Why do dolphins beach en masse?

Twenty-six dolphins have been found dead, having stranded themselves in shallow waters on the Cornish coast. Why do they beach themselves together?

A pod of 15 dolphins first became beached early on Monday morning, having swum up the river creek near Falmouth. Within a few hours, more than 70 of the animals were spotted in the same river as well as others nearby.

While rescuers are trying to send the surviving creatures back into deeper waters, post-mortems on seven of the dead dolphins have failed to reveal why they died. Further tests on the remaining 19 are due in coming days.

Meanwhile, several thousand miles away 100 whales have become trapped in a bay off Madagascar, with nearly 30 having died.

THE ANSWER

• Much less common than whales beaching
• Several theories abound
• Dolphins were maybe following a stranded member of the pod; chasing a prey; or being chased
• Or perhaps they were disoriented by sonar

Sarah Dolman of the Whale and Dolphin Conservation Society says there are important differences, not only between dolphins and whales, but between different types of whale. They roughly divide thus:

• Toothed whales - a group which includes smaller whale species and dolphins. "They are those that echo-locate and feed on fish," says Ms Dolman.
• Bigger whales - a group which "feed on krill and have a very different social structure".

"Dolphin species are highly social," says Ms Dolman - the point being that they operate a flat hierarchy and don't shoal around a natural leader. Whales, however, have "matriarchal society".

This matters because it's much more common for larger whale species (such as those in Madagascar) to become beached together. Partly this is due to their matriarchal nature, so that when the leader becomes ill and heads towards shore, the others are likely to follow them, says Ms Dolman.

Increasingly though, pods of whales have been found stranded together not as a result of ill health but due to sonars - underwater radar. This is likely to have been the problem in Madagascar, where the beached whales were of the melon head species, which is particularly susceptible to noise, she says.

WHO, WHAT, WHY?
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Dr Andrew Cunningham of the Institute of Zoology says that it's been proven recently that a number of the beachings of deeper diving whales have been linked to sonars. The animals become disorientated, and surface too quickly, developing a decompression sickness that prevents them from diving deeper again.

The case of the Cornwall dolphins is more perplexing, however.

"There's a lot of interest in this latest case in Cornwall because there hasn't been a mass stranding of dolphins since the use of sonar," he says. The last major stranding (of pilot whales which are in the same toothed whale group as dolphins) in the UK was in 1981, although there have been others elsewhere. So could it be a rare example of dolphins being disoriented by sonar?

"When it comes to dolphins, it's still not really understood."

There are several other theories about what happened in Cornwall, explains Dr Cunningham:

• one of the pod got into trouble and was followed by the others
• that they were chasing a shoal of fish and then couldn't get back into deeper water
• or perhaps that they were being chased themselves

As dolphins are known for being a very social species, it's often thought that a distress signal sent out by one, brings others nearby flocking to help. "They're like humans, in that if your house and family were under threat, you'd go and help them, rather than run the other way," says Ms Dolman.

So far, in the Cornwall case, the animals appear to have been healthy, which would imply that causes were human-related, but it will be days before all the post-mortems are completed.

RELATED INTERNET LINKS
Whale and Dolphin Conservation Society
Institute of Zoology
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