Shrews are tough little characters

LITTLE creatures are often overlooked, while big ones get all the attention. Shrews are a case in point.

These little mammals, with their hectic lifestyles, are more interesting than some with much higher profiles. I hardly ever get queries about them, but recently a reader asked if there are many shrews in Ireland, whether they hibernate and how they live.

Shrew mice or grass mice are not rodents. They belong, with hedgehogs and moles, to one of the most ancient of mammal groups, the insectivores.

The insect eaters first appeared 135 million years ago. Rodents would not arrive on the scene for another 60 million years.

Shrews, with their long snouts, don’t win beauty competitions. An unattractive appearance can give an animal a bad image and spawn ridiculous superstitions. It was believed, for example, that a cow would become ill if a shrew ran over its back. Mature ladies, of a nagging or quarrelsome disposition, are sometimes referred to as ‘shrews’; Shakespeare didn’t help the shrew by naming a misogynistic play after it. This animal is supposed to be very aggressive and able to deliver a painful bite but I have handled shrews on several occasions and none of them bit me. Nor are all of the cultural references negative: the adjective ‘shrewd’, which may mean ‘shrew-like’, has a positive connotation.

These tough little characters live in farmland, woodland, bogs and wet areas just about everywhere in the country. We have more shrews in Ireland than any other mammal but there is only one Irish species — the pigmy shrew — which is tiny, even by shrew standards. During the breeding season, a mature adult will weigh no more than seven grams, less than the weight of a 1 coin. Therein lies one of the great shrew mysteries: how did such a tiny creature manage to get to Ireland in the first place? Presumably, it came here from Britain, where there are three species of shrew. The common shrew, found there, weighs about 12 grams, while a water shrew might top the scales at a whopping 23 grams. At the end of the last ice age, it would have been very difficult for such tiny creatures to make their way across the rugged land-bridges which, we think, linked Ireland and Britain. Perhaps several shrew species arrived here but only the pigmy liked the place. Unlike their common cousins, pigmies prefer damper climates, like ours.

Shrews are unlikely migrants. Very sedentary, a pigmy will spend its entire life in a grassy patch no more than 40 metres square. Each individual defends its territory fiercely, living alone except when a female is rearing young. Aggression and social isolation are dictated by the needs of the hunt. The beetles, woodlice and spiders, which shrews pursue will flee or hide if they detect the slightest sign of a predator. An animal seeking such mobile prey had better be on its own; a posse of hungry predators on the rampage will soon give the game away. Shrews detect their prey mainly by smell and the long nose is constantly twitching and sniffing the air. Most of the animal’s brain goes to processing odours. Their hearing is good, but their eyesight is poor.

Getting sufficient food, when you are a shrew, is a matter of life and death. Tiny objects lose heat very quickly in cold environments and a shrew must eat constantly, throughout both day and night, if it is to keep body and soul together. Deprived of food, it will starve to death in less than four hours. The normal daily food intake is about 80% to 100% of body weight. For a pregnant female, the stakes are even higher: she may need to eat twice her own weight in a day.

The prospect of you or me consuming our own weight in food in a day is revolting but, actually, the comparison is not really valid.

A ‘human day’ and a ‘shrew day’ are of very different lengths. If you measure a ‘day’ by the number of heartbeats, rather than by the rotation of the Earth, you get a more accurate impression of the shrew’s world. The human heart beats, on average, about 100,000 times in a day, whereas a shrew’s heart might clock up two million beats in the same period. A person might live for 100 years. Only an occasional shrew reaches its first birthday and the oldest ones attain the ripe old age of 16 months. A day in our lives, therefore, is equivalent to months in a shrew’s. The animal’s food intake, when considered from this perspective, is not all that different from our own.

It’s odd that shrews don’t hibernate as this would help them deal with the cold and the food shortages of winter. Our other insectivore, the hedgehog, is a hibernator, so sleeping through the tough winter months must have been an evolutionary option which shrews chose not to pursue.

Males visit female territories in spring and summer. Romances are brief; ‘love them and leave them’ is the rule. A female shrew rears her two annual litters of four to seven babies on her own. The nest is a ball of grass, well hidden. It is very difficult to spot shrews but they produce a very high-pitched squeak, which children and some adults can hear.