Scientists working in Brazil recently witnessed a heartbreaking scene involving a female marmoset who sustained a mortal head injury after accidentally falling from a tree. They then watched in amazement as her partner of three-and-a-half years climbed down to comfort her as she lay dying.
For primatologists, this is a truly rare occurrence. Prior to this incident, the only compassionate care-taking behavior towards dying adult group members has been seen in humans and chimpanzees.
Here's the video of the incident — with a graphic warning after the 0:56 mark.
The researchers had been observing this group of marmosets for quite some time. They were familiar with all 12 individuals, including the two involved in the incident — the group's alpha female (F1B) and alpha male (M1B). The couple was in a long-term relationship and had already raised eight offspring between them (marmosets exhibit both monogamous and polygamous behaviors).
While observing the group, the primatologists watched F1B fall from a tree, hitting her head on an object on the ground. She lay in agony for 2.5 hours before passing away. Her partner didn't realize what had happened and didn't notice her lying on the ground for 45 minutes. But when he saw her, he immediately went to her, leaving two babies in the tree unattended.
For the next hour and 48 minutes, M1B sat alongside her, trying to interact with her. He hugged her, sniffed her, and emitted alarm calls (which is usually only done when a predator is nearby). He even tried to have sex with her (probably because he was suffering from stress, or he was trying to elicit some kind of response from her; and in fact, marmosets often use sex to reinforce their social bonds, much like bonobos). He also kept a look-out, shooing the young ones away (something that chimps have also been observed to do; scientists aren't entirely sure why).
Eventually, however, F1B died. A few months later, to the surprise of the primatologists, the male left the group and was never seen again.
Read the entire study at Primate: "Responses towards a dying adult group member in a wild New World monkey." Supplemental information via BBC.