Dolphins 'decompress like humans'

Stranded dolphins are common along the coast of Cape Cod in winter months

Scientists have found tiny bubbles beneath the blubber of dolphins that have beached themselves.

The bubbles were discovered by taking ultrasound scans of the animals within minutes of stranding off Cape Cod, US.

The team's findings help confirm what many researchers have long suspected: dolphins avoid the bends by taking long, shallow decompression dives after feeding at depth.

The study is reported in Proceedings of the Royal Society B.

Many biologists believe that marine mammals do not struggle, as human divers do, with decompression sickness - "the bends" - when ascending from great depths.

In humans, breathing air at the comparatively high pressures delivered by scuba equipment causes more nitrogen to be absorbed into the blood and the body's tissues, and this nitrogen comes back out as divers ascend.

If divers ascend too quickly, the dissolved nitrogen forms bubbles in the body, causing decompression sickness.

But marine mammals such as whales, dolphins, and seals are highly adept at dealing with the pressures of the deep.

They slow their hearts, collapse the tiny air-filled chambers in their lungs, and channel blood to essential organs - like the brain - to conserve oxygen, and limit the build-up of nitrogen bubbles in the blood that happens at depth.

However, veterinary scientist Michael Moore from Woods Hole Oceanographic Institute in the US, thinks that it is "naive" to think that diving mammals do not also struggle with these laws of chemistry.

Even marine mammals ascending from the deep must rid themselves of the gas that has built up in their tissues, or risk developing the bends.

If dolphins, he explained, come up too quickly then there is evidence that they "grab another gulp of air and go back down again," in much the same way a human diver would "re-tank and re-ascend" to try to prevent the bends.

"But there's one place you can't do that [if you are a dolphin] and that's sitting on the beach," Dr Moore told BBC News.

And so when he and his team scanned eight Atlantic white-sided dolphins and 14 short-beaked common stranded dolphins using ultrasound, they were not surprised to find tiny bubbles below the blubber of the animals.

Because three of the dolphins were scanned within minutes of their stranding, the team ruled out the possibility that the air pockets were a result of beaching, and instead think that they formed while the animals were still in the water.

Bends over

Sascha Hooker, a marine mammal ecologist with the Sea Mammal Research Unit in St Andrews, UK, commented: "This study is much less about why animals strand, and much more about using stranded animals to give us a bit more insight [into] what is going on inside live marine mammals.

"[What's] particularly interesting from this is that the animals that were released... survived.

"So it looks like these animals are able to deal with some bubbles."

She explained that studying the behaviour and physiology of diving animals is incredibly difficult because researchers cannot follow them down to the deep.

Stranded animals, therefore, offer researchers rare access to these expert divers to measure what changes they undergo to avoid the bends.