Mating frogs don't need to break the ice

A frozen pond full of frogspawn has the look of an old-fashioned bathroom window, with its bubbly diffractions of shape. I had to peer to see the eggs, suspended in aspic in the icy glow. They are dyed black with melanin to soak up the sun, and the jelly may be laced with some form of antifreeze: after all, frogs spawn in little lochans high on the hills, where winter is slow to fade.

A really hard spell can kill spawn that's left piled above the water, and an especially cold spring can let a fungus infection take hold (a spreading white mycelium, like threads of cotton wool, called Saprolegnia). Last week's Arctic flurries seem likely to have left their mark.

As for frogs themselves, it's the males, hibernating at the bottom of a pond, that can suffer in a really hard winter - less from the ice at the surface than from trapped toxic gases from rotting vegetation.

They cannot, on the other hand, survive being frozen solid. One I saw resting, spread-eagled, under a skin of ice in early February looked as relaxed as any sunbather.

I also noted a frog which, arriving at the pond for last month's mating orgy, climbed to the top of the highest rock at the edge and then leaped out over the water with an almost audible whee-ee! It splashed down among a clump of fellow males jockeying for access to a female the size of a tennis ball.

The sight of a dozen or more frogs being hauled around the pond by a single gravid female with her arms in the air is initially arresting but ultimately grotesque: one can feel quite slithery for watching. Sometimes, when the orgy is over, the water gives up dead victims of rape, literally hugged to death.

At its busiest, the pond held well over 100 frogs, thronging the water as thickly as Ganges pilgrims. Now they are dispersing into the garden's hedge-bottoms and vegetable beds, where I trust they are eating slugs (the original reason for the pond). In a few weeks, depending on the weather, the tadpoles will hatch and whip the water into a froth as they feed upon the last of the rotting spawn-jelly.

At the moment, about 40 clumps of spawn crowd the pond from edge to edge. I scooped one out in the kitchen colander with the idea of counting the eggs but quickly grew dizzy, guessing only at rather fewer than the 1,500 to 3,000 that scientists seem to have settled on. Only a few hundred of them will live to grow legs, and a mere handful will reach frog maturity. But this is said to be the safest time where predators are concerned, since only moorhens, newts and flatworms have much of an appetite for the eggs.

A Connemara reader, John Brittain, would add another predator. Walking near Clifden Castle, he saw a heron poised over a little flash of floodwater at the edge of a field and, being a keen fisherman, wondered what the heron's prey could be. "Investigating further, I found frog spawn with the embryos removed. In my opinion, it was deftly picking out a small but tasty appetiser for itself before the adult frogs came back to make up the main course." This may be so, but herons sometimes complicate the picture by squeezing out dollops of eggless jelly from some of the females seized in that massive scissors of a bill.

Everything eats tadpoles and froglets, from water-beetles, dragonfly nymphs and newts to ducks, thrushes and pygmy shrews. As adults, they are then pursued by every carnivorous animal, from foxes and hedgehogs to otters and stoats, and by owls, ravens and kestrels, along with the herons.

At any age, indeed, Rana temporaria is so much in demand as food that it seems unthinkable it should ever have been missing from Ireland's wildlife ecosystems. It is bad enough that the field vole, a staple of carnivore prey in Britain, should have failed to reach this island, but without the frog we would lack one of nature's most significant takeaway foods.

Early naturalist-monks, such as Donatus and Giraldus, vowed that Ireland had no frogs, snakes or toads, but many modern biologists find it hard to believe that frogs failed to make it over a post-glacial land-bridge. They resist the longstanding acceptance that our frogs were introduced, perhaps by the Normans, for food, or, famously, by a Trinity don called Dr Gwithers, who brought spawn from England in about 1696 and tipped it into a ditch beside the college.

One way to test the native antiquity of the frog in Ireland has been to radiocarbon-date fossil frog bones found deep in caves alongside fossils of mammals such as bear and wolf. But it is only recently that the amount of frog bone needed for the dating process became so small that precious museum fossils could be individually sampled.

In the most recent research, led by English nature zoologist Dr Chris Gleed-Owen, 10 samples of fossil frog bones in the collections of the National Museum were dated last year in the radiocarbon laboratory at Oxford University. They included bones excavated from bear caves in counties Sligo, Clare and Cork, and from a frog skeleton found in the Neolithic cemetery at Carrowmore, Co Sligo.

All the dates came out to less than 400 years old, seeming to prove yet again that frogs can beam their bones like time-travellers into ancient palaeontological contexts. Dr Gleed-Own, dismayed, but not surrendering (contamination of samples is always possible), believes the case isn't over yet.

Meanwhile, the continuing abundance of frogs in Ireland is good news, as in many parts of its European range, R. temporaria is in serious decline. Reports of mass deaths among several frog species across the world has led to the setting up of a Declining Amphibians Populations Task Force to co-ordinate research. Various kinds of pollution have been linked to frog mortality, among them pesticides such as carbaryl and high levels of nitrates in farmland ponds and ditches. But most scientists look to a synergy of causes, even including increased ultra-violet radiation from the damaged ozone layer.

Most immediate of threats to the frog is certainly the simple loss of breeding-grounds, as farm ponds are filled and marshy land built over. The spread of concrete could initially cost us frogs by the million. But, as in Britain, more suburban garden ponds may eventually offer a significant habitat, breeding urban frogs for urban foxes.