Dive a few hundred metres beneath the ocean's surface and the environment changes rapidly, becoming darker and colder as the pressure rises. It is amazing to think of warm-blooded mammals in such an inhospitable environment,yet this is the domain of whales and dolphins. Beaked whales are particularly at home in deep water, as Peter Tyack and his colleagues discovered when they tagged two species and monitored their dives. They found that beaked whales repeatedly make long dives as deep as 2 km in search of squid, making their average foraging dives deeper and longer than any other air-breathing animal(p. 4238).

It is no surprise then that little is known about beaked whales. `They are not deep divers, rather they are occasional surfacers,' says Tyack. He explains that the animals only surface between dives for a few seconds, making it difficult to track them. To study two beaked whale species' normal diving behaviour, Tyack and an international team of collaborators used a tagging device recently designed by Mark Johnson to measure various aspects of the animal's dive, including their depth and pitch as they glide through the depths.

The team found that both species of beaked whales have similar dive behaviour. First, a deep dive typically reaches depths of between 800 and 1000 m and lasts 45 min to 1 hr. After surfacing briefly, both species start a lengthy cycle of increasingly shallow dives before the whole process starts again. `Once they are deeper than 100 m, the rest of the dive is thought to have little impact in terms of nitrogen saturation of the blood,' explains Tyack as he describes how the lungs collapse due to water pressure, preventing gases passing between them and the blood. He suspects that these relatively small whales cannot store sufficient oxygen to fuel a dive aerobically, so they switch to anaerobic metabolism as their oxygen reserves dwindle. Tyack believes that the shallow diving between deep dives helps these whales clear the unfavourable by-products of anaerobic metabolism, such as lactic acid. He likens it to athletes who clear lactic acid from their overworked muscles by gentle exercise, to speed their recovery. Tyack also believes that these dives help beaked whales avoid predators such as killer whales and white sharks,which rarely venture deeper than 20 m.

So why do beaked whales expend so much energy plumbing the depths? Like other deep divers they dive to catch their favourite food: squid. Although the tags showed that they clearly hunt prey, they also revealed something unexpected. `After diving so far, beaked whales pass hundreds of prey before selecting one to hunt down', he says, but why and how they choose their target remains unclear.

Having detailed the enigmatic mammal's diving behaviour, Tyack is keen to point out that there is a serious reason for studying these animals beyond simple curiosity. He explains that recent mass strandings of beaked whales have been associated with military use of sonar. Autopsies showed that the carcasses had symptoms consistent with decompression sickness, challenging the view that the whales' physiology had evolved to protect against decompression. Could the whales prevent decompression through their normal diving behaviour?Tyack's whale tracking data suggests this is the case, and the team predicts that if sonar does cause decompression sickness, it is because it alters the normal dive behaviour of beaked whales in a way that leaves them vulnerable to stranding.

Tyack, P. L., Johnson, M., Aguilar Soto, N., Sturlese, A. and Madsen, P. T. (
). Extreme diving of beaked whales.
J. Exp. Biol.