Dozens of animal species adopt orphans, from chimpanzees and wolves to dolphins and even sloths. Some adopt only close relatives, others take in complete strangers, and a surprising number adopt across species lines entirely. The behavior shows up in primates, carnivores, marine mammals, birds, and rodents, driven by a mix of hormonal triggers, social bonds, and in some cases, what appears to be genuine altruism.
Chimpanzees and Other Primates
Wild chimpanzees are among the best-documented adopters in the animal kingdom. A long-term study of forest chimpanzees published in PLoS One found that half of all observed adoptions were carried out by males, and remarkably, only one of those males turned out to be the biological father. Genetic testing confirmed that in three out of four cases where researchers could check, the adopting males were completely unrelated to the orphans they took in.
These weren’t casual arrangements. One alpha male named Brutus adopted two infants who matured without the physical stunting typically seen in orphaned chimps. Another male, Porthos, adopted a young female named Gia and carried her for 17 months, even during dangerous encounters with rival groups. The commitment was real, though the study found that adoption didn’t broadly increase an orphan’s chance of surviving two years past their mother’s death. The benefits were uneven: some adopted orphans thrived, while others did not.
Other primates adopt too. In 2004, a researcher photographing bearded capuchin monkeys in Brazil noticed one capuchin caring for a young marmoset, a completely different species. The capuchin held the marmoset against its chest, let it ride on its back, and even cracked open nuts to feed it. Locals confirmed the arrangement had been going on for months. In 2021, scientists reported the first known cases of cross-group adoption in bonobos, where two separate females each adopted an infant from a different social group.
Wolves and the Role of Hormones
Wolf packs are built around cooperative pup-rearing, and that instinct sometimes extends to pups the pack has never encountered. In Yellowstone, biologists have observed that when a new breeding male takes over a pack, he will typically adopt the young pups already there rather than harm them. Packs will also, in specific circumstances, accept pups from rival families.
What’s notable is that adopted wolf pups don’t become second-class pack members. Jeremy SunderRaj of the Yellowstone Wolf Project confirms that all pups, adopted or not, are treated the same. The behavior is rare in the wild, but it happens consistently enough that conservationists have learned to exploit it. A technique called cross-fostering involves placing captive-bred pups into wild litters before they’re ten days old. With some careful preparation (including rubbing the new pups with scents from the den), wild packs accept them as their own.
The hormonal mechanism behind this is prolactin, a hormone that surges in all pack members during pup-rearing season in early spring through summer. Prolactin triggers nurturing behavior toward pups, and every adult in the pack experiences it, not just the breeding pair. This same hormone plays a parallel role across mammals. Research published in the Journal of Neuroscience showed that prolactin acts on the same brain region in both male and female mice to promote parental care. Male mice that couldn’t produce prolactin during interactions with pups failed to retrieve or care for them normally. In species where prolactin is tightly suppressed in males, like rats, paternal care is largely absent. In species with higher baseline prolactin, like mice and wolves, males are far more nurturing.
Red Squirrels and the Kinship Rule
Not every species adopts freely. Red squirrels are solitary animals that generally want nothing to do with their neighbors, but they will adopt orphaned pups under one strict condition: the orphan must be a close enough relative. Research published in Nature Communications found that red squirrels only adopt kin related to them above a coefficient of 0.125, which in practical terms means a niece, nephew, or grandchild at minimum. Anything more distant than that, and the squirrel won’t take the pup in. This fits neatly with a concept biologists call inclusive fitness: caring for a relative’s offspring helps pass along shared genes, so the cost of raising an extra pup is offset by the genetic payoff.
Dolphins, Whales, and Marine Mammals
Marine mammals produce some of the most unexpected adoption stories. In French Polynesia, a bottlenose dolphin who already had calves of her own was observed mothering a young melon-headed whale that appeared to have been abandoned. The whale adapted quickly, socializing with young male dolphins and imitating their behavior, including surfing and leaping in waves. Scientists noted this went well beyond the brief “kidnapping” of other species’ young that some female dolphins engage in during conflicts. This was sustained, genuine parental care.
Beluga whales have shown similar openness. Since 2016, scientists have tracked a pod of belugas that includes a lone narwhal, integrated into the group since it was young. In 2011, a pod of sperm whales took in a deformed bottlenose dolphin, possibly after it had been injured. The dolphin traveled, foraged, and played with the whales and their calves for over a week, despite sperm whales normally avoiding contact with other species.
Lions, Leopards, and Unlikely Predator-Prey Pairs
Some of the most striking adoption cases involve predators caring for species they would normally hunt. In 2002, a lioness in northern Kenya made headlines after she was spotted cuddling baby antelopes. Over the following months, she adopted five young antelopes and was seen lying beside them to ward off other predators. In 2020, a lioness in India adopted a two-month-old leopard cub, even though lions and leopards are natural competitors. The lioness, who had her own cubs of the same age, nursed and fed the leopard cub for weeks, treating it indistinguishably from her biological offspring. Scientists who observed the situation said she cared for the cub as if it were her own, though it eventually died.
These predator-prey adoptions are extremely rare and don’t typically end well for the adopted animal. They likely stem from a misfiring of strong maternal instincts rather than any evolved strategy, but they demonstrate how powerful the hormonal drive to nurture young can be.
Birds: Adoption, Kidnapping, and a Blurry Line
Bird adoption is common but complicated. Emperor penguins are a well-studied example, and their behavior blurs the line between adoption and something closer to kidnapping. Research published in Animal Behaviour found that 53% of penguin adoptions occurred after a chick was actively taken from another adult, while the rest involved brooding a chick found wandering alone in the colony. Penguins who have lost their own eggs or chicks are the most likely to adopt, driven by hormonal states that make them intensely motivated to brood.
Domestic hens adopt readily. Research shows that a hen’s behavior toward adopted chicks differs from her behavior toward her biological brood only on the first day. After that, mutual recognition kicks in and innate adoption responses take over. Sloths show a similar ease with adoption. There have been three recorded instances of three-fingered sloths adopting young two-fingered sloths, a different species entirely. Observers noted the comfort between the animals was immediate and natural.
Why Animals Adopt at All
From an evolutionary perspective, adoption is puzzling. Raising another animal’s offspring costs energy and time without passing on your own genes. Several explanations coexist depending on the species. For red squirrels, the math is straightforward: adopting close kin still benefits your genetic line. For wolves, the hormonal surge of prolactin during breeding season may make all adults receptive to any pup, biological or not, as an unavoidable side effect of a system designed to keep the pack’s own young alive.
For chimpanzees, some researchers point to social benefits. An alpha male who adopts orphans may strengthen social bonds within the group, gaining allies as those orphans mature. For the interspecies cases, the simplest explanation is usually a strong parental drive meeting an available infant at the right moment. A lactating lioness encountering a small, vulnerable cub may respond to it the same way she responds to her own young, regardless of species, because the hormonal and neural circuits driving her don’t distinguish between them.
What’s consistent across all these cases is that adoption in animals isn’t random. It follows patterns shaped by hormones, social structure, and relatedness, even when the individual case looks like pure compassion.