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A few dolphins keep the pod together
« on: 2004-08-12 20:04:09 » |
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Well-connected dolphins keep pods together
http://www.newscientist.com/news/news.jsp?id=ns99996267
Some animals are born socialites, it seems. Just a few genial dolphins keep dolphin societies together, and if they disappear, the social cohesion of the pod collapses.
That means that capturing wild animals, taking killer whales for displays in marine parks, for instance, could have unforeseen consequences for their companions left behind.
Ecologist David Lusseau of the University of Aberdeen, UK, studied the social lives of a community of 62 bottlenose dolphins in the Doubtful Sound, New Zealand, from 1994 to 2001. He kept track of individual animals via photographs showing the markings on their dorsal fins.
Lusseau then worked out which dolphins appeared together more often than you would expect by chance. And he assumed that these individuals spent significant time together, and were linked in the dolphin social network.
The resulting network initially showed no obvious structure. To get behind the confusion, Lusseau turned to physicist Mark Newman of the University of Michigan, Ann Arbor, who has pioneered mathematical techniques for probing complex networks such as the internet or the web.
Tightly knit
"The basic idea," says Newman, "is to look for groups within the network that are tightly knit", where there are many contacts between the members of one group, but fewer between members of that group and another one. When Lusseau and Newman tried the technique on the dolphin network, they found a surprise.
"We found two sizable sub-communities joined together rather tenuously by just a few common members," Newman says. These individuals were unexceptional in appearance, yet occupied central positions within the social network, so that their disappearance might split the entire network in two.
"Remarkably," says Newman, "this is exactly what happened. Some years into the study, two of these keystone individuals did indeed disappear, and the community split into two separate groups that went their own way." When these individuals returned later, the group reformed. Their research will be published in an upcoming issue of Biology Letters.
These results suggest that animal communities could be very vulnerable to the loss of a few key individuals. As Lusseau points out, for example, those catching killer whales for public displays or marine parks usually take several young females who can reproduce and live for many years.
If these animals played a crucial role in whale society, removing them could damage the cohesiveness of the remaining group.
Mark Buchanan
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