When lemurs grow old, their movements become slower and stiffer. They wobble on branches they could once grip with ease. Sometimes their tooth comb, a group of teeth used for grooming, falls out, which makes 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 does not want to tumble around but is content to sit together and assist in grooming. “Young ones can be too rambunctious,” Dr. Grebe said.
To her credit, Cheyenne never settles for just any geriatric lemur. A while back, the keepers tried to introduce Martine, a female collared lemur, to Chloris and Cheyenne. Chloris didn’t mind — a cordiality perhaps aided by her cataracts. “She doesn’t care what anything looks like,” Ms. Keith said. But Cheyenne showed her teeth, stared down the new lemur and eventually chased her off. Ms. Keith said that Cheyenne could be bossy but that Martine was notoriously fractious: “She was not putting out the right vibes for Cheyenne.”
Still, Cheyenne and Chloris are open to elderly singletons joining their enclosure in the D wing. Until a few months ago, the lemurs were cohabiting with Pedro, a very old mongoose lemur who loved kiwis, until he died.
Wild lemur populations are often sympatric, meaning they live in the same geographic area. But scientists have only rarely observed different species interacting with each other, according to Dr. Tecot. One 2006 study found that crowned lemurs and Sanford’s lemurs in Madagascar formed a polyspecific association, communicating and coordinating their activities over time. Pairing between lemurs of different species seems even rarer, if it happens at all. Dr. Tecot, who co-directs the Ranomafana Red-Bellied Lemur Project in Madagascar, has never seen any mixed-species pairings in the wild.
In captivity, these pairings can offer insights into how lemurs may form interspecies companionships, according to Ipek Kulahci, a postdoctoral researcher at the University of Notre Dame.
Cheyenne and Chloris, who will both turn 33 this April, do not have the energy to play anymore. But they still soak in the sun in their outdoor enclosure and stay warm in their sleeping baskets, which are padded with fleece blankets to cushion their old bones.
In recent years, Chloris has been having more forgetful episodes in which she seems unaware of where she is — her keepers call them “senior moments,” Ms. Keith said. But when Chloris returns to lucidity, in her good eye, she sees she is still with Cheyenne.