For all that the toads that I wrote about in this column a few weeks back have thick warty skins, amphibians in general are thin-skinned and very sensitive. That sensitivity is proving their undoing, and we should be paying much more attention to their demise than we are.

What exactly is an amphibian, and why should we take more notice of them?

Amphibians are cold-blooded vertebrates, most similar to fish and reptiles. Their claim to fame is their ability to exploit both aquatic and terrestrial habitats, and their name is derived from the Greek word amphibios, which means "living a double life." Three orders of amphibians survive today. These are the frogs and toads, collectively known as the Anura; the salamanders and newts, known as the Caudata; and the caecilians or Apoda.

Whereas the frogs and toads are tailless, squat and endowed with enormous hind legs suitable for jumping, the salamanders and newts have tails that are large in proportion to their body size, and their front and hind legs are almost the same. The Caecilians, strangest of the three orders, are limbless, as their scientific name "Apoda" tells us, and spend most of their lives burrowing.

Although enormously different to look at, the three groups do, of course, share major characteristics. Their skin is glandular, and unlike the skin of birds or mammals, has no hair or feathers; it typically has mucus-secreting glands that keep it moist, facilitating its vital role in respiration. All amphibians have two lungs and a three-chambered heart, and all produce gelatinous eggs without shells, which develop outside the body. For the most part, they also share a distinctly divided life history, beginning as aquatic larvae which ultimately metamorphose into the adult forms.

As with any definition, there are exceptions. Most amphibians lead a double life, dependent on water and on land at different stages, but some species are permanent land dwellers, and others completely aquatic.

The amphibian pedigree is a very ancient one, and the ancestors of the species alive today are considered to be the first creatures that successfully colonized the land from their homes in fresh-water pools and streams. They are also our own ancestors, of course, mammals having appeared much later in the long process of evolution.

Fossils found near Flagstaff, Ariz., have revealed that frogs that hopped were already present nearly 200 million years ago, at a time when dinosaurs would have been the better-known inhabitants of the land. Their early adaptation for jumping was so successful that it allowed frogs to spread across the great supercontinent of the period that we call Pangaea -- so successful, indeed, that those still surviving today are not much changed, despite the passage of many millions of years.

Many researchers, in fact, believe that frogs' unique ability to jump, suddenly, far and erratically, helped them to avoid predators so successfully -- including the dinosaurs. Now there are more than 4,000 species, almost as many species of frogs as there are of mammals.

Somehow, amphibians survived, passing through that critical bottleneck known as the K-T boundary, the break from the Cretaceous to the Tertiary when the vast majority of species on earth, including the dinosaurs, perished.

The irony is that despite surviving the era of the dinosaurs, despite passing through the extraordinary period of mass extinction at the K-T boundary, despite being so successful for so many millions of years, amphibians are now in trouble. Some species have disappeared completely in recent years. Many others are in rapid decline, and still others are showing up with bizarre deformities. Frogs in particular are disappearing from many parts of the world.

Various factors have been implicated, including acid rain, the thinning ozone layer and habitat destruction, but the common denominator is human activity.

You may think that this is just a lot of fuss over a few species of amphibians declining, developing deformities and disappearing, but consider history. Mining coal has long been a dangerous, albeit lucrative, activity. Those actually doing the digging recognized early on that in addition to rock falls there was a far more insidious danger: the presence of invisible but deadly gasses seeping from the coal seams. The simple solution was the earliest example I know of the use of a bioindicator.

Because of their light bone structure and their complex way of exchanging air between their lungs and the air sacs which help to make flight possible, birds are sensitive to gasses at concentrations far lower than those that affect humans. Carrying a canary down with them gave miners a highly visible early-warning system that saved many human lives.

Tough on the canaries though!

In our modern, sterile lives, we are perhaps too divorced from basic industries such as mining; otherwise, we might more readily acknowledge that we are staring another bioindicator in the face. When the canary keeled over in its cage the miners did not assemble a committee, plan a research project and promise appropriate action depending on the results. They got up and out of the mine.

Of course, the miners were able to quit the gas-filled galleries for a healthier environment up top, and then rectify the situation. We are not yet able to flee the surface of our planet. It is the only real estate we have. Yet we are staring at a dying canary, and still procrastinating.

Because they breathe (partially, at least) through their skins, and because they are dependent on water for part or all of their lives, amphibians such as frogs are very much more sensitive to pollution, toxic chemicals, radiation and disease than we are.

In contrast, we humans, with our internal lungs and greater size, are protected from such direct contact with airborne and waterborne problems. That is not to say we can avoid them, only that they take longer to affect us than amphibians. Consequently, amphibians are very good bioindicators, making visible to us significant environmental changes that we might otherwise overlook. They are a particularly potent early-warning system.

Ignoring the fate of the amphibians, we choose to behave like miners who, instead of running from danger, organize a committee to investigate their dead canary.

Amphibian declines, deformities and disappearances are not local and isolated. They are being recorded in many countries around the world. We are receiving numerous early warnings that there are serious imbalances in our ecosystem, and as safety notices urge, "ignore this warning at your own risk."

In June 1997, ABC News noted that, "If frogs begin showing signs of distress, it could be only a matter of time before other species are affected, including humans." Perhaps we already are being affected, but we are not taking the effects into account. Might not rising rates of skin cancer, asthma, allergies, depression and other ailments be part of a global issue?

Worth thinking about, before more of our canaries die.