The grey’s arrival in an area spells doom for the local red squirrels, writes
In an infamous act of environmental vandalism, a ‘scurry’ of American grey squirrels was presented to a bride during a 1911 wedding celebration in Castleforbes Co Longford. Released into the countryside, the invaders thrived. Slowly, but surely, they have extended their range. All eastern and northern counties are colonised and the Shannon, which blocked their westward march for decades, has been breached.
The grey’s arrival in an area spells doom for the local red squirrels. The red is usually described as a ‘native’ but it was probably introduced; Ireland was already an island before suitable coniferous woods became available 9,000 years ago. Exploited for its pelt, this high-wire acrobat became virtually extinct here in the 18th century. Today’s squirrels are descended from Scottish ones brought in to restore the population.
Their recent history in the Phoenix Park is typical. Grey squirrels were seen there, for the first time, in 1978. The last sighting of a red one was on St Patrick’s Day 1987. Why do reds lose out to the invader? Having lived here for millennia, reds should be perfectly adapted to local conditions, whereas the invader, used to broadleaved habitats in north America, had to change its ways to survive. Greys don’t attack reds, nor do they compete with them directly. Bigger, and more at home on the ground, they prefer mixed woods to the coniferous ones favoured by reds. So why can’t they coexist?
The red’s digestion may be a factor. The grey’s stomach is better able to cope with the toxins broadleaved trees produce to upset the stomachs of squirrels eating their seeds. In mixed woodland, therefore, the reds might lose out when food is scarce. That the reds do better in pure coniferous stands supports that idea. However, infectious disease is a more likely culprit. Greys carry a squirrel-pox virus but they don’t fall victim to it. Reds have no such immunity. Picking up the disease from the greys, they die.
Now, researchers at Exeter University have identified another possible factor in the squirrel civil war; ‘intellectual deficit’ among the reds. Hazelnuts were placed in boxes with transparent lids. Some boxes were easy to open; it was just a matter of raising the lid. Others required ingenuity on the squirrel’s part. Levers had to be activated to gain access. Both types of box were presented to wild greys at Exeter’s Streatham campus and to reds in woods near Brodick Castle in Scotland.
Reds and greys readily opened the boxes with simple lids, but the greys were better at opening the more complex ones. About 91% of grey squirrels managed to work the levers and get at the peanuts, whereas only 62%
of the reds succeeded. The researchers conclude that red squirrels are less efficient than greys at foraging and food extraction. They may lose out to greys when competing for food in lean times. There was, however, a ray of hope for the reds: a few, who had mastered the complex lid mechanisms, went on to solve subsequent tasks more quickly than did the greys and they changed tactics immediately when an attempt failed.
Are greys innately more intelligent than reds or it is just that, as an alien species, they had to hone their cognitive skills to survive? Greys, the researchers say, show ‘superior behavioural flexibility’. This ‘may have
facilitated their invasion success, but it may also have resulted from selective pressures during the invasion process’.
Pizza Ka Yee Chow et al. ‘A Battle of Wits? Problem-solving abilities in Invasive Eastern Grey squirrels and native Eurasian Red Squirrels’. Animal Behaviour. 2018.