These heartwarming images show how a female chimpanzee has adopted her ill sister’s newborn infant at a zoo in Switzerland.

Kitoko the chimpanzee is currently looking after her own baby but she has also apparently adopted her sister Fifi’s son after she suffered from mysterious health issues at Basel Zoo in Switzerland.

The zoo said in a statement that the birth of Fifi’s son had gone well but noted the mother had “handed her baby over to other members of the group unusually early”.

Despite continuing to suckle her newborn, after about two weeks, “Fifi became weak and developed a limp in her hind legs. Although the zoo vet gave Fifi intensive care, she showed no signs of improvement. The vet could not find any cause for Fifi’s symptoms.

“At the end of July, the zookeepers noticed that Kitoko, Fifi’s sister, was caring for the newborn the majority of the time and had even begun to suckle it. A few days later, Kitoko gave birth to her own baby boy, who she initially appeared to ignore.

“However, father Kume, 18, and other members of the group insisted that Kitoko should look after the young male. Experienced mother Kitoko has been taking care of the two little ones ever since. Both are doing well and developing normally. The young female is called Sangala and Kitoko’s son is named Sabaki.”

The zoo said that they are working with veterinarians, as well as gynaecologists and cardiologists who work in human medicine, in a bid to find out what is wrong with Fifi. The zoo said that “the veterinary team is still working to establish a diagnosis”.

It is not all that uncommon for a chimpanzee to adopt another’s baby, and this also occurs in the wild, according to the zoo. They explained: “Chimpanzees will sometimes also adopt babies in the wild. Baby chimpanzees are dependent on their mothers for the first six years of their lives.

“If the mother dies prematurely, the young animal’s chances of survival in the wild fall dramatically. If other members of the group adopt the orphaned little one, their chances of survival remain high.”

But they also said that orphans who are adopted in the wild are often older than Fifi’s son. They said: “There are only two known instances in the wild of young aged under two being adopted.”

They added: “Scientific studies have shown that the chances of adoption in the wild are higher among related animals, and that adoptions by sisters of the deceased mother are particularly successful.”

But they also said that the fact that Sangala was adopted by Kitoko “is most likely thanks to the circumstances in the zoo. In the wild, the dying mother would have distanced herself from the group and taken her little one with her.

“The available resources and group dynamic at Basel Zoo enabled Kitoko to take on her sister’s baby. As Kitoko was expecting a baby of her own, she was prepared to look after the little one.”