Attitudes about animal numerosity have changed dramatically since the mid-twentieth century when many researchers believed only humans had enough gray matter to think quantitatively. They cited as an object lesson the 1907 case of Clever Hans, the horse that supposedly could solve arithmetic problems and would tap out his answers by hoof; as it turned out, he was responding to unconscious cues from the people around him.

Since then, researchers have approached the field with caution and rigor, seeking to identify the specific evolutionary pressures that might spur the need for numeric judgments in any given species.

Social carnivores like spotted hyenas, for example, live in fission-fusion societies, collectively defending their territories against rivals but in ever-shifting groups of widely roaming members. “You can never predict who you’ll find in which group,” said Sarah Benson-Amram, an assistant professor of zoology and physiology at the University of Wyoming. “You might be alone or in a group of ten.”

Because hyena jaws can pulverize zebra bones, encounters between competitors can prove deadly; every spotted hyena must have a good sense from moment to moment of how many there are of us versus how many of them. Dr. Benson-Amram and her colleagues tested hyena numerosity skills in the field, playing back the recorded whoops of spotted hyenas living in South Africa and Namibia to hyenas in Kenya.

The Kenyan carnivores reacted to the whoops of strangers as predicted, approaching the hidden source of the sound when they had a home team advantage, retreating when they heard a few too many distinctive voices in a row. Or sometimes, when the local hyenas were outnumbered, they’d call for backup.

“I always loved it when the hyenas would whoop and others would come running in,” Dr. Benson-Amram said. “It was very dramatic, very aggressive, and I appreciated this ability to recruit other group members to the cause.”