Post by grrraaahhh on Jun 10, 2011 9:33:43 GMT -9
BBC article extract:
Historically, they say, brown bears were persecuted in Europe for centuries before their gradual elimination from much of western Europe. For example, they write, brown bears disappeared in Denmark 3500 years ago, in Britain during the Middle Ages, and in the German lowlands by 1600.
In contrast, bears have remained over vast areas in North America. Where they were persecuted, the end came quickly; south of Canada brown bears rapidly collapsed between 1850-1920 with most remnant populations disappearing between 1920-1970 as modern weapons came into use.
Bears have recovered across both continents to different degrees. But the nature of their bounce back is also different.
European bears live in areas of much higher human density, yet they have an annual population growth rate of around 15%.
American bears, which have more space, have growth rates between 4-8%.
The question is why, and Zedrosser’s team have found the answer.
Their research reveals that European bears now have litters 2.8 years apart, and first give birth at 5.3 years, on average. American bears take 3.6 years between litters and wait till they are almost 7 years old before reproducing.
So European and American bears are evolving different life histories, with European bears giving birth more often and at a younger age.
The evidence suggests the trigger for this change was the different “persecution histories” of the two populations.
Because European bears were hunted at relatively high levels over centuries, the hunting created a selection pressure on the bears, which responded by investing more in reproduction relative to body mass. In short, hunting selected for bears that gave birth early and more often.
In northern North America, brown bears were relatively untouched. Further south, the population crashes were so quick as to have had little effect yet on the bears’ life history.
This finding is important.
While it may not be widely known, scientists are already aware that hunting can change the morphology, or body size or shape of species.
www.bbc.co.uk/blogs/wondermonkey/2011/06/ghosts-of-persecution-past-and.shtml