Who's your cousin? Amazon River dolphins, above, might be more distantly related to Ganges River dolphins than marine mammalogists thought.

A Dolphin Joins the Whales

YOKOHAMA--Scientists have long puzzled over where to place the four species of river dolphins on the family tree of marine mammals. Now a new genetic analysis suggests that the apparently similar dolphins aren't so closely related after all. At least one species is closer kin with sperm whales than with the rest of the river dolphins.

The new analysis supports a growing number of morphological and molecular studies suggesting that the family relationships among the toothed marine mammals are more complicated than researchers once thought. Beaked whales and sperm whales, for instance, both live in the open ocean, are deep divers, and feed on squid, but the fossil record doesn't provide much support for the idea that they evolved from a common ancestor. The relationships among river dolphins, named for their home rivers--the Ganges, Yangtze, Amazon, and La Platta--have also been contested.

To untangle the family tree, molecular biologist Norihiro Okada of the Tokyo Institute of Technology and colleagues gathered DNA samples from 14 cetacean species. They identified SINEs, or short interspersed elements of repetitive DNA, that are inserted randomly through the genome. From these the researchers constructed the sequence in which whale, dolphin, and porpoise diverged. The order was: sperm whales, Ganges River dolphin, and beaked whales, followed by the remaining freshwater and marine dolphins, Okada reported here at the Evolution and Adaptation of Marine Mammals conference on 12 March.

Not everyone buys it. Christian de Muizon, a paleontologist at the National Museum of Natural History in Paris, has serious doubts about Okada's interpretation of where Ganges River dolphins fall. While the fossil record suggests some traits that back up Okada's conclusion, he says, most of the evidence points to the proximity of beaked and sperm whales on the family tree. Still, the new findings are sufficiently intriguing, de Muizon says, that he'll take another look at his specimens.