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.

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

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 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.