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Deer moms come to the rescue—sometimes

May 30, 2007
Courtesy University of Alberta
and World Science staff

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.

Mule deer fawns with their moth­er. (Cour­te­sy Yel­low­stone Nat'l Park)

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. 

“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 Be­hav­iour.

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 de­fence 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 de­fence be­cause they rely on fight­ing to fend off preda­tors year-round, Lin­gle added. White­tails and many oth­er spe­cies re­strict ag­gres­sive de­fence 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 de­fence, Lin­gle said.

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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 they 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. “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 Behaviour. 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 defence 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 defence because they rely on fighting to fend off predators year-round, Lingle added. Whitetails and many other species restrict aggressive defence 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 defence, Lingle said.