Mothers in one deer species seem remarkably generous in defending other parents’ kids, a study has found—but another deer species displays much less gallantry.
The two species responded differently to fawns’ recorded distress calls, according to Susan Lingle, who conducted the research as a postdoctoral fellow at the Universities of Alberta and of Lethbridge, both in Canada.
Lingle used speakers to broadcast calls of fawns under threat, such as when they face a coyote attack, toward adult deer.
Whitetail deer mothers ran to help only in response to their own species’ call, and only when their own offspring was out of sight, she reported. But mule deer mothers answered calls of both species’ fawns, even when their own fawn stood next to them so they had no reason to believe their own was in trouble.
Mule deer fawns with their mother. (Courtesy Yellowstone Nat'l Park)
“The fact that mule deer ran to the speaker when their own fawn was standing next to them safe and sound revealed they do not help other fawns because they mistake them for their own,” she said.
“It was surprising just how indiscriminate mule deer females were. For example, the females that weren’t even mothers also ran to the speakers to help fawns. That would not be expected if females were simply trying to protect their own fawns.”
The findings appear in this month’s issue of the research journal Animal Behavior.
Mule deer came to the speaker and stayed there as long as the distress calls played, twisting and turning as they confronted perceived attackers, Lingle said. Whitetail mothers came near the speaker briefly, then tended to withdraw right away.
While the findings seem to hail mule deer as superior mothers, their motivation for protecting other fawns is likely based not on altruism but on survival, said Lingle.
“Having a rigid and aggressive response to the simple sound of a fawn distress call may ensure effective defense of a female’s own offspring, even though this means the female invests time and energy and puts herself at risk by helping many other animals. In contrast, a whitetail mother waits to assess whether a fawn is her own before she steps in to defend it. As a result, whitetail fawns suffer considerably more predation during the first months of life than do mule deer fawns.”
Mule deer may have developed a more effective aggressive defense because they rely on fighting to fend off predators year-round, Lingle added. Whitetails and many other species restrict aggressive defense to just the youngest fawns. Whitetails rely on flight rather than fight for most of their lives, so this may hamper their ability to mount an aggressive defense, Lingle said.