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Grouchy grouse? Aggressive males may keep grouse populations from growing.

Angry Old Males Accused of Causing Crashes

Testy males may be responsible for crashes in British red grouse populations. When male red grouse become more territorial, they throw population growth into reverse, new research suggests. But the finding is controversial because of earlier evidence that parasites cause the crashes.

The boom and bust of animal populations--from lemmings to snowshoe hares to red grouse--has long been a theoretical football for ecologists. Central to the debate is whether they are caused by "extrinsic" influences such as predators or food supply, or by "intrinsic" factors such as territoriality. Research in the late 1990s showed that reducing parasites could boost female fertility and eliminate population crashes in red grouse, an important game bird in the United Kingdom. But some researchers suspected intrinsic factors also played a role.

To see if territoriality could explain fluctuations in red grouse populations, François Mougeot of the Center for Ecology and Hydrology in Banchory, Scotland, implanted 153 old male grouse with testosterone, to mimic the surge that happens when males feel crowded. The treatments made the males more aggressive, driving away young males and reducing the density of breeding pairs by 50% compared to untreated populations nearby. Because young males fled and remaining birds presumably produced fewer offspring, treated populations declined by up to half over 1 year, whereas untreated populations continued to burgeon, the team reports in the 13 February issue of Nature.

Ecologist Andy Dobson of Princeton University in New Jersey says previous research has established that parasites are almost always responsible for population nosedives, in grouse and many other species. Testosterone is known to depress the immune system, so the new results may actually be the result of a testosterone-induced increase in parasites, he says. Such research is nevertheless important to uncover the mechanisms by which parasites cause cycles, he says.

But Mougeot says unpublished data show that the testosterone implants had no effect on parasites. He admits that parasites can crash some populations, but says intrinsic processes may be responsible elsewhere. "Controlling parasites certainly increases grouse numbers, but might not be sufficient to prevent population declines. These could still be caused by populations becoming crowded and males aggressive and intolerant," Mougeot says. Land managers might best prevent crashes both by treating parasites and removing aggressive males, he suggests.

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Mougeot's site