Road accidents seem to be behind the natural selection of black squirrels in US cities, as they may be easier than grey ones for motorists to spot and swerve to avoid.
Grey squirrels (Sciurus carolinensis), known as eastern gray squirrels in the US, have been introduced in countries throughout the world, but their native range is eastern North America. Despite their name, they聽have actually a variety of colours. The grey version is the most common, but there are also black forms and rarer white ones.
Pockets of the black squirrels appear in a variety of places, such as the city of Syracuse in New York state, where they make up half of聽S. carolinensis聽numbers, says聽聽at the Hobart and William Smith Colleges in Geneva, New York state.
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In 2023, Cosentino and his colleagues suggested that predators more easily spotted the black squirrels in rural areas, so natural selection was in woodlands, but that doesn鈥檛 explain why black squirrels are doing better than greys in the city.
Now working with another team, Cosentino did 50 roadkill surveys across autumn 2022 and spring 2023, covering a total of 2600 kilometres in and around Syracuse to see which squirrels were being killed by motorists. 鈥淩oadkill is a really common way for squirrels to go in cities,鈥� he says.
The researchers also looked at more than 100,000 photos of squirrels in the area to work out the prevalence of the black and grey morphs.
鈥淲ithin about 10 to 12 kilometres from the city centre, we found that the melanics are about 30 per cent underrepresented among roadkill,鈥� says Cosentino.
He and other researchers have already found that people than they do grey squirrels. 鈥淚f drivers can spot them even a second earlier, that could be advantageous,鈥� says Cosentino.
His team is now suggesting that as black squirrels are more easily seen and avoided on roads than grey ones, road deaths are driving their natural selection in urban Syracuse, while predation is working against them in rural areas.
It can鈥檛 be ruled out that behavioural mechanisms are also at play, though, such as black squirrels potentially taking fewer risks with road crossings, says Cosentino.
鈥淚t鈥檚 an interesting speculative聽hypothesis, the evidence they produce is consistent with the idea, but that is about as far as we can confidently go with correlative data,鈥� says at Oregon State University.
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