Male Dolphins Leave Sponging to the Females

Dec. 10, 2008— -- Sexual stereotypes are not the preserve of humans. Male dolphins, it seems, are not interested in learning how to use a sponge, but their sisters are.

Dolphins were first seen carrying sponges cupped over their beaks in Shark Bay, Australia, in the 1980s.

Janet Mann of Georgetown University in Washington, DC, and colleagues have now reviewed data collected during 20 years spent monitoring this group of dolphins and found that, while mothers show both their male and female calves how to use sponges, female calves are almost exclusively the only ones to apply this knowledge.

"The daughters seem really keen to do it," says Mann. "They try and try, whereas the sons don't seem to think it's a big deal and hang out at the surface waiting for their mothers to come back up."

Solitary spongersSponger dolphins shuffle their beak around in the sand, apparently using the sponge as protection. When they ferreted out a hidden fish, the dolphins drop the sponge and catch the prey.

The researchers even dived down with sponge-capped hands and tried the technique themselves - successfully ferreting out spotted grubfish.

Out of the 19 dolphins that were born to sponging mothers during the study period, and whose sex was known, 11 were female and eight were male. Ten of the 11 females, but just two of the eight males became spongers themselves, generally within two to three years.

The study also reveals that sponger dolphins tend to be loners - spending more than 80% of the time on their own or with one calf. And despite their ability to use sponges as a hunting "accessory", they do not seem to have a competitive advantage over dolphins that don't: females in both groups produce roughly the same number of calves.

Secret larderThe difference between the two seems to be where they choose to forage. Spongers hunt only in underwater channels with sandy bottoms. Their foraging grounds are a few metres deeper than the sand flats where other dolphins hunt, meaning they spend more energy on feeding. "But ultimately they're doing as well as others," says Mann.

Mann believes they may be exploiting an otherwise unused larder, as the density of females in channels is lower than out on the sand flats.

She says her study is the first to look at the advantages that using tools brings to non-human animals. Chimpanzees have been seen putting sticks in anthills to collect ants and using spears to hunt bushbabies. But whether or not tool use gives them a survival advantage has not been measured.

"Sponging appears to be a costly behavior, so the fact that spongers do not do worse in terms of reproductive success is quite interesting," says Michael Krützen an expert in cetacean behaviour at the University of Zurich, Switzerland. "It seems they can exploit an ecological niche that offsets their higher costs."