Winter cold spawns ingenious survival tactics

ANOTHER LIFE:THE FROGS were often up in the pond and singing for my birthday, but the month is almost gone without a sign of them. A badger-shaped furrow through the dead grass at the rim suggests another sort of inspection and perhaps a disappointed, furry shrug. But give it time: the western frosts haven't been the worst, and a few days of around 10 degrees C could prompt some spawning. That, at least, was what seems to have conjured the first report of the year, from a wetland at Inchybridge, Timoleague in west Cork on January 23rd. A supporting photograph of the spawn duly appeared on the Nature's Calendar website, a splendid online monitor of phenological events (naturescalendarireland.com).

Another measure of how well our frogs have survived the winter could come from the school-oriented "Hop-to-it" survey run by the Irish Peatland Conservation Council (ipcc.ie) every spring since 1997. As one of Europe's last healthy strongholds of Rana temporaria, the welfare of this island's frog population is of more than passing ecological interest.

While the IPCC is chiefly concerned with bogs (where many shallow pools must have been frozen hard for long periods), most of the annual sightings are of frogs in garden ponds. And since a good many frogs, mostly male, overwinter in the mud at the bottom, there could be many casualties – not, as a rule, from being frozen in ice, but from slow suffocation as the level of water falls and it loses dissolved oxygen.

A lid of ice on a pond rarely thickens to the bottom –­ in fact, it helps insulate the deeper water from further cooling. But it seals in toxic gases produced by last autumn’s rotting plants, and a covering of snow further shuts out the light needed for plant photosynthesis that generates oxygen.

Not only frogs, but fish and aquatic insects and larvae suffer a similar mortality.

Female frogs generally overwinter on land, in crevices under logs and in stone walls, converging on the ponds to mate and spawn. Their travels are usually nocturnal, so the hard night frosts will have kept many under cover, still in their winter torpor, and some will, indeed, already have frozen to death.

THERE HAVE BEEN exceptional, balmy springs to tempt the first queen bumblebees out in mid-February, but this is clearly not one of them.

In fact, the queens take care not to be fooled by the odd sunny day into breaking their winter sleep. They choose a north-facing bank in which to dig their hibernaculum (nice word) and only emerge when the soil temperature rises to cue their awakening.

To find bumblebees, butterflies and craneflies enjoying the flowers of the High Arctic summer was one of the surprises of the East Greenland wilderness (indeed, the similarity of a bumblebee’s buzz to the distant approach of a long-delayed helicopter is something still impressed on my mind). They have all survived, in one way or another, winter temperatures of minus 50 degrees C or more. Most are dark-coloured and more or less hairy, all to help absorb the sun’s radiation.

The help of “anti-freeze” chemicals in winter survival of insects, even in temperate environments, is well-known but only part of the story: the familiar glycerol, for example, will freeze at around minus 5 degrees C. Such chemicals are used in two kinds of strategy, each depending on how cold the winter gets.

In really cold areas, such as the Arctic, insects can survive being frozen solid by controlling where ice crystals form in their bodies and using the anti-freeze to keep their cells and organs undamaged. In generally milder winters, such as Ireland’s, many insects use the chemicals to become “supercooled” but not frozen. They may empty their guts and reduce the water in their bodies to reduce the minute particles around which ice can form.

Such adaptations come from natural selection to match the prevailing winter conditions. Exceptionally severe conditions will kill even well-adapted insects, and degrees of protection must vary from one region to another. For earthworms, certainly — while not insects — there are big regional differences in biochemical strategies and how much frost they can endure. Our worms dig a bit deeper to avoid the cold, while Greenland’s let themselves freeze internally, but not where it matters.

It would be nice to suppose that Connacht’s midges, perhaps relatively unadapted, have been decimated by the frosts (this with apologies to swallows, frogs and bats). Bog pools, after all, are a major midge habitat, and many must have been crusted with ice more or less since Christmas. But there are midges even in the Antarctic, and Greenland’s High Arctic, as I recall with a shudder, has the densest clouds of mosquitoes in the world.

EYE ON NATURE

When I was taking a short cut through Moore Street, Dublin, to get my train home, a large seagull flew over my head, swooped down to a tray of fish, picked up a fairly large fish, but dropped it. I didn’t wait to see if it came back for its meal. Mairéad Furlong, Wexford

A rook has begun to visit our bird feeder which hangs by a ring from a crossbar on a pole. He takes the ring in his beak, shakes the feeder to spill the contents on to the lawn, then hops down and shovels the seeds in with a sideways sweep of his beak.

Lesley Wishart, Portballintrae, Co Antrim

I came across a mouse that had met his demise stuck in the jaws of a giant cockle on Portmarnock beach. (photo enclosed). Could a gull have dropped the shellfish and the mouse tried to eat it?

Sarah O’Farrell, Baldoyle, Dublin 13

That sounds like a likely scenario.

I saw my first otter in the Garavogue River, Sligo, where the river meets the sea. It swam around and then scampered up to a grassy area near me. When he realised I was there, he jumped back in and promptly swam away.

Alison English, Old Cartron, Sligo

Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo. Email: viney@anu.ie. Include a postal address.