In December 2018, researchers at the Gir National Park in India stumbled upon a lioness who appeared to have adopted a baby leopard as one of her own. The little male cub, who was around two months old, was seen nursing from the lioness, feeding from her kills and playing with her two biological cubs, who were around the same age as the leopard. This rare case of interspecies foster care left the researchers entirely befuddled; pulished in the journal Ecosphere, they describe the lioness’ behavior as plainly “bizarre.”

From an evolutionary perspective, caring for the offspring of another animal doesn’t make much sense. Raising young—nursing them, gathering food for them, making sure they stay safe—requires a lot of time and energy, and is typically done in the interest of propagating one’s own genes. It’s not unheard of for animals to look after non-biological offspring of the same species, but “such acts directly help in boosting the [caregiver’s] lifetime reproductive success,” the study authors write. Female cheetahs, for instance, are known to adopt orphaned male cubs that, once they reach adulthood, form large coalitions with the mother’s own offspring.

Before the lioness and her leopard cub pounced onto the scene, there had been just two other documented instances of interspecies adoption. In 2006, scientists described the adoption of a marmoset by a family of wild capuchin monkeys. More recently, a bottlenose dolphin mother was observed caring for a melon-headed whale calf over the course of more than three years. But in these cases, according to the researchers, “none of the foster parents and adoptees ... belonged to mutually competing species.” Lions and leopards, by contrast, compete for the same resources in the wild—and are usually not very fond of one another.

“They are at perpetual odds,” Stotra Chakrabarti, study co-author and animal behavior researcher at the University of Minnesota, tells Cara Giaimo of the New York Times. Lions kill both adult leopards and their cubs, while leopards are prone to attacking unguarded lion cubs.

And yet, the mother lion, her lion cubs and her spotted leopard baby, all got along just fine. Researchers thought the blended family would last only briefly; in 2017, an African lioness in Tanzania was seen nursing a leopard cub, but the association lasted for just one day and was “not considered as a formal adoption,” the study authors write. Over the course of a 45-day observation period, however, the researchers saw the leopard cub hanging out with its foster family on 29 different days.

The relationship only seems to have come to an end when the leopard baby died. In February 2019, his body was found near a watering hole, with no signs of injury suggesting that he had been attacked. A necropsy, in fact, indicated that the cub had been suffering from a congenital femoral hernia, which means it was born with a bulging blood vessel in its groin that ruptured, likely causing his death.

The circumstances that led to this unusual animal adoption are not entirely clear. One day after the leopard cub was seen with the lioness, a female leopard was sighted at the same location; she may have been the cub’s biological mother, though researchers could not say for certain whether she was lactating. Perhaps she abandoned her baby, who was subsequently adopted by the lioness. But why?

It is possible, the researchers suggest, that the lioness’ response was prompted by her inexperience. At five or six years old, she was a relatively young mother. Her first litter of two cubs had died very young, and so her more recent litter marked her first foray into parenting. What’s more, “given that she was a lactating mother with cubs of her own, her maternal and hormonal instincts could have overridden her recognition or the lack thereof for an unusually spotted cub,” the researchers write.

The distinct behavior of Asiatic lions may have made this unexpected alliance possible. In contrast to African lions, male Asiatic lions do not tend to live with females unless they are mating or sharing a large kill. Females also tend to exist on their own for a few months after giving birth, which perhaps allowed the lioness and leopard cub to exist in happy isolation. How would the leopard have fared if his adopted family had interacted more with adult lions? It’s an intriguing question that can’t be answered, due to the cub’s premature death.

“It would have been fantastic to see, when the leopard cub grew up, how things would be,” Chakrabarti tells the Times. “But it didn’t happen.”