Squirrel scientists tackle the adoption conundrum
Why would an animal adopt an orphan and give it the same level of care it would give to its own offspring? What place does such apparent altruism hold in an evolutionary scheme by which the continuation of one's own genes is paramount?
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A red squirrel cares for a young pup. Red squirrels have been observed adopting orphaned pups from a relative's litter.
(photo: Ryan W. Taylor)
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Oxford theoretician William D. Hamilton became obsessed by these questions and in 1964 released his own special take on kin selection. Jamie Gorrell, a PhD candidate at University of Alberta, sums up Hamilton's thinking this way: "You only help someone if they are related enough to overcome the cost." That means, in effect, that by helping a relative you are supporting the continuation of your genes.
Thanks to what became known as Hamilton's Rule, it is mathematically possible to relate the degree of support to the cost of supporting and the degree of relationship.
Hamilton was obsessed with altruism and "baffled" that no one else seemed to think it was a pressing area for study, says Gorrell. "His great struggle was to get his work accepted." Eventually, however, the simplicity and clarity of his theory won through and it showed up in classrooms and textbooks around the world.
There were no hard numbers to plug into the theory until very recently. Researchers have sought the proof in creatures like ground squirrels whose large, complex communities add confounding social dimensions to the equation. Then Gorrell and his colleagues working with the Kluane Red Squirrel Project just west of Haines Junction made a momentous discovery among some not-so-social rodents.
"I remember that day well," says Gorrell. The scientists had climbed a tree to survey a new litter of squirrel pups in a nest. One pup was clearly larger and older than the other four. And it had been marked previously by squirrel surveyors; it had already been counted in another nest.
"Wow! What's going on?" the researchers asked themselves. "It threw us for a loop and I thought maybe we had made a mistake," says Gorrell. "Once we thought about it, we realized that the extra pup, that had already been counted, came from the nest of a neighbouring female."
"We think she might have been taken by a predator." Goshawks, lynx, coyotes and hawk owls are among possible culprits.
The scientists had been keeping tabs on the squirrels in that area for two decades, trying to understand squirrel demographics and the effects of habitat stress – such as climate change and spruce beetle infestation – on reproduction. What's learned about squirrels can be applied to other animals as well. It's also easier to study squirrels than grizzlies, for instance, for some obvious reasons, including the fact there are usually more squirrels and more generations of them, than bears, in a small area.
The scientists check on pregnant squirrels, count and mark the young... and, since the early '90s, they clip tiny tissue samples to be searched for genetic signatures back in an Edmonton laboratory. Complex family trees are established. As it turned out, the adopting mother of that "Wow!" pup was its older sister.
"In my mind, I saw this happening once and thought this was a pretty neat story," says Gorrell. It was neat enough that earlier researcher might be expected to have similar cases. Old data books were perused, dots connected, and four other definite cases of squirrel pup adoption came to light.
The big question was why? Why spend time and energy looking after someone else's kid? "It doesn't make sense in terms of natural selection, which is about passing on your genes in competition with everyone else doing the same," says Gorrell.
"There are two ways to pass on genes: You can have kids of your own or you can help your relatives have kids," he says. All five cases of adoption the scientists discovered were between relatives. "But we thought it was a fluke," he adds.
Back to the data books. There were no cases of nonrelatives being adopted. The adopting parent was always as close as a sister, grandmother or aunt.
So how do the squirrels know one orphan is a relative and another not? Squirrels chatter constantly. They rattle, says Gorrell. "What we think is happening is that the adopting female recognizes the chatter of a neighbour... one day the communication breaks down. The neighbour is silent. Another squirrel wanders over, perhaps curious about why the noisy territorial defence has ceased, and discovers orphans."
From having heard the defence rattle of the now-departed mom, the adopting mom is able to deduce the pups are related and takes one home. She feeds it and treats it just like her own – without discrimination – then sends it off with her own at summer's end to seek its fortune, its territory and nesting site. It appears she will only adopt one pup. That's all she can handle without shortchanging her own litter.
A squirrel comes by its special signature chatter genetically, says Gorrell. High-pitched parents, for instance, would have high-pitched children. Because it's in the genes, the chatter signature won't change. This is contrasted with the "prior-association" method by which birds acquire a song – that is, by overhearing it from the parents in the nest. Birds, unlike squirrels, can be retrained to a different call.
After three years of sharing the forest with his furry subjects, from the summer heat to the minus 40 of winter, Gorrell as become entranced and looks forward to more seasons among them. "We're staring to believe they're much more intelligent than we give them credit for," he says. After pondering that statement for a bit one comes to suspect that we have even more to learn from our small, noisy neighbours.
Meanwhile, it's a pity William Hamilton died a decade before the Yukon red squirrel adoptions were discovered, says Gorrell. The Oxford theoretician would have appreciated the hard, vindicating numbers.
- More information on the Kluane Red Squirrel Project is available at www.redsquirrel.ca.
