Some signs of female camaraderie are easy to spot. Lionesses suckle each other’s cubs. Female spotted hyenas greet each other through elaborate ceremonies of mutual trust, lifting a leg and exposing their famously penislike genitals to their snuffling sisters and their bone-crushing jaws.

Elephants touch trunks, share food, play lifeguard for the day. Dr. Soltis cited the time a female elephant rescue the wayward baby of her closest friend after it stumbled headlong into the elephant submersion pool, by hauling the panicked calf out with her trunk. Hey Hortense where RU? Got Dumbo. Bring towel.

Sometimes displays of female friendship become heated, hyperbolic, a monkey chant for the home team. Marina Cords of Columbia University has spent more than 30 years studying the blue monkeys of Kenya, 10-pound primates that, their name notwithstanding, are really charcoal gray.

She has seen many violent territorial disputes between neighboring monkey groups, in which the adult females line up to fight in the treetops, the adult males mostly hang back to watch, and the young monkeys scamper obliviously below. The females scream, lunge, bite, rip the flesh of an enemy’s calf down to a bloody frill round the ankle. And when the battle ends, the salon sessions begin.

“There’s a frenzy of grooming among the females in the same group,” Dr. Cords said. “You see them huddling together in clusters, with individuals scooting from one huddle to another, as though everybody is trying to groom as many individuals as possible.” They comb and pluck with their fingers, soothe scabs and wounds with their lips.

Through grooming, the monkeys decompress, and remind one another that their fates are still linked. After all, should a group of blue monkeys grow too large it will split into factions, and the sisterly comrades of today may be flaying you a new pair of anklets tomorrow. Shall we groom?

In other cases, affiliative behaviors are subtle and difficult to track. For years female chimpanzees were viewed as asocial, content to forage alone or with dependent offspring while largely ignoring other females of their group. The males may be legendary kin-based allies, born and reared together and wedded to their natal turf. But as the so-called dispersing sex, female chimpanzees must leave their birthplace at puberty and seek asylum in another group, which means being surrounded by unrelated females all competing for the same goods. What’s to like about that?