The two marmosets—small, New World monkeys—had been a closely bonded couple for more than 3 years. Then, one fateful day, the female had a terrible accident. She fell out of a tree and hit her head on a ceramic vase that happened to be underneath on the forest floor. Her partner left two of their infants alone in the tree and jumped down to apparently comfort her, until she died an agonizing death a couple of hours later.

According to the researchers who recorded the events with a video camera (see video above), this is the first time such compassionate mourning behavior has been observed outside of humans and chimpanzees, and it could indicate that mourning is more widespread among primates than previously thought.

Humans mourn their dead, of course, and some recent studies have strongly suggested that chimpanzees do as well. Scientists have recorded cases of adult chimps apparently caring for fellow animals before they die, and chimp mothers have been observed carrying around the bodies of infants for days after their death—although scientists have debated whether the latter behavior represents true grieving or if the mothers didn’t realize their infants were really dead.

But there has been little or no evidence that other primates engage in these kinds of behaviors. Indeed, a recent review of the evidence led by anthropologist Peter Fashing of California State University, Fullerton, concluded that there were no convincing observations of “compassionate caretaking” of dying individuals among other nonhuman primates, such as monkeys.

The sad story of the two marmosets unfolded on 7 February 2005. A team led by animal behavior researcher Bruna Bezerra, now at the University of Bristol in the United Kingdom, had been watching the marmoset group since 2001 in their northeast Brazil home, and the pair had been the dominant male and female since observations began. When the female fell out of the tree, her partner engaged in a number of behaviors, including embracing her, sniffing at her, chasing other monkeys away, sitting by her, and trying to copulate with her. He also emitted alarm calls normally used when a predator is near. And several months after her death, the male disappeared from the marmoset group, never to be seen again.

The team points out that marmosets, like humans but unlike chimps, form male-female couples to bear and raise infants—a relationship known as pair-bonding—and that this might explain why the male was particularly affected by the female’s death. “To the best of our knowledge, this is the first time” such behavior “has been reported for a New World monkey,” says Bezerra, whose team reports its findings this month in Primates. She adds that it was “clear to me” that the male was engaging in caretaking of his dying partner, although she adds that some of his behaviors—such as emitting alarm calls and trying to mate with her—might have been signs of stress rather than compassion.

Fashing says that the new observations demonstrate that “like chimpanzees, common marmosets are capable of interacting with dying group mates. It is especially interesting because marmosets are cooperative breeders with unusually strong male-female bonds,” and “we would expect that these animals would be especially affected by the loss of their primary mating and social partner.” Nevertheless, Fashing cautions that because the report is based on just one observation, it is premature to draw conclusions about how widespread such behavior is in marmosets and other cooperatively breeding primates.

James Anderson, a psychologist at Kyoto University in Japan who has studied mourning behavior in chimps, agrees. “There isn’t a strong case made for describing the reactions as akin to those seen in humans and chimpanzees,” he says. Rather, the male marmoset’s behavior, including the attempts to copulate with the female, could indicate that he was excited and aroused by her lying on the ground, “rather than any kind of compassion.”