Why do whales beach themselves?

BY Robert Lamb / POSTED December 22, 2009
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A large crowd watches as rescue workers try to free more than 50 pilot whales stranded on Chapin Beach in Dennis, Mass., on July 29, 2002. (AP Photo/ Julia Cumes)

Last night I had the privilege to attend another Science on Tap lecture at the Georgia Aquarium, this time featuring Dr. Gregory D. Bossart, the aquarium’s chief veterinary officer. Bossart discussed a subject that he’s devoted much of his career to: the mysteries surrounding stranded marine life.

Whales and dolphins have been beaching themselves since time out of mind. As far back as 327 B.C., Aristotle contemplated the subject. In modern times, beached whales have become a regular media circus as well-meaning crowds gather to try to aid these dying titans of the deep.

As Bossart laid out, there are essentially two varieties of stranding:

Single stranding: These instances involve a single whale or dolphin washing up on the beach. In 95 percent of the cases, these are animals in their final days, dying from disease, injuries or age-related maladies. In some cases, though, the animal is a stray calf. As a dolphin calf depends on its mother for its first two years of life, it doesn’t stand a chance in the wild without her and, lost, often beaches itself. Likewise, if the mother is sick or wounded, the calf will follow its parent into shore and beach itself as well.

Mass stranding: These involve large groups of animals beaching themselves on the shore. We’re talking tens or even hundreds of whales. A 1918 mass stranding on the Chatham Islands in New Zealand involved roughly 1,000 animals. Not only more tragic for the numbers involved, these events are far less understood than those involving single animals or mother/calf pairs.

The perplexing fact about mass stranding is that they often involve otherwise healthy animals rather than sick whales washing ashore to die.

Bossart recounted one particularly heart-wrenching stranding involving 64 rough tooth dolphins. A crowd of locals tried to help and pushed all the animals back into the surf — only so they could wash up a little farther down the shore. When Bossart arrived on the scene, there was nothing to do but euthanize all but seven dolphins. Necropsies (animal autopsies) revealed that most of the animals had been in fine physical shape, and the seven survivors were eventually released back into the wild, where they appeared to thrive.

Bossart said that the predominant theory with these examples is that, out of an entire group, a single leader dolphin becomes ill or disoriented. These are, after all, very social animals. When one leader runs ashore, the others follow. If humans push them all back to sea, they then follow the same leader back to shore.

That’s not the only theory regarding mass stranding, however. In my next post, I’ll talk about some of the more troubling cases in which we may be to blame. Read Are we to blame for beached whales and dolphins?

Thoroughly fascinated? You, too, can attend the Science on Tap lecture series. All the details are here. In the weeks ahead, the Georgia Aquarium will  release details on upcoming lecture topics.

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