Dolphins go sponging for food

BIRDS DO IT, great apes do it and of course humans. Now a species of bottlenose dolphin has joined the select club of animals able to use a tool to achieve a goal.

A small subset of the Western Australia Indian Ocean bottlenose dolphin uses sponges to forage for food on the sea floor. It also emerges that the dolphins who pursue this activity become "workaholics", putting in long hours as they root about for food.

Details of the research were released on Tuesday evening online on the freely available PLoS One. The Public Library of Science publishes on the web leading research that is immediately available and provided free to everyone.

A team led by Prof Janet Mann of Georgetown University has studied a group of dolphins in Shark Bay for more than two decades.

"It turns out the brainiacs of the marine world can also be tool-using workaholics, spending more time hunting with tools than any non-human animal," says Prof Mann. "This is the first and only clear case of tool use in a wild dolphin or whale."

Not all of the bottlenoses use the technique. Prof Mann found just 41 dolphins in a population numbering into the thousands that used marine sponges as a foraging tool on the sea floor.

They use the sponge to clear away sand and debris to expose burrowing prey. The technique is demanding however, given those dolphins who adopt it tend to spend more time hunting and diving and tend to dive for longer periods than the bottlenoses who do not use sponges for foraging.

Those who use sponge tools in this way tend to be more solitary than their peers, the researchers noted. They also spent more time in deep-water channel habitats and less time socialising. This did not however affect their ability to produce offspring.

Most of the spongers were female and they were able to transfer the behaviour to their offspring, but what the offspring did with it largely depended on the sex involved.

Prof Mann and her co-authors found that female calves readily started sponging, but male calves seldom used sponges and if they did, they began much later. "While a few males carry sponges, they seem to be slow-learners in this regard," she says.

In part this is a reflection of the later behaviour of the dolphins, she suggests. Daughters readily adopt the social and foraging behaviours of their mothers, while sons tended to seek out other males, doing something akin to young human males - hanging out with the lads.

"We believe these early sex differences foreshadow the long-term reproductive interests of males and females, with males being focused on alliance formation, necessary for successful mating, and females focused on foraging skills, necessary to meet the demands of three to eight years of nursing each calf," Prof Mann says.

This too sounds alarmingly human.

Prof Mann's work is part of a larger project to understand social networks and factors related to female reproductive success in the Shark Bay dolphin community.

• The PLoS One link to the story is online http:// dx.plos.org/10.1371/journal.pone.0003868