It’s not so surprising when a mother responds to the distress calls of her own babies, or that she’ll take note of the cries of other members of her species. Or even that a human mom would perk up at a crying kitten (who could resist?). But why would a deer notice a human baby crying?
Mule deer and white-tailed deer will respond to the distress calls of babies outside their own species, including humans, as long as those cries fall within the same frequency range as young deer, Susan Lingle of the University of Winnipeg in Canada and Tobias Riede of Midwestern University in Glendale, Ariz., report September 5 in the American Naturalist. Such sensitivity may be shared across mammals, they say.
Among mammals, newborn distress vocalizations often share traits such as a rich harmonic structure and a simple pattern of frequency variation, differing instead in fundamental frequency, duration and other features. And frequency is important, at least for mule deer and white-tailed deer. Moms of these species will only respond to the distress calls made by fawns that aren’t their own if the frequency falls within the range for its own species. Lingle and Riede wanted to see if the deer would respond similarly to babies from other species.
They started with mule deer, which are easier to work with. A researcher would crawl, hiding in the terrain and vegetation, to place a speaker about 100 to 200 meters upwind of a deer mom. Then they would retreat to some 25 to 50 meters away. Then they played the distress calls of young yellow-bellied marmots, sea lions, fur seals, domestic cats and dogs, silver-haired bats, several ungulates (including other deer species) and humans. One or two more researchers sat 500 to 1,000 meters distant, armed with binoculars and spotting scopes, recording what the deer did. A positive response would be for a deer to approach the speaker — similar to how they would react to hearing the cry of their own young.
When the frequency of the call fell outside the range of the mule deer fawns, the scientists played both the normal call and one manipulated to be within the correct range. The deer also heard control sounds, such as a meadowlark song, and sounds associated with predators. And if the mule deer responded by approaching the speaker, the researchers repeated that part of the experiment with white-tailed deer moms.
Female mule deer came within 10 meters of the speaker when they heard the calls of young fur seals, marmots, cats and ungulates that fell within the deer young’s frequency range, and within 25 meters for calls of cats, sea lions, bats and humans. Similarly, white-tailed deer also approached the speaker when distress calls in the frequency range of deer newborns were played. The deer mothers didn’t approach when control or predator sounds were played, or in response to distress calls of young outside the deer young’s frequency range, which included marmots and eland.
“These findings suggest that acoustic traits of infant distress vocalizations that are essential for a response by caregivers, and a caregiver’s sensitivity to these traits, is shared across diverse mammals,” the researchers write. The results also might explain why frequency of infant distress calls varies so much among mammals — it’s another way for species tell each other apart.