Pet ownership, where one species intentionally cares for another primarily for companionship, is traditionally considered unique to humans. This behavior involves a deliberate expenditure of resources, time, and energy on an animal that offers no direct, measurable benefit to the caretaker’s survival. However, the natural world reveals various instances of animals forming complex, sustained interspecies relationships. Scientists question whether these bonds truly parallel human pet-keeping or are simply byproducts of other biological drives.
Conceptualizing Interspecies Bonds
To analyze complex interspecies interactions, scientists use established terminology that moves beyond the anthropocentric idea of “pet.” Most relationships fall under the umbrella of symbiosis, which describes a close, long-term association between two different species.
Mutualism is one form of symbiosis where both species gain a benefit from the interaction, such as the relationship between a cleaner fish and a larger predator it services. Commensalism is a second type, where one species benefits while the other is neither harmed nor helped. A classic example involves barnacles attaching themselves to a whale, gaining a mobile home and access to food without affecting the whale. These symbiotic arrangements are rooted in clear ecological advantages, distinguishing them from the altruistic appearance of pet-keeping.
The most challenging relationships involve true altruism, where one animal expends energy to care for an individual of a different species with no discernible benefit. This behavior, often observed in cross-species adoption, seems to defy the typical evolutionary pressure for self-preservation and genetic propagation. Such instances require a different explanatory framework than simple symbiotic exchanges.
Documented Cases of Cross-Species Care
Sustained cross-species care has been documented across the animal kingdom, often involving mammals. A lioness in Kenya, named Kamuniak, repeatedly adopted newborn oryx calves, a common prey species, over several years. She was observed protecting the calves from other predators and acting in a maternal role, a behavior that contradicts her natural predatory instincts.
Marine mammals also exhibit this unusual behavior. A bottlenose dolphin in French Polynesia was seen mothering a young melon-headed whale, caring for it for at least three years. The young whale was fully integrated, socializing and adopting the behaviors of the dolphin pod.
Other pairings include a three-fingered sloth in Costa Rica nurturing a baby two-fingered sloth, species separated by over 30 million years of evolution. In 2021, scientists documented the first case of cross-group adoption in great apes, where two bonobos adopted infants from different social groups. These bonobos provided comprehensive care, including carrying, grooming, and sharing food with the young adoptees.
Biological Drivers of Observed Behavior
These instances of cross-species care are often attributed to the powerful biological mechanism of misdirected parental instinct. Hormones like oxytocin and prolactin, which flood the system during and after pregnancy to facilitate nurturing, can sometimes override the ability to distinguish between one’s own offspring and a vulnerable young from a different species. This hormonal surge transforms an animal’s typical aversion to foreign infants into an irresistible attraction to nurture.
An animal’s lack of species-specific infant cues can also contribute to the misidentification of an adoptee. If the adopted young is roughly the right size and exhibits general distress signals, the nurturing animal’s instincts can be triggered despite the physical differences. This mechanism is particularly strong in mammals, which have highly conserved neural pathways for parental motivation.
In some cases, especially those involving older animals, the behavior may be linked to social learning or a desire for social bonding. The sustained care for an unrelated, non-beneficial individual suggests an ability to recognize and respond to the distress or need of another creature. These complex emotional and cognitive responses indicate that the drive to nurture extends beyond species boundaries, providing a biological basis for behaviors that resemble the human act of keeping a pet.