A goat frolics with a baby rhinoceros. A pig nestles up to a house cat. A rat snake makes nice with the dwarf hamster originally intended as its lunch.
Few things seem to capture the public imagination more reliably than friendly interactions between different species — a fact not lost onAnheuser-Busch, which during Sunday’s Super Bowl will offer a sequel to “Puppy Love,” its wildly popular 2014 Budweiser commercial about friendship between a Clydesdale and a yellow Labrador puppy. The earlier Super Bowl spot has drawn more than 55 million views on YouTube.
Videos of unlikely animal pairs romping or snuggling have become so common that they are piquing the interest of some scientists, who say they invite more systematic study. Among other things, researchers say, the alliances could add to an understanding of how species communicate, what propels certain animals to connect across species lines and the degree to which some animals can adopt the behaviors of other species.
“Even one example raises the possibility that there’s something interesting going on here,” Dr. Burghardt said.
Science has not entirely ignored unusual interactions between species. Biologists have described relationships formed to achieve a specific goal, like the cooperative hunting between groupers and moray eels. And in the mid-1900s, Konrad Lorenz and other ethologists demonstrated that during critical periods after birth, certain birds and other animals would follow the first moving object they saw, whether animal, human or machine, a phenomenon known as imprinting. Dr. Lorenz was famously photographed with a gaggle of “imprinted” geese trailing behind him.
Yet until recently, any suggestion that interspecies relationships might be based simply on companionship would probably have been met with derision, dismissed as Pixar-like anthropomorphism. That has changed as research has gradually eroded some boundaries between homo sapiens and other animals. Other species, it turns out, share abilities once considered exclusive to humans, including some emotions, tool use, counting, certain aspects of language and even a moral sense.
To be sure, some scientists remain skeptical that the examples of cross-species relations offer much more to science than a hefty dose of cuteness.
Clive Wynne, a professor of psychology at Arizona State University, said that the videos he has seen all portray interactions that take place “in a human-controlled environment.”
“To me, that’s what kind of removes what would otherwise be interesting,” he said. “Because it ceases to be directly a story about animal behavior and becomes a story about human impact on the environment, like the difference between gardening and the beauty of natural landscape.” But others see fertile ground for investigation even in bonds formed in captivity or other domesticated settings. “There are so many questions,” said Barbara Smuts, a primate researcher at the University of Michigan who in 1985 shocked some of her colleagues by applying the word “friendship” to describe bonds between female baboons. “We know this is happening between all sorts of species. I think eventually the scientific community will catch up.”
In the meantime, there is no shortage of stories about animals that have reached out across species barriers, some supplied by researchers like Dr. Smuts, who described watching her dog, Safi, an 80-pound German Shepherd mix, forge a friendship with a donkey named Wister on a ranch in Wyoming in the 1990s.
At first, Wister charged and kicked at the dog, recognizing her as a potential threat. But gradually, Safi coaxed Wister to interact, performing repeated play bows and running up and down along the fence of the corral where the donkey was penned.
Eventually, as Dr. Smuts wrote in a 2001 essay, “Each dawn, after being released from his corral, Wister would stand outside our door and bray until I let Safi out, and then they would play and wander together for hours.”
The dog taught the donkey to pick up a stick and carry it in its mouth, although, Dr. Smuts said, “he looked like he didn’t quite know why he was walking around with a stick in his mouth.”
The two animals also appeared to work out a common language. When Wister, several times Safi’s size, accidentally kicked the dog during play, the donkey would stand stock still, as if to say, “I didn’t mean it.” Safi, for her part, would jump up and nip Wister’s neck, appearing to signal, “That hurt.” Then the two would pick up playing where they had left off.
Barbara J. King, an anthropologist at the College of William and Mary, said that she hoped researchers would begin to collect examples of cross-species interactions to build a database that would merit scientific scrutiny. “I think we’re not even at the point of being able to extract patterns because the database is so small,” she said, adding that the topic could also benefit from a rigorous definition of what constitutes a “friendship” between members of different species.
Dr. King suggested some criteria. A relationship, she proposed, must be sustained for some period of time; there must be mutuality, with both of the animals engaged in the interaction; and some sort of accommodation must take place in the service of the relationship, whether a modification in behavior or in communication.
