Another Life: Our stoats are a rare link to an ancient ecosystem

Stoats are our now-and-then neighbours, glimpsed in ripples across the road or at frozen moments on top of a wall. I have watched one swimming in a pool on the shore and another at full stretch after rabbits on the duach. In a recent hard winter we had some dainty close-ups through the kitchen window, as one fed from bird crumbs outside on the sill.

Stoats don’t really care if we’re looking or not; they’re beautiful, single-minded hunters. Once, in a low-budget winter in Connemara, I took a rabbit off a stoat when they tumbled together at my feet on a bog road, and it followed me along the walls all the way home. Its angry vibes pierced my back, for, as Giraldus Cambrensis wrote in the 12th century, the stoat can be “vindictive and relentless in its wrath”.

Stoats have been around so long in Ireland that you'd think there's nothing left to know about them, apart from not being the "weasels" that rural Ireland commonly supposed. But even zoologists who have made a special study of them are fascinated by all the questions that remain. Dr Paddy Sleeman of University College Cork, for example, who wrote a book about them, led a so-called Irish Stoat Match at Fota last month – a talk-in with fellow scientists on the trail of some of the answers – and two more such gatherings are coming up. "There are huge gaps in our knowledge," he says.

Some of the gaps are simple, as in how many Mustela erminea hibernica the Irish countryside supports. There has been an estimate of 160,000, but that was just a guess based on British figures.

The maps of the ongoing Irish Mammal Atlas (see biodiversityireland.ie) show big regional differences in sightings prior to 2010 and those reported since then. But you have to be careful in drawing conclusions.

Perhaps Northern Ireland has, indeed, lost an awful lot of stoats just lately, as intensive farming and rodent control have reduced the animals’ food supply – or perhaps just fewer people have been looking there than in the Republic. There’s a big splash of new records around Galway and Waterford, for example, because of important new surveys to measure the stoats’ presence.

In Galway, Vincent Wildlife Trust, in partnership with ecologists at NUI Galway, set 600 small plastic tubes, lined with dabs of glue and baited with rabbit meat, along the hedgerows and stone walls of the county. When the DNA of pulled-off hairs was analysed at Waterford Institute of Technology almost half the locations proved positive for stoat.

A further trial around Waterford shows stoats at a third of sites. Along with sightings from wildlife wardens, forestry workers and the public, a more accurate mapping and population estimate should result.

Special hibernica
What's so special about the Irish stoat is the hibernica at the end of its name and the history and physical differences that go with it. It is a distinct and separate Irish subspecies of the European stoat, certainly here before the last ice age and again about 11,000 years ago. (We know this from fossil bones in Co Cork caves.)

As a rare link to an ancient ecosystem, it is adapted to the cold but not to sitting snow, which is why it doesn’t need to go white (ermine) in winter. This may also link to the wavy line between the brown back and white belly, shown in most of our stoats, so different from the straight line of stoats in the rest of Europe.

Size also matters – sometimes rather oddly, as in that stoats in the north of Ireland are smaller than those in the south. Paddy Sleeman thinks this might be because southern stoats have rabbits to eat much earlier than those in the north.

But diet has often raised questions about the stoat’s survival. What on earth did they eat after the ice ages when the lemmings were gone? And what was around in postglacial Ireland: were stoats then fish-eating, coastal dwellers, as some sightings, even now, suggest?

Today they climb trees after birds and burrow into rubbish dumps, after rats, but exactly what they eat becomes important again, as new small mammal species – bank vole and greater white-toothed shrew – spread across Ireland to offer potential new prey, or reduce and outcompete the native shrews and field mice. As stoat casualties mount on the roads, more stomachs will find their way to the relevant laboratories.

Meanwhile, the Irish mustelid family has offered interesting grist to evolutionary biologists, studying the way size is graded between species in the group – say, from stoat to mink to pine marten, to otter and then to badger. It seems that the males of each species are smaller than the females of the next larger species.

There’s so much still to know.