A whale's closest relative on land is probably a hippopotamus, scientists have recently concluded.

Such a conclusion reflects a shifting attitude. Previously, many scientists believed whales, mammals gone to sea and uniquely adapted to that foreign environment, resembled no living land mammals.Ancestors of modern whales were probably large land creatures called mesonychids that lived about 60 million years ago. From the lagoons and estuaries where they dwelt evolved intermediate aquatic animals that have since branched into the nearly 80 whale species now recognized.

The mammals most akin to whales belong to a group that stayed ashore, including hippos. Also in the group are deer, sheep, bison, pigs, camels and cattle.

"Whales, we are realizing, are mammals first, ocean dwellers second," writes James M. Darling in the current National Geographic.

Darling, executive director of the West Coast Whale Research Foundation, who earned his doctorate studying humpback whales and their unique songs, writes: "I think the humpback song, much like the antlers or horns of hoofed animals, could be a secondary sexual characteristic of males to display dominance."

He sees similarities in the mating acitvities of humpbacks and Rocky Mountain bighorn sheep. Biologists describe behavior between whale mothers and calves as comparable to moose, musk-ox and caribou.

Another scientist sees parallels between bottlenose dolphin and lion societies. And another suggests the sperm whales' mating system is much like that of African elephants.

All these conclusions are evidence of scientists' most important finding about whales over the past two decades: No longer must we kill whales to study them.

Still, whales continue to die at the hands of humans both for commercial and scientific reasons. The numbers are declining. Fifteen years ago, 45,673 whales were killed. The number fell to 6,623 by 1985. In 1986 the International Whaling Commission declared a five-year moratorium on commercial whaling.

Subsistence hunting is still done by Alaska natives and by aborigines of Canada, Greenland and Siberia. Last year, Norway made about 300 commercial kills of minke whales; and, under the guise of scientific research, Japan targeted 300 minkes, Iceland 80 fin whales and 20 sei whales. In retaliation, the United States has barred Japan from catching fish in U.S. waters.

Meanwhile, Darling writes, research at Cambridge University suggests molecular analysis of small skin samples might provide vital information on whale populations, "a technique that, if proven, could put the secientific whalers out of business for good."

Living whales represent a sort of last frontier for researchers. Although biologists have successfully radio-collared and tracked most endangered land animals, that hasn't worked with whales. They can't be tranquilized, and they lack necks and convenient appendages to hold transmitters. The sea plays havoc with electronic gear.

Bruce Mate of Oregon State University thinks he's on the brink of a solution: landing a small helicopter gently on a whales's back, then releasing a coffee-cup-size transmitter that would implant itself with stainless-steel sutures, much like a barnacle.

Mate, who has had some success implanting transmitters on stranded or otherwise accessible whales, predicts, "In a few years, several species will be satellite-tagged. Field researchers with laptop computers will be able to pick up a pay phone and get the animals' locations and dive data for the previous week."

Even without radio telemetry, scientists continually broaden their knowledge of whales. In a 20-year study of about 330 killer whales in British Columbia waters, for example, a Canadian mammalogist has learned that killer-whale society is the most stable known of any social mammal.

Although seemingly imperturbable, whales suffer all manner of torments from such pests as birds pecking parasites from their backs, or barnacles and other hitchhikers that can festoon a whale like a moving island. A more serious threat may be posed by myriad species of internal parasites.