Nature's great egg timers

A NICE idea, the earth rolling and rocking the wall. In Mayo, unlike, New Hampshire, cold rarely lies that long, or bites deeply enough to stir the soil.

What it does is freeze the film moisture on the stones and prise them apart, just far enough to slip when a thaw comes. Our stones are "dogs heads" of sandstone, ground and polished by a glacier. They are perilously balanced at the best of times; now you have to watch out for tumbled stones all, along the road to the mountain.

The county council, on the other hand, inventing brave new walls along widened scenic roads, sticks the dogs heads together with a core of concrete. You can spot this on hillsides by pipes set into the wall at intervals to bet water through. I don't mind it, though Frank McDonald might: Georgian facades and non slip Famine walls are part of what we are.

To wildlife, the walls of the west stand in for hedge rows, especially where brambles, hawthorn and goat willow press up against them. Even the new, hard hearted walls will give a grip to lichens and mosses and make crevices for spiders and beetles. Wrens and pipits, on the other hand, may find them sadly short of nesting holes.

Stoats, too, will be baffled by walls which, while serving their hunting patterns perfectly well (like snipers on rooftops), lack the interior labyrinth in which a shrew or a rabbit may be consumed at leisure, or cosy eaves in which to have their young.

Stoats offer a striking instance of something called delayed implantation, an odd biological device by which some mammals match their reproduction to the optimum times for mating and giving birth.

It suits seals, for example, both to have their young and mate again at the one brief time in the year they spend on land. But since the female's gestation period is eight, months, she delays, the implantation of the fertilised egg and development of the embryo to match the annual haul out.

Other mammals have different evolutionary reasons for delaying, implantation, and the result tug, breeding arrangements can seem quite bizarre.

Most of Ireland's female stoats will spend this month actively developing a litter of embryos, to give, birth in March and April, having actually been fertilised last summer 280 days previously. The really weird part begins in the nest, where the female's current mate will copulate not only, with her but with all the infant females in the litter. They are quite receptive to this, even before their eyes are open, in fact, the oestrogens triggering their reproductive systems may reach them in their mothers milk.

The resident male is often the infants' father which is why, says UCC zoologist Paddy Sleeman, in his Whittet book on the mustelids, to call a man "a bit of a stoat" is considered such an insult. On the other hand, the stoats' short lifespan makes it quite likely that the mother's current mate is a new arrival.

The point of the paedophilia seems to be to squeeze an extra litter into the female stoat's brief life, about two and a half years at most. It's hard to see this ferocious little carnivore in the role of prey, but foxes, dogs and cats are among its executioners. So are owls, kestrels and peregrine falcons indeed, the black tip to the stoat's tail seem to have evolved specifically to confuse aerial attackers.

The stoat's strange breeding pattern probably originated as a response to cyclic population explosions among their quarry, burrowing rodents such as voles, and lemmings - a glut of prey needing to be eaten. In the 47 mammal species that show delayed implantation, it is often a way of timing births to match the abundant food of spring or summer. It can also put off pregnancy - or even cancel it - when nourishment is low.

Ireland's badgers, for example, now giving birth in their setts, may well be having fewer young, than usual. Badgers have two mating periods - in spring and autumn and "save up" the embryos for development from mid December onwards. This is the time of lethargy in which badgers are living off their fat and this reserve depends very much on the earthworm supply in the previous summer.

Hot, dry summers are bad for earthworms and can leave the badgers emaciated. In a study in Oxfordshire, following a dry summer, all the adult females got pregnant, but only the fattest produced their litters. Suckling young is physically costly, and it seems that pregnant badgers that don't feel up to it may re absorb all or part of the litter back into their tissues.

What happens in Ireland in more normal years has been studied by zoologists from UCD. They used hundreds of badgers from a mass clearance by the Department of Agriculture in 600 square kilometres of Co Offaly. The study found that most of the badger sows in Offaly mate in late March, and early April. About two thirds achieve successful implantation by early January and only half of those go on to suckle their young. And beyond such basic information lie strange and subtle mechanisms by which badger groups regulate their birth rate, and decide which females will make the best mothers.

It has been shown that it can take 10 years for, incoming badgers to repopulate a cleared area to its original density. But if groups of badgers are left, and reproductive success depends on population density, their numbers could build up again far more quickly. The "control" of badgers seems destined to become a biological battle of wits.

On a visit to the beach at Brittas Bay recently, I found the enclosed shell (white, oval, brittle, convex). I have been wondering what it is as I have not been able to find any information on it anywhere.