Cultural life of whales may cut diversity.

Cultural heritage--traditions that affect behavior--may be a powerful force in whale evolution, much as it has been in human history, a biologist suggests.

Proposing culture to explain genetic diversity in species other than humans is perhaps unprecedented, notes Hal Whitehead of Dalhousie University in Halifax, Nova Scotia. Yet in the Nov. 27 Science, he suggests that whale traditions could explain a puzzle: why in some of their DNA, four whale species have only a tenth of the diversity that other whales and dolphins have.

"It's like the idea of guns," Whitehead says. Human cultural groups that made guns, rode horses, or sailed across oceans overran or outcompeted other people. As victors spread and losers perished, the original human diversity dwindled.

A similar force could steer whale evolution, Whitehead suggests. The species with skimpy diversity--killer, sperm, and the two pilot whale species--have unusual opportunities for developing powerful cultural influences. Whales are smart, live for decades, and pick up behaviors, such as songs, from family members. Unlike most other whales and dolphins, female relatives of these four species stay together for life in what is known as a matrilineal social structure. Males are far less social, although some pilot and killer whale males, too, stay with their parents.

"In one group, the females get a bright idea," Whitehead speculates. The change could improve anything from defense to baby-sitting. Whales with the improved technique would raise more young, who pick up the new trick. "Gradually, the ones with the bright idea take over the world, so to speak," Whitehead says.

In the Science paper, he reviews reports of 19 whale and dolphin species' diversity in the DNA passed from mother to offspring in cell structures called mitochondria. Other explanations for the low diversity among matrilineal whales have not satisfied Whitehead.

The most common scenario, the genetic bottleneck, postulates that these whales were once on the brink of extinction, leaving a small DNA pool for the renaissance of the species. Such models require population drops to about 1,000 whales for 1,000 generations, yet Whitehead sees no evidence for such disasters.

Other researchers have mused over whether low diversity could come from mass strandings on beaches, in which family groups vanish together. Some whales do this but not all the matrilineal ones, Whitehead points out.

Culture as an explanation for limited DNA variation seems a plausible hypothesis to Scott Baker of the University of Auckland in New Zealand, who has studied the genetics of humpback whales. He agrees with Whitehead that recent population declines for the matrilineal animals have not constituted a worldwide genetic bottleneck. However, Baker wonders whether ancient brushes with extinction might still be showing effects.

Marine ecologist Jim Estes of the University of California, Santa Cruz notes that learning from Mom can have profound effects that don't necessarily show up in mitochondrial DNA. The sea otters he has studied seem to pick up their mothers' specialty in food gathering, such as tackling sea urchins instead of abalones.

A specialist in cultural evolution, Peter J. Richerson of the University of California, Davis points out that "in recent years, evidence that social learning is important in nonhuman animals has been growing."