Loch Ness: otters and old logs

''Sightings'' of the Loch Ness Monster - and of its cousins in Chesapeake Bay and Lake Champlain - continue to stir curiosity. But those who prefer reason to gullibility should consider the fact that many sightings of Nessie, at least, could be due to frolicking otters, logs propelled by biogas, distant motor boats , and other such ordinary phenomena.

Of these, the gas-propelled logs probably are the most bizarre. They would be trunks or limbs of rot-resistant species that had fallen into a deep lake and settled to the bottom. Gases formed by slow decay would accumulate in the log until they made it buoyant enough to shoot to the surface. There, relieved of the water pressure, the gas would escape explosively to create a bubbly froth while the log would protrude like the neck of a great serpent.

Robert P. Craig, a retired Scottish electrical engineer, has investigated this possibility extensively. He explained recently in New Scientist that Scots pine (Pinus sylvestris) would do nicely.

Under the water pressure at the bottom of a deep loch, the tree's abundant resins would be squeezed out to form a tough gas-trapping, water-resistant skin. Craig notes that Scots pine grows along Lochs Ness, Morar, and Tag, which supposedly have monsters. Loch Lomond, another deep loch, lacks both.

Last June, in a three-part series published in New Scientist, British naturalist Maurice Burton also suggested that gas-propelled debris might account for some monster reports. However, he focused more strongly on otters because he has concluded that their general appearance in the water and playful habits fit many Nessie descriptions.

He notes that, in Scotland, otters may grow to be six feet long with records of 7.5 to 8 feet. Large otters frolicking on the surface could look serpentlike. What is more, lines of otters could look like a long serpent when the lead otter rears up to look around.

Such appearances could easily fool unwary observers. Indeed, Burton identifies two of the most celebrated Nessie photos with otters. One of these, taken in 1933, shows a longish body thrashing around on the water. Lantern slides made directly from the original negative clearly show an otter, Burton says. The other photo, taken in 1934, shows what is purported to be Nessie's long neck. This, Burton says, seems merely to be an otter's tail.

Yet other mistaken identities involve motorized dinghies. Seen from afar, these appear to be a dark animal head moving through the water. Burton tells of credulous watchers refusing to believe they were not seeing a monster even though one person with them could see the boat through binoculars.

Of course, not all ''monster'' sightings in Loch Ness or elsewhere are otters , motorboats, or old logs. On the other hand, the evidence for real monsters is very thin - ambiguous photographs and hard-to-interpret eyewitness accounts. There's no need to speculate about monsters when prosaic explanations seem more likely.