“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.”