’s going on here? Before independence, baby grey kangaroos go through a period when they spend time inside and outside their mother’s pouch. Following out-of-pouch forays, mothers normally sniff their young before allowing them back in, but King’s team suspect that during a kerfuffle they may skip the sniff test, allowing a vulnerable joey to quickly clamber in before fleeing from danger. Once inside the wrong pouch, the young may take on the mother’s odour, making them smell confusingly like her own progeny.
So poor offspring recognition is the prime cause of “accidental” adoption in this species – and it’s true of others, too. We take for granted our ability to recognise individuals, but not all animals share this skill. They often use cues like sound, smell and sight, but they may only be able to discriminate between broad categories such as male or female, familiar or unfamiliar, and dominant versus subordinate. What’s more, although many animal parents have evolved the ability to discern their own young from others’, , paving the way for mistakes and cheating.
Disaffected youth
The consequences of adoption following mistaken identity can be dire. In the case of the grey kangaroos, the true offspring of adopting mothers were abandoned and died, unless a direct swap of joeys occurred. But 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 in close proximity to those of unrelated bugs. Mother burrower bugs tend their developing eggs before they hatch, then feed their offspring nutlets from weedy mint plants. Finding mint nutlets is a competitive business, so not every mother bug gets her fair share. And if the delivery rate isn’t up to scratch, discerning to join a better-fed brood.
“Discerning young may ditch their mothers to join a better-fed brood”
whose offspring, if poorly provisioned, may leave home in search of better parents. If they can inveigle their way into a nest where the chicks are smaller than they are, they gain an advantage when it comes to obtaining food.
Blue-footed booby chicks also have a tendency to put themselves up for adoption if their parents are poor providers. Being born the youngest to parents that struggle to feed their offspring puts a booby chick in danger of being bullied to death by its sibling. So, if the colony is dense and conditions in other nests are good, . Yet life in the foster nest is tough because the chick is usually slightly smaller than the resident chicks. So the stakes for leaving home are high, but the risks of staying may be even higher.
Slim pickings aren’t the only reason to seek new parents. Pierre Bize at the University of Bern in Switzerland and colleagues found that Alpine swifts born into nests heavily infested with bloodsucking louse flies actively sought adoption. The more parasites were present, the more likely the chicks were to move to a new nest. By , reducing the detrimental effects to their growth and development.
Adoption among geese is even more intriguing. Studying wild barnacle geese on Svalbard in the Norwegian Arctic, researchers at the University of Groningen in the Netherlands noticed widespread adoptions during the goslings’ first week of life. At first they thought this was accidental. But observing the rate at which predators like Arctic foxes and gulls were picking off these feathered fuzzballs, they discovered that . Back at the university, Jan Komdeur found that , preferring to throw in their lot with parents of a high social rank, which may also improve their chances of survival. “We think it is the chick deciding when to the leave the family, and which family they want to join,” he says. Their freedom to do so is aided by the fact that geese seem to find it hard to distinguish their own goslings before they are about nine days old.
While goslings may choose new parents, other adoptees have little say. Perhaps the most devious examples of adoption involve parents “kidnapping” the young of others to act as a living shield for their own. This seems to underlie adoption in the convict cichlids studied by Wisenden, who is now at Minnesota State University in Moorhead. The fact that adoptees tend to be the smallest brood members led him to suspect that adoption wasn’t simply a random process. revealed that the true offspring were more likely to survive predation in broods that had extra, smaller fry than in broods with no adoptees. “[A predator] has got one shot, and it chooses the easy prey,” says Wisenden. So by adopting offspring smaller than their own, parents are setting them up to be targets. “It’s ruthless.”
How convict cichlids acquire their sacrificial adoptees isn’t clear. “Parents are always actively searching around and if they see babies, and they don’t eat them, they will lure them into their brood,” says Wisenden. Adoption is apparently so beneficial that almost everyone is doing it. Applying genetic paternity analysis to convict cichlids in Costa Rica, Wisenden recently found that .
Not all adoptive parents are dupes or villains, though. In some cases, taking care of someone else’s kids can be a win-win scenario. Among tessellated darter fish, for example, males tend the young alone. A prospective father will set up his territory, clean the underside of a rock, wait for a female to lay her eggs there, and then fertilise them. “At a certain point that male, who is the father of the young, will leave,” says Suzanne Alonzo of the University of California, Santa Cruz. Then, a smaller, unrelated male typically takes over the territory, picking up where the first male left off: cleaning the rock surface, aerating the eggs and defending them from other fish and predators. Females prefer males that are looking after healthy eggs so, despite his size, the adoptive dad may get lucky and fertilise some eggs of his own.
In the animal kingdom, a panoply of adoption stories illustrates that there is no single reason for adoptions to occur – and that there are still plenty of puzzles to ponder. But in the tessellated darter, at least, adoption makes perfect sense. “In this species, [it] is a sexually selected trait,” says Alonzo. In other words, females love a doting dad, so .
“Males have evolved the drive to adopt because it increases their sex appeal”
Brooding on it
The internet is a natural home for bizarre alliances and baby animals, so it’s hardly surprising that stories of cross-species adoptions go viral. The unlikely couplings of gorilla and kitten, hippo and tortoise, elephant and sheep, snake and hamster, and lioness and baby oryx have all hit the big time. These animals tend to be either captive or domestic.
In the wild, instances of animals intentionally adopting young from another species are rare. Many of the reported cases involve birds. They include , , and a abandoned by their parents.
An inkling of what might be going on here comes from the story of a king penguin that adopted a skua chick from under the noses of its parents. Skuas will eat penguin chicks and sometimes adults too, so this was risky. Nevertheless, the foster parent stood its ground against attack from the chick’s true parents.
put it down to misplaced broodiness. Penguins need to maintain their parental drive during long absences to hunt for food, so have high levels of the “parenting hormone” prolactin. This might also be why penguins have been seen brooding pebbles, and why . Misdirected broodiness might also explain some of the stranger interspecies adoptions you see online.
This article appeared in print under the headline “Cuckoo in the nest”
