US: Why dolphins carry sponges

Researchers from the University of Georgetown in Washington DC became the first to study the relationship between tool use and fitness in wild animals after investigating a subset of Western Australia's bottlenose dolphin population and their use of marine sponges as foraging tools. Only some dolphins held a sponge on the end of their beaks to dislodge fish hiding in sand on the sea floor.

While rare in wild animals, tool use is of widespread interest to researchers because of its relationship to animal cognition, social learning and culture. Measuring the costs and benefits of tool use has been difficult, largely because if tool use occurs, all population members typically exhibit the behaviour.

"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 lead researcher Janet Mann, a Georgetown professor of biology and psychology who has been studying the Shark Bay dolphins off the WA coast for more than 21 years.

"This is the first and only clear case of tool-use in a wild dolphin or whale," Mann says. She started systematic data collection on the sponging behaviour in the dolphins in 1989 and found 41 dolphins in a population of thousands used marine sponges on their beaks as a foraging tool.

The dolphins use sponges to find hidden prey in the sandy sea floor and spend more time using their sponge-tool than any non-human tool user documented to date. "Spongers" are also 'workaholics' spending more time hunting, diving, and diving for longer time periods, than other females in the population. They also tend to be solitary.

Yet the researchers found that even with these potential immediate costs, such as less time socialising, calving success of sponger females was not significantly different from non-spongers.

Mann and her co-authors also report a clear female-bias in the development of sponging. Almost all the spongers are females and they transmit this behaviour to their offspring. While a few males carry sponges, they seem to be slow learners in this regard, Mann says.

The researchers note that all female calves started sponging before they were weaned, whereas male calves rarely used sponges and, if they did, it was after weaning. They suggest that while daughters show a strong tendency to adopt the social and foraging behaviours of their mothers, sons appear to be less interested in maternal behavior and are more concerned with finding other male associates.

* Findings of the study were published in the 10 December edition of the journal PLoS ONE