Like many humans, most young animals approaching adulthood tend to leave their parents and siblings and strike out on their own. They want to avoid competing with relatives. They want to avoid incest. In certain species, they want to avoid nagging.
But a new paper published in Thursday's Science shows there's at least one species that bucks this trend. Prairie dogs, especially female prairie dogs, stay home. They tend to only leave their native territories when all of their relatives are gone.
The paper's author, John Hoogland, didn't notice the pattern for decades, and that's surprising. He knows prairie dogs better than almost anyone.
"Frequently I refer to prairie dogs as my little people," Hoogland says. "They have distinct personalities, just the way people do."
Hoogland's office is at the University of Maryland's Center for Environmental Science, but this week he drove for two days to reach his field study site in a remote corner of New Mexico's Valles Caldera National Preserve.
He'll start this field season just as he has the past 30 — he'll trap each and every prairie dog in his study colony. He'll tag their ears and — just so he can recognize them from far away — he'll dye their fur with numbers and symbols.
"We've now tracked over these years well over 30,000 prairie dogs," Hoogland says. "Probably closer to 40,000."
He and his team have studied nearly every aspect of prairie dog life — alarm calls, mating, infanticide. But there was one subject Hoogland didn't much care about.
"I had very little interest in dispersal," he says.
Dispersal — when animals leave the place where they were born — is a really important topic for ecologists. Hoogland says a quick Internet search will yield thousands of books and hundreds of thousands of papers all about dispersal.
In all these studies, the names of two authors stand out.
"Robert May and William Hamilton are superstars," Hoogland says. "We're talking bout Mantle and Maris for baseball, Michael Jordan and Scotty Pippen for basketball. They had a very testable, provocative hypothesis."
The pair's hypothesis was this: If you compete with your family members, the family won't do as well and its genes are less likely to be passed on to subsequent generations. To avoid that intrafamilial competition, animals will leave home as soon as they can.
This hypothesis seems to have been borne out over and over in many species — in fig wasps and in lizards and in mice. Hoogland just assumed prairie dogs followed the same pattern. And he never would have taken a closer look if it weren't for the social dynamics of human mating pairs.
"My wife said to me — we talk about prairie dogs all the time — and she said, 'You know what? That dispersal by females isn't happening very often,' " Hoogland says. "And I remember saying to Judy, I said, 'There's nothing there. I'm not going to waste my time doing that.' ... Well, my wife can be very persistent."
And so he crunched the numbers. Hoogland's 30 years of data showed that prairie dogs weren't leaving home.
You often hear about females of various species remaining with their family groups throughout life, but even in those situations, they'll usually only stick around if there aren't too many relatives crowding up the place. Yet here, among the females of three prairie dog species, the opposite was true, Hoogland realized. The more relatives, apparently, the merrier.
Male prairie dogs do tend to leave their home burrows to find mates, the data show. But even they were slightly more likely to stay home if lots of their relatives were around.
Why do prairie dogs buck the trend? Scientists aren't entirely sure, but Ana Davidson, a prairie dog researcher and conservationist at Stony Brook University, says the benefits of prairie dog family life outweigh the costs of competition.
For example, prairie dog family members can help each other maintain their homes — the burrows they rely on for warmth and refuge from predators. They can also groom each other, removing parasites that carry disease.
"That grooming is part of the connection that the animals create with each other," Davidson says.
It's PDA — prairie dog affection. Research shows that if a new mother dies, her female relatives will nurse her babies. And if a prairie dog sees a predator approaching a close relative, it's more likely to sound the alarm than if the predator is threatening a stranger.
All of these behaviors help to support prairie dogs' communal lifestyle. But Davidson says the creation of large communal colonies is exactly what made prairie dogs a nuisance to humans and led to their trapping and poisoning. The close quarters also facilitate the spread of exotic diseases. Prairie dogs now inhabit only 5 percent of their historic range.
Davidson hopes that work like Hoogland's will help humans understand prairie dogs and better conserve them.