Biology by the numbers
by Cherie Winner | © Washington State University
Robert Wielgus has studied grizzly bears in Idaho, cougars in British Columbia, and lynx in Washington. So when the Washington State University expert on large carnivores took a sabbatical two years ago, of course he went to . . . Paris.
Wielgus went to France to help restore grizzly bears to the Pyrenees Mountains. It was a project made to order for Wielgus, who began his career as a field ecologist but who now uses math to gain insight into carnivore behavior.
"I do field work here all the time," Wielgus says. "Then I go there and just immerse myself in mathematics. I've worked with them [the French] for a long period of time, and together we do amazing things."
Along with colleagues at Paris's Mathematical Eco-Evolutionary Theory Group, Wielgus helped persuade the French government to transplant five grizzlies from Slovakia to an area in the western Pyrenees where the native bears have suffered high mortality in recent years.
Grizzlies, called brown bears in Europe, still roam the mountains along the border between France and Spain, but their numbers plummeted from about 200 a century ago to five in 1995. The addition of three Slovakian bears in 1997 stabilized the grizzly population in the central Pyrenees. A separate population in the western part of the range, however, has continued to struggle.
In 2004, French authorities wanted to know whether it would be worthwhile to bring in more foreign bears. If the habitat wouldn't support them, or if human-caused mortality was just too high in that area, there was no point putting money and effort into another transplantation.
Wielgus's analysis showed that neither habitat nor humanity was the problem. The problem was not enough bears, and especially not enough lady bears.
In a small population like the one in the western Pyrenees, says Wielgus, the strongest males hoard females. One big boar might lord it over three sows, breeding with each in successive years. His territory becomes home to cubs of various ages as well as their mothers. It's a sprawling, relatively peaceful family.
But if the patriarch dies, other males try to take over his realm. Disaster ensues. A female caring for cubs won't mate, and since the new males have no interest in becoming foster dads, getting rid of the cubs becomes job one. The formal name for this behavior is sexually selected infanticide.
"They don't even eat these cubs," says Wielgus. "Just kill 'em, shake 'em, and throw 'em away. They're not doing it for the food value."
They're doing it to get a faster start on a family of their own. When a female loses her cubs, she comes into heat again quickly. That clearly benefits the new male, but the population as a whole suffers. Not only are the cubs lost, but the females often become so stressed that their productivity drops. Sow bears in the western Pyrenees average just one cub every three years. At that rate, says Wielgus, the population is doomed.
The way to recovery for these bears is to boost their overall numbers, particularly the number of females. In larger groups, with plenty of females to go around, the males become downright easygoing.
"Everyone just copulates with everyone," he says. There are too many male neighbors to defend against; and since none of the males knows which of them fathered which cubs, they leave all the youngsters alone.
Wielgus was one of the first biologists to discover sexually selected infanticide in bears. His work in the northern Rockies in the 1990s helped overturn the long-held belief that trophy hunting helps carnivore populations by removing surplus males and giving females and cubs greater access to resources. Wielgus showed that removing large males has just the opposite effect.
"Every time you kill a big male, you whack the kids of [up to] three females," he says. In fact, the sexually selected infanticide that results when large males are removed can damage populations enough to drive them to extinction.
Wielgus and his Parisian colleagues used a mathematical model called a Monte Carlo simulation to predict what would happen with the Pyrenees grizzlies over the next 30 years if different numbers of bears were added. They found that bringing in at least seven bears, six of them female, by 2007, would likely secure the group's future.
The French government's decision to bring in five female bears in 2006 is a big step toward recovery, Wielgus says. He and his colleagues will continue to press for more bears to be added, to further boost the population's chances for survival.
In the meantime, says Wielgus, the project gives students in his quantitative ecology class a whole new perspective on a subject that can seem abstract and dry compared to the big, fierce beasts they're excited about. "This is an example for my students, that this isn't just a bunch of esoteric theoretical mathematics," he says.
"Basically, this is mathematics saving species."
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