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Adolescent chimpanzees adopt their younger siblings if their mothers die. The older siblings keep a close watch on the youngsters, protect them from threats, and give them lots of comforting snuggles.

The finding adds to the evidence that chimpanzees can understand when others are suffering, and to some extent can help them. In line with this, a second study shows that chimpanzees have a strong emotional response when they see that another chimp, or a familiar human, is injured.

鈥淎fter individuals lose their mothers, they do receive attention from other individuals in their group, who are often their siblings,鈥� says Rachna Reddy of the University of Michigan. She has now studied the adoptions in detail.

Orphaned

Reddy was tracking a group of about 200 chimps in Kibale National Park, Uganda, when they were struck by a respiratory disease between December 2016 and February 2017. During the outbreak, 25 chimps died and 13 lost their mothers.

Chimps spend their first five years being carried and nursed by their mothers, and another three staying close to them. They are then . Of the orphaned chimps, nine were under the age of 12, meaning they were adolescents or younger.

Reddy followed four pairs of young chimpanzee siblings, where the older of the two effectively adopted the younger. The older chimps were 10-17 years old, while the younger siblings they adopted were 6-7 years old.

Compared to the period before their mothers鈥� deaths, the pairs spent much more time together. They also groomed, reassured and consoled each other. 鈥淚t was like they were both seeking physical contact all the time,鈥� says Reddy. 鈥淭hey were sort of inseparable.鈥�

Reddy was particularly struck by how vigilant the older siblings became. While travelling, they would regularly stop and look behind them to check on their younger siblings. The older siblings became noticeably upset if they lost track of the youngsters. 鈥淚t鈥檚 common for young chimps to whimper if they get lost, but sometimes these older individuals would,鈥� says Reddy.

Caring primates

The behaviour makes evolutionary sense, because by helping their siblings the older chimps are ensuring that many of their own genes survive, says Reddy.

We cannot be sure what is going through their minds, but Reddy says the younger siblings were clearly scared and upset, and wanted to be with someone they trusted. Meanwhile, the older siblings seemed to take the situation extremely seriously. 鈥淚鈥檓 hesitant to use a word like 鈥榬esponsibility鈥�,鈥� says Reddy, but they were responding to their siblings鈥� distress. 鈥淚t might not be in a very sophisticated way, but they understand something about need and they anticipate that.鈥�

In line with this, chimps respond when one of their number is injured: they may inspect or lick a wound, or wait for a chimp that is moving slowly because of injury. Yutaro Sato of Kyoto University in Japan and his colleagues have now found that chimps respond on an emotional level to seeing an injury.

They showed six captive chimps a human with whom they were familiar, who had a prosthetic wound from which fake blood was oozing. Thermal imaging cameras revealed a temperature drop inside the chimps鈥� noses, which is known to be associated with a strong emotional reaction 鈥� especially negative emotions.

鈥淏oth studies are related to chimpanzees鈥� ability to understand others鈥� distress,鈥� says Sato. He says chimps can spot obvious signs of distress, like screaming or visible injuries, but they seem to miss subtler cues that most humans would notice.

Fostering a teenager

Consoling individuals in distress has been observed in other social species, and it seems to have a long history in primates. In a third study, Liz Campbell of the International Fund for Animal Welfare in Azrou, Morocco saw a remarkable incident while monitoring Barbary macaque monkeys.

A three-year-old male called Pipo was hit by a car and seriously injured. His group left him behind and he was alone for two days, much of which he spent screaming. Then another group of macaques found him and took him in for four months while he recovered. Eventually Pipo returned to his own group.

While Barbary macaques often foster infants, Pipo was almost fully-grown 鈥� yet he was still accepted. Campbell says this is good news for conservationists, as it may mean that even slightly older monkeys in captivity can be released into wild groups. It also illustrates that the macaques have similar empathic abilities to the chimps. 鈥淭hey do recognise that this juvenile was distressed and injured,鈥� says Campbell.