I n their annual ABC survey, Fortean Times magazine found 200 local newspaper reports of alien big cats (ABCs) across the country, from Surrey pumas to Beasts of Bodmin. These cats are notoriously elusive; after a spate of media coverage the beast usually vanishes. In a few cases an animal is caught or killed, and from these we can gain clues about what is really out there.

There are 37 different species of cat, but they share many physical similarities. By contrast, the single species of domestic dog varies in shape from great danes to dachshunds. A Rottweiler is not simply a large Jack Russell; but a puma is remarkably similar to a scaled-up Abyssinian.

This can make judging size difficult. In 1996, police marksmen in County Tyrone cornered an animal after repeated ABC sightings. They concluded it was a young lion and probably dangerous, and shot the animal. In fact, it was a caracal cat. These reach a weight of about 10kg, the size of a large fox. Real lions, on the other hand, average around 200kg.

In 1999 there was the Beast of Barnsley. A number of witnesses reported seeing a lion and locals were advised to keep children and pets indoors. The culprit turned out to be Rocky, a retriever-Rottweiler cross whose coat had been shaved because of eczema. At roughly 30kg, Rocky was about one-seventh the size of the real thing.

In May this year, a Mrs Johnson of Cricklewood called police when she found a strange animal sitting on the wall of her back garden. She described it as "the size of an Alsatian", and suggested it might be a leopard.

When staff from London Zoo sedated the creature with a blowpipe they found they had a young female European lynx. The lynx, now called Lara, is recuperating in London Zoo; where she came from remains a mystery. When captured she was somewhat underweight at a svelte 11.5kg. Although lynxes have relatively long legs, to equate her with an Alsatian (typically around 35kg) is like suggesting that Kylie Minogue (44kg) is the size of Luciano Paravotti (130kg). To describe Lara as a leopard (60kg) would be inflating La Minogue to the dimensions of a pygmy hippopotamus (200kg).

In 1983 the panther-like Beast of Exmoor caused so much concern that marines were called in to hunt it down. In her book on the beast, Di Francis recounts how a marine sniper had the animal in his sights, but did not fire because he thought it was too far away for a safe shot.

Afterwards, he was puzzled when he paced the distance and found that the beast had been easily in range. His mistake was probably in assuming it was a large animal; it is likely the animal was much smaller and much closer than he thought.

We take our ability to judge size for granted. However, it is not a matter of innate ability as much as experience. We rely on all sorts of cues from our environment when judging size and distance. A magnum of champagne five metres away could be mistaken for a normal bottle three metres away, but by picking up on various clues of perspective, we can usually judge distance and hence size.

This can sometime mislead us, as in the famous Muller-Lyer optical illusion (two parallel lines of equal length, one with arrowheads at each end pointing outwards, and the other with similar arrowheads pointing inwards). Although the two lines are the same length, the inward-pointing arrows make one line look bigger by fooling our sense of perspective. Or try this experiment. Hold one hand about two feet from your eyes and the other half as far and shift your gaze between them. The image of each seems to be the same size; even though one is twice as close, unconscious knowledge that the two are the same size causes us to seem them the same.

We can judge size and distance well enough with ordinary objects, but presented with something strange, our ability to estimate becomes unreliable. If you believe that what you are seeing is a big cat, you will tend to overestimate its size. This affects even those who should know better. After a spate of panther sightings near Balmoral, a gamekeeper shot at an animal that ran into his headlights, only to find it was a tortoiseshell cat.

The situation is compounded by the presence of felines that are smaller than a panther but bigger than a normal cat. The cait sith is a ferocious creature from Scottish folklore, a black cat with a white mark on its throat. It was long considered mythical, as there are no black wildcats in Britain. Then a black cat almost four feet long was shot near the village of Kellas in West Murray.

Genetic testing and subse quent specimens revealed that the Kellas cat was not a new species but a complex hybrid of wildcat and domestic cat. The gene for dark colouration, known as melanism, is known to be linked to increased size. The origin of other Kellas characteristics - it is a strong swimmer - remains mysterious.

Ray Charter, the head keeper of big cats at London Zoo, has taken numerous ABC calls. Lara was the first genuine big cat he has seen: the others have usually been large domestic cats. Some cat breeds are bigger than others: the Bengal is a hybrid of domestic cat and Asian leopard cat and males may be nine kilograms or more.

Lara was not adapted to life in the wild. Nor was Felicity, a rather tame puma captured in Scotland in 1980. There is little evidence that wild ABCs are a threat to farm animals or humans.

The idea that we need to shoot first and ask questions later is misguided when the animal in question is more likely to be a moggie than a man-eater.

Of course, some ABCs are neither. In November, Kim Simmonds of Linton Zoo went to investigate the body of a striped animal lying by the side of the road in Cambridgeshire. The animal in question was actually an imitation tiger print seat cover.