As lemurs get old, their movements become slower and stiffer. They wobble on branches that they once could easily grip. Sometimes their tooth comb, a group of teeth used for grooming, falls out, making it harder for them to keep their fur fluffy on their own. So the best companion for a geriatric lemur is another geriatric lemur, someone who doesn’t want to rush around but is content to sit together and help with care. “Youngsters can be too wild,” said Dr. Grebe.

To her credit, Cheyenne is never satisfied with a geriatric maki. Some time ago the zookeepers tried to introduce Martine, a lemur with a collar, to Chloris and Cheyenne. Chloris didn’t mind – a cordiality that might have been helped by her cataracts. “She doesn’t care what something looks like,” said Ms. Keith. But Cheyenne showed her teeth, stared at the new maki, and finally chased her away. Ms. Keith said that Cheyenne might be bossy, but that Martine was notoriously fragile: “She didn’t bring out the right mood for Cheyenne.”

Even so, Cheyenne and Chloris are open to older singletons who join their enclosure in the D-wing. Until a few months ago the lemurs lived with Pedro, a very old mongoose lemur who loved kiwis until he died.

Wild tarsier populations are often personable, which means they live in the same geographic area. According to Dr. However, Tecot scientists have rarely observed how different species interact with one another. A 2006 study found that crowned lemurs and Sanford’s lemurs in Madagascar formed a polyspecific association that communicated and coordinated their activities over time. Mating between lemurs of different species seems to be even rarer, if at all. Dr. Tecot, co-leader of the Ranomafana Red-Bellied Lemur project in Madagascar, has never seen mating of mixed species in the wild.

In captivity, these pairings can provide insights into how lemurs can form interspecies camaraderie, according to Ipek Kulahci, a postdoctoral fellow at Notre Dame University.

Cheyenne and Chloris, who will both turn 33 this April, no longer have the energy to play. But they still enjoy the sun outdoors and stay warm in their sleeping baskets, which are padded with fleece blankets to cushion their old bones.

In recent years, Chloris has had more forgetful episodes where she doesn’t seem to know where she is – her keepers call them “older moments,” Ms. Keith said. But when Chloris returns to clarity, she sees in her good eye that she is still with Cheyenne.