In some popular online videos, Dr. King noted, these criteria are clearly missing. In a YouTube clip depicting a hamster on the back of a snake, for example, it is unclear if the two are best buddies or whether the snake is simply not hungry.
Another video, showing a lioness in Kenya “adopting” a series of antelope calves, provides an idea of how perception may differ from reality. In the video, a conservationist who observed the lioness says, “Many people felt this had to be a message from God.” She adds, “This was the lion and the lamb laying down together.”
But Craig Packer, a lion researcher at the University of Minnesota who also appeared in the video, said in a recent interview that the lioness was likely toying with her prey before killing it.
“She was just keeping it around,” Dr. Packer said. He added that amicable interactions between normally hostile species are unlikely to take place in the wild, where “they would end in tears every time.” Even in captivity, however, sustained bonds evoke interesting observations. Marc Bekoff, a professor emeritus of ecology and evolutionary biology at the University of Colorado, Boulder, said examples that involve animals raised together from a young age illustrate the openness present in many species for some time after birth.
“It shows that young animals are really open doors,” said Dr. Bekoff, who has long studied animal emotions.
That certainly appears to be the case at the San Diego Zoo and Safari Park, where since 1981 trainers have been pairing cheetahs with dogs at an early age. The dogs have a socializing effect on the skittish cats, the zoo has found, allowing trainers to take the cheetahs to public events as “ambassador animals.”
Janet Rose-Hinostroza, an animal trainer in charge of cats at Safari Park, said that a particular canine personality type is needed to make such relationships work. In selecting puppies, she said, she looked for ones that were easygoing, and neither too dominant nor too submissive.
The dog and the cheetah gradually work out a way to play together, Ms. Rose-Hinostroza said. Dogs like to wrestle, but cheetahs’ preferred play is the chase — a way to hone their predatory skills.
“The cheetahs are like, ‘No, no, you need to be the gazelle!’ ” Ms. Rose-Hinostroza said.
It is probably no coincidence that many of the better-known animal pairings involve dogs, which have honed the art of cross-species communication through millenniums of having lived with humans. The dogs at the safari park, each housed with a cheetah, are adept at reading body language and take a dominant role with their feline companions — Donna J. Haraway, a professor emerita at the University of California, Santa Cruz, and the author of “When Species Meet,” suggested that the dogs function almost as “social psychologists.”
And sometimes that means figuring out how to speak the other species’ language.
When one dog, Clifford, had trouble persuading his feline companion, Majani, to play, he adopted a new tactic, Ms. Rose-Hinostroza said. Having learned from a trainer how to fake a limp, Clifford tried it out on the cheetah, looking much like a wounded gazelle. The disability, she said, proved irresistible to the cheetah, who came down off its perch to join the game.
But it is grooming, not playing, that cements a dog-cheetah friendship, Ms. Rose-Hinostroza said. Initially, the young cheetahs are terrified by the puppies’ attempts to play, but gradually the two animals begin to trust one another, and at some point, the cheetah begins to lick and groom the dog.
“When you see that happen, you go, ‘Yes, the cat actually likes the dog now,’ so that’s a good day,” she said.
Communing between species, researchers said, can inspire speculation not just about the animals but about the humans that are so fascinated by them.
Dr. Bekoff, for example, said that videos of interspecies interactions offer a way for people to connect with a natural world from which they feel increasingly detached.
“People are really craving to be ‘re-wilded,’ ” he said. “They’re craving to be reconnected to nature, and it’s these odd examples that are really seductive.”
Others see in the meeting of dog and doe, goat and rhino, tiger and bear, an ideal of peaceful connection that humans too often find elusive.
At Haller Park in Kenya — where Mzee, a 130-year-old tortoise, tends to Owen, an orphaned baby hippo — a man visiting the park with his child gazed at the unlikely couple and remarked, during a documentary about the pair, “If two very different creatures get along like this, then why cannot Iraqis and the British, Americans, Palestinians, the Israelis not get on?”
Or as Dr. Haraway, put it: “In a situation in which terrorism is cultivated from every angle and we are taught to fear practically everything, why should anybody be surprised that there’s a profound desire for the pleasures of the peaceable kingdom?”
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