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Loner Dolphins Produce as Many Offspring

Bianca Nogrady, ABC Science Online

Photos

Dec. 10, 2008 -- Female dolphins that use marine sponges to help them forage for prey spend less time socializing with others, but still produce the same number of offspring, according to new research.

Researchers have long known of a bottlenose dolphin population in Western Australia's Shark Bay that use sponges as foraging tools, but they have been puzzled as to why only some of the dolphins use the sponges and why most are female.

In a study published on the PLoS One Web site, the team from Washington's Georgetown University reveal how the sponge-using female dolphins spend more time foraging than other dolphins and dive for longer in deep water habitats.

But despite spending less time socializing, the female dolphins that use sponges are equally successful at calving as non-sponge-using dolphins.

The absence of any clear advantage, or disadvantage, from sponge use might explain why the practice has not spread throughout the entire population, according to Rob Brooks of the University of New South Wales and editor of the research paper.

"When there's food out there and you need to compete for it, individuals may choose their foraging strategy in a way that maximizes their own individual impact," said Brooks.

The sponge-using dolphins cover their snout to help ferret out sand-burrowing prey from the sea floor.

The study's authors suggest they may have adopted the technique to, "more effectively access prey in channel [deep water] habitats compared to non-tool users, thus exploiting an otherwise unused niche."

Therefore the first dolphin to develop the sponge strategy would have had a clear advantage over other dolphins.

But as the strategy spread from mother to daughter, competition increased and the advantage shrinks to the point where the behavior reaches a kind of equilibrium that is "evolutionarily stable," Brooks said.

The other unusual feature of this behavior is that it is transmitted from mother to daughter only, and does not spread to other members of the dolphin group or male offspring.

The researchers suggest this is because the male offspring are more concerned with getting to know other male dolphins and forming alliances necessary for successful mating in the future.

Tool use among non-human animals in the wild is rare, and attracts considerable interest among animal researchers.

"I think we are always interested in questions that suggest that animals are more similar to us than we might have previously thought," said Brooks.

Related Links:

ABC Science Online

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