A sharp cry pierces the air. Soon a worried mother deer approaches the source of the sound, expecting to find her fawn. But the sound is coming from a speaker system, and the call isn't that of a baby deer at all. It's an infant fur seal's.

Because deer and seals do not live in the same habitats, mother deer should not know how baby seal screams sound, reasoned biologists Susan Lingle of the University of Winnipeg and Tobias Riede of Midwestern University, who were running the acoustic experiment. So why did a mother deer react with concern?

Over two summers, the researchers treated herds of mule deer and white-tailed deer on a Canadian farm to modified recording of the cries of a wide variety of infant mammals—elands, marmots, bats, fur seals, sea lions, domestic cats, dogs and humans. By observing how mother deer responded, Lingle and Riede discovered that as long as the fundamental frequency was similar to that of their own infants' calls, those mothers approached the speaker as if they were looking for their offspring. Such a reaction suggests deep commonalities among the cries of most young mammals. (The mother deer did not show concern for white noise, birdcalls or coyote barks.) Lingle and Riede published their findings in October in the American Naturalist.

Researchers had previously proposed that sounds made by different animals during similar experiences—when they were in pain, for example—would share acoustic traits. “As humans, we often ‘feel’ for the cry of young animals,” Lingle says. That empathy may arise because emotions are expressed in vocally similar ways among mammals.

Psychologist David Reby of the University of Sussex in England, who studies the evolution of communication, is not surprised by these findings. From an infant's perspective, it is advantageous to attract any potential caregiver that could increase its chances of survival. And for parents, Reby says, “it is probably more advantageous to respond to anything that vaguely resembles a baby distress call.” If a predator is involved, a parent cannot waste time deciding whether the baby in need of help is its own. The costs of ignoring the cry are too high.

These results might also explain some instances of cross-species adoption in the wild. If a mother has recently lost her own infant and still has maternal hormones circulating, Lingle says, she may be primed to care for a ward when she hears its call—no matter what it looks like.