Worrying at the country rat

THE brown rat dining at our bird-table is the very picture of health, each hair shampooed and shining, little ears perked up in furry shells. It sits comfortably poised upon its haunches, holding an oatflake daintily between its paws, like a child making a biscuit last: its eyes sparkle, its whiskers tremble in the breeze.

The bird table stands in a niche in the hedge, with a canopy of wire netting to ward off rooks and magpies. When I bang on the window and let out a roar, the rat backs off into the fuchsia and waits to see if I mean it. The moment I subside to my desk it will re-emerge, so I must keep on banging and gesticulating until it gives up and retires down the brush - for the time being.

A waiting blackbird hops up from the shadows to take its place at the oatmeal box, jumping in with both claws and eating with panicky swipes of its bill. showering the flakes in all directions. This messy feeding is what brought the rat up in the first place. The rodent was actually more personable and interesting to watch, if one could ignore the scrape of that indecently naked tail.

Country rats are altogether different from T. S. Eliot's urban stereotype "Dragging its slimy belly on the bank/While I fishing in the dull canal". Among clean leaves and sanitary shadows, they are living, as it were, more in context. Even when they tunnel through my compost heap they are helpfully letting the air in to let nature do its work.

That doesn't mean, of course, that a rustling at night in the wall between me and the hot-press is anything but unsettling. It is then that I think of rats piddling leptospirosis into the compost And even if they don't carry that potentially fatal bacterium of Weil's disease nearly as often as people used to think, they are apparently rife, at least in British farmyards, with pathogens causing half a dozen other dreadful things, from cryposporidiosis to han-taan fever.

So yes, we poison them, putting down the bait under lengths of turned-down guttering with a rock on the top. I'm still not satisfied that this is strictly rational behaviour - it's not my X-million tons of grain a year the rats of the world are eating - but taboos have always been good for the chemical trade.

If we didn't poison the rats, what would happen? How soon does one compost-heap-full become a "horde"? Population dynamics are a big and enormously subtle part of ecological theory, shading all the way into the shifting sands of chaos theory.

David Macdonald, head of the Wildlife Conservation Unit at Oxford University, takes rats as an example of the "Malthusian nightmare" in his fine new book European Mammals: Evolution and Behaviour (Harper Collins £15.00 in the UK).

In ideal conditions, female brown rats become sexually mature at two months old, and thereafter can have litters as often as once a month, with about nine babies in each. Exponential mathematics thus projects 108 offspring and 450,000 descendants per year. But rat populations, like those of most mammals, are either stable or fluctuate within clear bounds. What, beyond crude parameters of food supply, and death by owl, stoat, fox or Warfarin, actually controls them?

Macdonald spends several pages (to be read very attentively indeed) on the conflicting explanations applied in the past. One school focused on erratic fluctuations in numbers caused by external factors (such as weather). The other looked to regulation which was dependent on internal density: a population fall would bring a baby boom; too high a population would be corrected by social pressures or starvation, and so on.

The newer population models are much more complicated than either approach, their outcomes sometimes reckoning even with the patterns of chaos theory called "strange attractor". In this conceptual revolution, says Macdonald, apparently random fluctuations can arise from processes dependent on the density of rats: events within the population can produce cycles which teeter on the edge of chaos.

To deal with the 450,000 rats however, he relaxes, with some gratitude, into the familiar density-dependent world. Here, as the offspring eat the available food the theoretical age of maturity slips and litters are spaced more widely crowding causes declines in breeding and smaller litters, attracts more predators and fosters contagious disease. When all this and more is taken into account, the initial pair of rats may produce less than 200 descendants annually, and only a few of these will survive to reproduce.

Nonetheless, says Macdonald, quite enough rats survive to make them one of the world's most serious pests. He deals extensively with the remarkable duel between man and rat in the use of slow-acting anti-coagulants such as Warfarin. Under the pressure of 30 years of selection, the rats, in many populations are showing metabolic resistance. Some have actually increased in size - a genetic advantage which continues to hold good even when the Warfarin is absent.

Behavioural resistance is, says Macdonald, "a more recent joker in the pack". Even in normal circumstances, wild brown rats are extremely wary of unfamiliar food, or food in an unfamiliar place - a trait called neophobia, enhanced by centuries of reason for mistrust. Even a perfectly wholesome pile of wheat, encountered where it shouldn't be, may be treated with caution for weeks. The new, one-dose cakes which we laid down remained under the drainpipe for a couple of weeks.

Perhaps from the infant experience of taking food from their mother's mouth, brown rats have developed one way of judging whether a new food is safe to eat: if its smell is on the lips of another rat, they'll chance it. But those in some regions just can't be tempted to taste a poisoned bait even one, like Warfarin that doesn't take effect for days. In exercising such caution, as David Macdonald points out, rats are actually forfeiting the opportunism by which their species has prospered.