The patter of tiny feet

The first chill winds bring a patter of tiny feet behind the plasterboard that lines the old part of the house. Or indeed, not so much a patter as a Stockhausen symphony of rattles, bangs and crashes, interspersed with pauses not quite long enough to allow a drift into sleep. I lie there inventing scenarios which might accommodate the noises: mice as piano-movers, mice on stilts.

It is as much this nightly disturbance as any scavenging of crumbs or raids on the dog's biscuits that prompt us to set traps. If I lived alone, indeed, a quite different relationship might evolve. But as it is, Ethna's deft fingers set the springs, and mine dispose of the dead. Among the house mice, Mus musculus, the occasional field mouse, Apodemus sylvaticus, offers a longer tail by which to lob the sad, velvety corpse across the hedge.

Why we should be more regretful about killing one kind of mouse rather than another is, I suppose, readily rationalised: the field mice are merely misadventurous visitors, while the house mice, moving back indoors from the hedge-bottom, are all set to stay for the winter, nest and multiply.

How rodents strike you depends a lot on context. I recently watched in charmed fascination as a field mouse climbed the hawthorn outside my study window and joined a cock chaffinch on the peanut-holder: bird on one side, mouse on the other, swinging gently in the breeze.

I have also watched a rat sitting up to groom itself on the bird table and thought how immaculate, even dainty, it looked: give it some fur on its tail, like a squirrel (also, of course, a rodent) and country rats, at least, might be quite tolerable. One has just dug a burrow deep into the raised bed where the sea kale grows, leaving a quite remarkable heap of soil at the entrance; others regularly tunnel into the compost heap, for warmth as much as food, thus helping to speed up the compost's decay.

And yet the aversion to rats runs deep. Last winter a very large one, pushing through a hole in the wooden floor of the toolshed, got stuck half-way and expired. I continued to use the shed for spade and fork, averting my eyes and holding my breath, until the malign, shadowy effigy disappeared.

Prof James Fairley, now retired to his native Belfast from the Zoology Department of UCG, has had more to do with rodents than most mammalogists. But while allowing that Apodemus, the field mouse, has a certain appeal, he is typically unsentimental about the "blighters" that "infest" houses.

Mus musculus, he writes in his new, self-published book, A Basket of Weasels, "is a restless, erratic and peripatetic feeder, nibbling a little here and then moving on to snack again somewhere else, defecating frequently and continually dribbling urine, so polluting far more food than it devours . . . However, some people will tolerate the companionship of a few mice, small size, nursery tales and cartoon films all having their pernicious influence."

Some Irish naturalists have been more indulgent. The great Charles Moffat, quoted by Fairley, set a quite different tone in the Irish Naturalist's Journal in 1929: "Most of us have, no doubt, been occasionally entertained by 'singing mice', and been struck by their tameness when pouring out their little melody - a melody often listened to, and rightly enough, with pathetic interest. It seems to be now beyond doubt that the song is an involuntary performance, due to some disease or derangement in the respiratory organs of the little singer." Ireland's house mice no longer seem to sing, at least not on the record - which makes me wonder if Mus musculus could have caught TB from humans.

While house mice wander abroad on the land in summer, competing with the field mice, winter finds them concentrated in and around buildings, with few of their kind in between. Since most domestic populations are probably descended from a few ancestors, how inbred do they become? Dr Fairley caught the mice for a DNA research project that mapped the comparative isolation of mouse-groups, reluctant even to cross a road between farms.

Their inbreeding may also help to explain unusual coat colours, a phenomenon used in the past to suggest the presence of distinct sub-species.

The most celebrated example were the "sandy mice" discovered on Dublin's North Bull at the end of the 19th century. But, like the black rabbits I have seen disappear against the dark peat of a Mayo island, the colouring probably owed more to inbreeding than any selective adaptation to foil predators.

Considering the range of carnivores that prey on mice, including fox, mink and otter, it is remarkable that only the stoat has ever been found to have Mus musculus in its stomach - this, again, must underline its scarcity away from human habitation. Rats, on the other hand, are a carnivore staple, especially at this time of year when numbers are high and young rats are dispersing. They are specially attracted to roadsides with spilled grain or sugar beet, where even herons may gather to kill and swallow rats whole.

I have previously commented here on the surprising predominance of young women in Irish research into bats, largely due to James Fairley's encouragement. To his own surprise, the only serious investigation of Ireland's wild rat populations ("seldom to the taste of people who have chosen to research our wild mammals") is also by a young woman - Dr Fidelma Butler, at UCD, in the early 1990s.

Her fieldwork was carried out at two pig farms in Co Kildare, where the animals were fed on swill, and disused machinery and rubbish heaps were scattered around the buildings to give them shelter. She trapped, gutted and skinned 366 rats over the course of a year, checking on their health and breeding condition (both, in the circumstances, excellent and yielding litters averaging nine young every few weeks). An incidental finding is that those bare, scaly tails take the brunt of injuries as male rats fight for access to a mate.

Dr James Fairley is at 15, Luxor Gardens, Belfast, BT5 5NB; his book, A Basket of Weasels, deals with a wide range of furred wildlife and costs £21 or £17 in UK.