When you hear that raspy, female voice coming down the hall, you know that your chain-smoking boss Susan is about to show up in your office. This ability to match a voice with a concrete identity is a complex mental task, thought to be the domain of only a few animals, including humans. But new research in horses suggests that they too evolved the skill.
Most creatures have ways to recognize members of their own species. Hamsters sniff, birds listen for a song, and wasps look at facial markings. But researchers have struggled to nail down whether the animals are simply recognizing something familiar or whether they are connecting the stimulus to a specific individual.
Behavioral biologists Leanne Proops, Karen McComb, and colleagues at the University of Sussex in the U.K. adapted a technique normally used to study human infants, who cannot verbalize their thoughts. In a common experiment, babies who stare longer at certain objects are assumed to be perplexed by them. Applying this idea to domestic horses, the researchers made a horse watch as a member of its herd was paraded in front of it and subsequently hidden behind a barrier. Proops then played a prerecorded horse whinny over a loudspeaker and watched the observing horse's response. When the call came from a different horse from the herd than the hidden one, the viewing horse seemed confused, looking up at the barrier faster and for 20% longer than when the whinny belonged to the paraded horse, the team reports online this week in the Proceedings of the National Academy of Sciences. The horses are not just reacting to the sound of the whinny, says Proops, who notes that the viewing horse could identify six different horses from its herd: "They understand that every voice is paired to a face."
"Horses live in large herds, so this study suggests that they can differentiate between many, many different individuals," says entomologist Elizabeth Tibbetts, who studies social behavior in wasps at the University of Michigan, Ann Arbor. Horses need to recognize individuals to establish and maintain their social hierarchies, she says. McComb suspects that a wide variety of other animals also share the ability. It may be particularly important for animals that live in groups unrelated by blood and that need to understand each other's behavior to get along, she says.