How life adapted to the cold Irish winters

AT the end of the last Ice Age, animals and plants crept northwards to colonise the messy moraines left behind when the ice melted.

They got to the part of Europe that would become Ireland when it broke off from the continent and they were faced with a problem — the winter.

It wasn’t just that it was still cold. It was also that they were now so far north the winter days were extremely short. But evolution works because of the adaptability of life forms, and strategies were invented to solve the problem. The trees, particularly deciduous, developed a type of hibernation. The average tree has the same amount of biomass below ground as above. The living part of the tree retreated into its root system, where, insulated by the soil, it was dormant until the weather warmed up, the days lengthened and photosynthesis became worthwhile.

Some smaller plants did the same. Others became annuals, over-wintering as tough seeds primed to germinate in the spring.

Birds had multiple choices. Some migrated south to where the days were longer and warmer. Others stayed here but adapted their behaviour. Most of the insect eaters scrounged a winter living as vegetarians, getting their protein supplies from seeds and fruits.

Some birds, such as jays (above), concealed stores of non-perishable food for use in winter. Birds of prey were not so challenged. Hunting was easier with no leaves on the trees and with prey items weakened by hunger. Owls profited from the longer nights.

Land mammals living on an island didn’t have the option of migration, though some moved from the uplands down into sheltered valleys for the winter. The bats and the hedgehog, faced with a dearth of the invertebrates they fed on, chose hibernation. The pygmy shrew, which has a similar diet, kept on going at a frenetic rate and put up with a short life expectancy — the average pygmy shrew lives for 11 to 13 months.

Hibernation is not black and white — there are many shades of grey. Biologists are fond of saying hedgehogs hibernate in Ireland but squirrels don’t. Hedgehogs come out of their comatose state frequently in the course of the winter. They urinate, feed, re-make their beds and sometimes move to a new location. And when the weather is bad, squirrels become dormant for days or even weeks at a time, waking up occasionally and nibbling on food they’ve stored during better days.

Frogs, toads and newts did something similar to mammal hibernation. In Ireland, they mostly do this on dry land. But because they have a handy ability to absorb oxygen through their skins as well as from their lungs, they can do it under water. They also have chemicals in their bodies similar to car anti-freeze, so they don’t turn into blocks of ice in very cold weather.

Insects adopted a range of strategies. A few winged species, mostly butterflies and moths, migrated like birds. A few others hibernated like hedgehogs — queen bumble bees and tortoiseshell butterflies do this. Many more adapted in the same way as annual plants. They have short lives as adults and over-winter as eggs, larvae or pupae.

Most Irish fish species become torpid in winter. Their metabolism slows down and they eat far less food. The tench was introduced into this country from warmer places and it engages in something quite similar to hibernation, burying itself in mud or thick vegetation and dozing off when the water temperature is low.

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