Sponging dolphins keep it in the family

In Shark Bay, off the Western coast of Australia, a unique population of bottlenose dolphins have a unusual trick up their flippers. Some of the females have learned to use sponges in their search for food, holding them on the ends of their snouts as they rummage through the ocean floor.

To Janet Mann at Georgetown University, the sponging dolphins provided an excellent opportunity to study how wild animals use tools. Sponging is a very special case of tool use - it is unique to Shark Bay's dolphins and even there, only about one in nine individuals do it. The vast majority of them are female. A genetic analysis revealed that the technique passes down almost exclusively from mother to daughter, and was invented relatively recently by a single female dolphin, playfully named "Sponging Eve".

Dolphins tend to sponge only in deep water, which is why little has been done to study this behaviour since its discovery a decade ago. Now, Mann has published the first detailed analysis of dolphin sponging. She watched every dolphin who knows the technique and analysed how much time they spent on it and what it meant for their success at raising calves. This incredibly thorough analysis revealed that sponging dolphins are the most intense tool-users of any animal, except for humans.

Mann confirmed that the dolphins were using the sponges to root out potential meals. On days when the sea was exceptionally clear, she could see the animals swimming slowly along the sandy bottom while wearing their spongy muzzles and disturbing the sand. When they spotted something, they dropped the sponge and probed about with their beaks, often surfacing with small fish that they quickly swallowed. Meal in throat, they retrieved their unusual hunting aid and started again.

The technique worked for humans too. Mann's team tried it themselves for four hours with sponges over their hands, and consistently ferreted out the same species of fish - the spothead grubfish. Before the sponges were used, the fish were completely invisible to the divers but once revealed, they were easily spotted, tracked and found again when they reburied themselves. A single photo of a sponging dolphin with a fish in her mouth, while blurry, suggests that they too could be after grubfish.

Mann thinks that sponging has allowed dolphins to effectively hunt for elusive prey in a difficult environment - deep water channels. There, female dolphins are relatively rare and about half of those who are present are spongers, suggesting that it's not the best habitat for a dolphin unless it devises some cunning tricks.

The sponges are clearly important to dolphins, for they will carry them around for later use. Indeed, Mann found that these specialists did little else in the way of hunting, using their sponges for 96% of their foraging time. The quest for food also consumed almost half of their time, while most other dolphins typically spend about a third of their time foraging.

Even if you just consider the time that they spent actively using sponges, Mann estimates that the spongers spend 17% of their waking hours on this one activity, which makes them the most intense tool-user of any animal, save us humans. The runner-up - the Galapagos woodpecker finch - spends just 10% of its active life using tools and chimps and orang-utans spend a measly 3% of their time. Mann suggests that these other species have more diverse diets that they can acquire through a range of means, while the sponging dolphins have learned to become specialist grubfish-hunters.

Spongers are also more solitary than the average bottlenose, spending more time alone and having fewer associates than other adult females. But this extra investment in foraging didn't cost them anything in the long run though, for their success at raising calves was the equal of their peers.

Female dolphins were far more likely to pick up sponging than males and they did so at a much younger age, even before they had been weaned from their mothers. Mann used her knowledge of the dolphins' family trees to show that the behaviour is mainly passed down from mother to daughter. In over 2,000 hours of observation, Mann never saw the child of a non-sponger use the technique. In contrast, at least half of the children of sponger females learned how to do it.

Mann thinks that this bias exists because females are more likely to stick within the same areas as they grow up, and benefit from picking up the tactics of their mothers. Male dolphins roam more widely as they mature and need to focus on setting up long-term alliances - they cannot afford to learn a specialised procedure that could limit both their range and their social time. There's actually a similar pattern in chimps, where females are more likely to learn foraging techniques such as "fishing" for termites using sticks.

But why then is the technique limited to family lines? Why don't other females learn it from their peers? Mann thinks that it's because daughter dolphins have a stronger tendency to copy their mothers than other dolphins do. And since the spongers tend to be more solitary anyway, their children may be the only ones to see enough of the behaviour to be able to copy it successfully.

The statistics back that up - in 83% of the sponger sightings, the animal was either alone or accompanied by a daughter or mother, while in just 6% were they accompanied by a non-sponger. Sponging, then, is an all-or-nothing behaviour - learning it is too complicated and restrictive for any individual that lacks the time and impetus to do so.

There are probably many other undiscovered dolphin behaviours that fit the same pattern. These animals are highly intelligent, adaptable, and can survive in a broad range of ocean environments. They also have a prolonged infancy when they spend a lot of time with their mother and are extensively exposed to her own peculiar quirks. Together, these traits conspire to shape an animal that is very likely to evolve a myriad of different cultural traditions.

Reference: Janet Mann, Brooke L. Sargeant, Jana J. Watson-Capps, Quincy A. Gibson, Michael R. Heithaus, Richard C. Connor, Eric Patterson (2008). Why Do Dolphins Carry Sponges? PLoS ONE, 3 (12) DOI: 10.1371/journal.pone.0003868

More on dolphins and tool use:

• Boto dolphins woo females with chat-up vines
• Chimpanzees make spears to hunt bushbabies
• The chimpanzee Stone Age

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