If you've ever found yourself moved by the sound of a mewling kitten, or a whimpering pup, you know that our species can and does respond to the cries of other animals – but newly published findings suggest this quality is not unique to humans.
Above: The author's dog, doing her best impression of a helpless, sniveling baby seal | Photo Credit: R.T.Gonzalez
In the latest issue of The American Naturalist, biologists Susan Lingle, of the University of Winnipeg, and Tobias Riede, of Glendale Arizona's Midwestern University, report that mother deer will hurry in the direction of distress calls uttered by other infant mammals, including fur seals, humans, domestic cats, marmots, and, with a little tweaking, even bats. Via New Scientist:
[Lingle and Riede] recorded the calls made by infants from a variety of mammal species when separated from their mother or otherwise threatened. They then played the recordings through hidden speakers to wild mule deer ( Odocoileus hemionus) out on the Canadian prairies. They found that deer mothers quickly moved towards the recordings of infant deer, but also towards those of infant fur seals, dogs, cats and humans, all of which call at roughly the same pitch. Even the ultrasonic calls of infant bats attracted the deer mothers if Lingle used software to lower their pitch to match that of deer calls. In contrast, they found the deer did not respond to non-infant calls such as birdsong or the bark of a coyote.
"Our results suggest that acoustic traits of infant distress vocalizations that are essential for a response by caregivers, and a caregiver's sensitivity to these acoustic traits, may be shared across diverse mammals," write the researchers. In other words, the qualities that make a distress call a distress call may be common across a range of mammals (even ones separated by tens of millions of years of evolution). The instinct to respond to such calls may also be deeply rooted in our evolutionary history.
Jaak Panksepp, a neuroscientist at Washington State University, tells New Scientist that the deer in this study "can perceive the emotional content of another animal's separation call" (emphasis added), but that's almost certainly stretching this study's findings past their empirical limit. Inter-species infant vocalizations may stir feelings of love, affection, or compassion in you and me, but animal emotions are notoriously difficult to nail down. A more measured hypothesis would be that mammals evolved their sensitivity to infant vocalizations early on, as a means of ensuring the survival of their offspring. After all, whether it possesses the capacity for emotions or not, the continued existence of one's genes is central to the raison d'être of every organism on Earth.
The hypothesis that distress calls evolved to improve fitness raises an interesting question: How might this study pan out, if it were reproduced in animals known to prey on the offspring of other mammals? Sure, an infant's cry can attract the attention of a caring parent – but it can also act as a beacon for predators.