Dig this – dolphins who go sponging for their food

Published on Friday 6 March 2009 19:51

IN THE year of the 200th anniversary of Charles Darwin's birth, two extraordinary examples of animal adaptation are under study in the southern oceans.

The first investigation began when a fisherman in the Shark Bay area of Western Australia spied a bottlenose dolphin with a "tumour" on its nose. This turned out to be a sponge, carried by several members of an extended family of Shark Bay dolphins.

While foraging for food, these mammals first find a basket sponge that fits comfortably over their nose and then use it to disturb the sand on the ocean floor and oust buried fish.

Once the prey is routed, the dolphin drops the sponge and gives chase. Intriguingly, "sponging", as the technique has been termed, is mainly used by female dolphins, who pass the skill from mother to daughter.

Overall, about 11 per cent of female bottlenose dolphins from Shark Bay hunt with sponges, and in all cases where the mothers are known to do so, the daughters, too, are spongers. The dolphins using sponges predominantly forage in deep water channels, where more than half of all dolphins use them.

Spongers spend more time foraging than non-spongers, and either hunt alone or accompanied only by their calves. Their calving rates are just the same as non-spongers.

Researchers led by Professor Janet Mann, of Georgetown University in the United States, speculate that the strong female bias in sponging has developed because daughters stay close to their mothers for up to eight years, learning her foraging and social behaviours, while their brothers range more widely after weaning, establishing social contacts with other young males.

Furthermore, male dolphins can probably not afford to adopt a foraging technique that is more time-consuming and a solitariness that might restrict their access to females.

The use of foraging tools is thought to indicate high intelligence and is rare among non- primates. With the exception of humans and chimps, this is the first known example of the habitual use of a tool for hunting other vertebrates. Dolphins have long been known to be intelligent and, in captivity, they can be taught to recognise themselves in mirrors, copy human actions and respond to commands.

Tool use can now be added to these skills, providing a flexibility of foraging techniques that should help their ability to adapt in areas where humans are having an impact.

The second strange story from the deep is about the four-eyed spookfish, recently hauled out of the ocean between Samoa and New Zealand. Like every other vertebrate, this fish has only two eyes, but each has two distinct parts, allowing them to see both up and down at the same time.

Spookfish (Dolichopteryx longipes) live in the twilight zone at 200m to 1,000m below the surface, where visibility is poor. Many inhabitants of this region, including spookfish, have developed tubular eyes on top of their heads.

Similar to human eyes, these use lenses to focus the sun's rays on to light-sensitive retina, and this produces silhouette images of animals swimming above. But both predators and prey also lurk in the darkness beneath, where about 80 per cent of creatures emit their own light, called bioluminescence.

Some twilight-zone residents have developed pouches extending sideways from the retina, which give an unfocused image of light-emitting animals below, but, uniquely, the spookfish has evolved a far more sophisticated structure. Its retinal pouch contains a mirror made of thousands of tiny crystals that reflect incoming light on to the retina. And, as the crystals are all angled slightly differently around the mirror, light from any downward direction produces a well-focused image.

The eye is often used by those who doubt Darwin's theory of evolution as an example of an organ so complex it could not have evolved by random selection. Nonetheless, the eye has evolved independently at least 40 times.

The wide range found in nature shows varying degrees of sophistication, from simple light-sensitive pits in limpets to compound eyes in insects and our camera-like eyes. Now the unique spookfish eye can be added to this list.

Spookfish eyes have clearly evolved by Darwinian natural selection for life in the shadowy deep.

&#149 Dorothy H Crawford is professor of medical microbiology at Edinburgh University.