Mark Jennings is becoming something of a celebrity, which is a bit unusual because the Davis man does not sing, dance, tell jokes, write thrillers or appear on TV. He's a free-lance biologist and his area of specialty is frogs.

Jennings is one of the main characters in a new non-fiction book by free-lance journalist Kathryn Phillips called ``Tracking the Vanishing Frogs: An Ecological Mystery'' (St. Martin's Press, 1994, $22.95).

Phillips, who lives in Ventura, met Jennings at a 1990 conference in Irvine called to investigate mysterious declines in various frog populations worldwide.

Jennings' co-researcher, Marc Hayes, gave a talk at that conference that caught Phillips' attention. She got to know ``the two Marcks'' and began studying the herpetological literature. She also accompanied them into the field, which in the case of frog research meant hours of slogging along, through and in creeks and bogs - mostly at night.

Jennings and Hayes have spent more than a decade collaborating on research to find out why California's native frogs are disappearing. They were responsible for proposing that the California red-legged frog be listed as an endangered species by the U.S. Fish and Wildlife Service.

The agency is due to make a final determination on the frogs' status in 1995.

Jennings doesn't spend a lot of time in his Davis home - he's too busy studying frog populations in the field. We connected by phone last week while Jennings was on his annual trip to western Riverside County where he and Hayes (author Phillips calls them the ``Cagney and Lacey'' of biology) were checking a population of red-leggeds.

I asked Jennings what he thought of Phillips' book.

``I enjoyed it - it was well-written,'' he said. ``It makes easy reading for the lay public and will hold the interest of people who know nothing about frogs.''

I agree with Jennings' assessment. Phillips covers the history of frogs and their current problems with an eye toward the illuminating detail, like this passage:

``Some toad toxins are rumored to cause hallucinations. These hallucinogenic qualities have become legendary among people who find sport in taking mind-distorting substances, and have led to a still-uncommon practice called toad-licking. Toad-licking - which is exactly what the term implies - is a dangerous business. The toxins in amphibians, including toads and newts, are extremely potent. Drinking bug spray would probably be safer,'' writes Phillips.

As to the bigger picture, the decline of frogs, Phillips offers no easy answers. The northern leopard frog of the Colorado Rockies and parts of Canada is in decline; California natives like the red-legged and the mountain yellow-legged also are disappearing, the arroyo toad of Southern California is being lost along with its special sandy beach habitat, and the famous golden toads of Monteverde, Costa Rica, haven't been seen since 1989.

But not all frogs are disappearing. Pacific tree frogs are abundant in parts of California. The main reasons for the decline of other populations include human population growth and the destruction of habitat by developers, campers, off-road vehicles, logging, cattle-grazing, weekend gold miners, and an insensitive Forest Service.

Scientists aren't even sure if declining amphibians can be considered an ecological early warning signal like miners' canaries. They may represent a late warning. Or they may be reacting to a specific, undiscovered pollutant. Naturally, I asked Jennings if he'd heard about the latest Davis brouhaha concerning frogs.

``Uh-oh,'' he said cautiously.

I explained that UC Davis Professor Charles Goldman had asked the city to provide inexpensive culverts - like the toad tunnels common in England - to help our amphibians cross from the Core Area pond on Second Street to the other side of Pole Line Road.

Jennings said these ``mitigation corridors'' have been tried in other places. Sometimes they help, sometimes they don't.

``I support it if Davis wants to support its urban wildlife,'' said Jennings, as good a politician as he is a scientist.

No matter how you feel about toads and frogs - positive, negative or disinterested - I guarantee you'll enjoy ``Tracking the Vanishing Frogs.''

You may even be moved, like hobbyist Alan McReady of Sacramento, to dig a small seasonal pool in your backyard and repopulate your neighborhood with native frogs.