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Squirrels found to adopt orphans

June 1, 2010
Courtesy of the University of Guelph
and World Science staff

Those squir­rels you of­ten see fight­ing over food may not seem al­tru­is­tic, but new re­search has found that the crit­ters will some­times take in or­phaned rel­a­tives.

The study by An­drew McAdam at the Uni­vers­ity of Guelph in Can­a­da and col­leagues re­vealed that red squir­rels will adopt pups that have lost their moth­er. 

Al­though it hap­pens rare­ly, it’s sig­nif­i­cant be­cause while such adoptions are typ­i­cal among spe­cies that live in ex­tend­ed family groups, it’s much less com­mon among aso­cial an­i­mals, such as squir­rels, sci­en­tists said.

A fe­male North Ameri­can red squir­rel moves a new­born pup to a new nest. (Cour­tesy U. of Guelph)


“So­cial an­i­mals, in­clud­ing li­ons and chim­panzees, are of­ten sur­rounded by rel­a­tives, so it’s not sur­pris­ing that a fe­male would adopt an or­phaned family mem­ber be­cause they have al­ready spent a lot of time to­geth­er,” said McAdam, an ev­o­lu­tion­ary bi­ol­o­gist. 

“But red squir­rels live in com­plete isola­t­ion and are very ter­ri­to­rial. The only time they will al­low an­oth­er squir­rel on their ter­ri­to­ry is the one day a year when the fe­males are ready to mate or when they are nurs­ing their pups.”

The study is pub­lished in the June 1 ad­vance on­line is­sue of the jour­nal Na­ture Com­mu­nica­t­ions.

Over two dec­ades, the re­search team has come across five cases of adoption “out of the thou­sands of lit­ters that have been born since the proj­ect be­gan,” said McAdam.

Ja­mie Gor­rell, a doc­tor­al can­di­date at the Uni­vers­ity of Al­ber­ta, Can­a­da, and co-author of the stu­dy, iden­ti­fied 34 cases of po­ten­tial adoption over 20 years. An adoption is pos­si­ble only if the moth­er dies and a near­by squir­rel is al­so nurs­ing, the re­search­ers found. “Re­lat­ed­ness plays a crit­i­cal role in wheth­er a neigh­bour­ing squir­rel will adopt or not,” said McAdam.

In all five adoption sce­nar­i­os, the pups were nieces, nephews, sib­lings or grand­chil­dren to the adoptive moth­er.

“From an ev­o­lu­tion­ary per­spec­tive, the phe­nom­e­non of adoption raises the ques­tion of why an an­i­mal would adopt in the first place giv­en that it jeop­ar­dizes the sur­viv­al of their own off­spring,” said McAdam. “Un­der the right con­di­tions, an an­i­mal can prop­a­gate more cop­ies of its genes by help­ing rel­a­tives to raise their off­spring than by pro­duc­ing off­spring of their own. So in some cases it might be a good bet to adopt and ac­cept these costs.”

By ex­am­in­ing the breed­ing records of thou­sands of squir­rels over the past 20 years, McAdam was able to cal­cu­late the costs of adoption.

“What we found was that squir­rels will only adopt an or­phaned pup when the costs of adoption are low and when the or­phans car­ry a large per­cent­age of the same genes such as sib­lings, nieces or nephews rath­er than more dis­tant rel­a­tives.”

Re­mark­ably, squir­rels are able to as­sess which pups are re­lat­ed or not, he added. As squir­rels rarely in­ter­act, they learn who their near­by rel­a­tives are by hear­ing their un­ique calls, he said. If they fail to hear a rel­a­tive’s calls for a few days, they may in­ves­t­i­gate. “We sus­pect that, if they find pups on the ter­ri­to­ry, they remem­ber that their neigh­bour was a rel­a­tive and car­ry the pups back to their nest. This would be quite in­tel­li­gent be­hav­iour for a squir­rel.”


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Those squirrels you often see fighting over food may not seem altruistic, but new research has found that the critters will sometimes take in orphaned relatives. The study by Andrew McAdam at the University of Guelph in Canada and colleagues revealed that red squirrels will adopt pups that have lost their mother. Although it happens rarely, it’s significant because while such adoptions are typical among species that live in extended family groups, it’s much less common among asocial animals, such as squirrels, scientists said. “Social animals, including lions and chimpanzees, are often surrounded by relatives, so it’s not surprising that a female would adopt an orphaned family member because they have already spent a lot of time together,” said McAdam, an evolutionary biologist. “But red squirrels live in complete isolation and are very territorial. The only time they will allow another squirrel on their territory is the one day a year when the females are ready to mate or when they are nursing their pups.” The study is published in the June 1 advance online issue of the journal Nature Communications. Over two decades, the research team has come across five cases of adoption “out of the thousands of litters that have been born since the project began,” said McAdam. Jamie Gorrell, a doctoral candidate at the University of Alberta, Canada, and co-author of the study, identified 34 cases of potential adoption over 20 years. An adoption is possible only if the mother dies and a nearby squirrel is also nursing, the researchers found. “Relatedness plays a critical role in whether a neighbouring squirrel will adopt or not,” said McAdam. In all five adoption scenarios, the pups were nieces, nephews, siblings or grandchildren to the adoptive mother. “From an evolutionary perspective, the phenomenon of adoption raises the question of why an animal would adopt in the first place given that it jeopardizes the survival of their own offspring,” said McAdam. “Under the right conditions, an animal can propagate more copies of its genes by helping relatives to raise their offspring than by producing offspring of their own. So in some cases it might be a good bet to adopt and accept these costs.” By examining the breeding records of thousands of squirrels over the past 20 years, McAdam was able to calculate the costs of adoption. “What we found was that squirrels will only adopt an orphaned pup when the costs of adoption are low and when the orphans carry a large percentage of the same genes such as siblings, nieces or nephews rather than more distant relatives.” Remarkably, squirrels are able to assess which pups are related or not, he added. As squirrels rarely interact, they learn who their nearby relatives are by hearing their unique calls, he said. If they fail to hear a relative’s calls for a few days, they may investigate. “We suspect that, if they find pups on the territory, they remember that their neighbour was a relative and carry the pups back to their nest. This would be quite intelligent behaviour for a squirrel.”