Dolphin mothers pass tool use to daughters
Australian dolphins learn to hunt with sponges stuck to their noses.
Bottlenose dolphins are known to be smart, but a study of tool use has emphasized just how clever these mammals can be. Female dolphins in an Australian bay seem to be learning from their mothers how to stick marine sponges on their noses to help them hunt for fish, researchers say.
"It is the first documented case of tool use in a marine mammal," says Michael Krützen of the University of Zurich, Switzerland, who led the study into how the trick is passed from one generation to the next. Rather than being an inherited trait, the tool use is probably being learned by daughter dolphins from their mothers, the researchers report in the Proceedings of the National Academy of Sciences1.
Sponge-using dolphins (Tursiops truncatus) were first described in 1997 in Shark Bay, 850 kilometres north of Perth, Australia2. Since then, all dolphins known to use this tool have come from the same bay, and the vast majority have been female. Direct observations have been rare, but researchers think the dolphins use the marine sponges to disturb the sandy sea bottom in their search for prey, while protecting their beaks from abrasion.
The knack of learning to use tools from fellow creatures is thought to be very rare. Chimpanzees (Pan troglodytes) have been seen to use two stones to crack open nuts, for instance, and this is thought to be a culturally acquired trait3. In other instances tool use seems to be inherited. New Caledonian crows (Corvus moneduloides), for example, use twigs to gain access to food in nooks and crannies of trees, and can do so without having been taught by another crow4.
Daughters of Eve
To see whether the dolphin behaviour was inherited, Krützen and his colleagues analysed DNA from 13 spongers, only one of which, Antoine, was male, and from 172 non-spongers.
They found that most spongers shared similar mitochondrial DNA, which is genetic information passed down from the mother. This indicates that the spongers are probably all descended from a single "Sponging Eve". The spongers also shared similar DNA from the nucleus, suggesting that Eve lived just a few generations ago.
But not all the female dolphins with similar mitochondrial DNA use sponges. And when the researchers considered ten different means of genetic inheritance, considering that the sponging trait might be dominant, recessive, linked to the X-chromosome or not, they found no evidence that the trait was carried in DNA. "It's highly unlikely that there is one or several genes that causes the animals to use tools," says Krützen.
Andrew Whiten, a researcher who studies cultural tradition in chimpanzees at the University of St. Andrews, UK, says the work is very thorough. "Krützen and his colleagues have done a painstaking genetic analysis," says Whiten. But he cautions that there is as yet no evidence that dolphins can pick up tool use by observation.
Krützen points out that young dolphins spend up to four or five years with their mother, giving them lots of time to pick up the trick. "We know they are seeing it all the time," says Janet Mann, a co-author of the study from Georgetown University in Washington DC. In general, dolphins are known to imitate each other very well, Krützen adds.
Mann says the males probably learn sponging from their mothers as well, but do not engage in it when older, perhaps because they are too busy pursuing fertile females to engage in complicated foraging. She hopes to catch the dolphins in the act of learning sponge use from their mothers soon.
Krützen plans to study whether the sponge users have any advantage over non-spongers. A preliminary study of the fat content in dolphin blubber suggests that spongers get food that other animals do not, Krützen says.
- Krützen M., et al. Proc. Nat. Acad. Sci. USA, doi:10.1073/pnas.0500232102 (2005).
- Smolker R. A., Richards A., Connor R., Mann J. & Berggren P. Ethology, 103. 454 (1997).
- Boesch C., et al. Rev. Ecol.-Terre Vie, 32. 195 - 201 (1978).
- Kenward B., Weir A. A. S., Rutz C. & Kacelnik A. Nature, 433. 121 (2005).