Sunday, 28 July 2019

Small British Mammals: Squirrels

Red squirrel
The majority of rodent species look, more or less, like mice or rats, depending on their size. But there are a number of exceptions among the many, many, kinds of rodent in the world. Probably the best known such exceptions in Britain are the squirrels.

By far the most common tree squirrel in Europe, and perhaps the most widespread species of squirrel anywhere in the world, is the red squirrel (Sciurus vulgaris), sometimes called the "Eurasian red squirrel" to distinguish it from the North American animal of the same name. This lives almost everywhere in Europe, apart from the Mediterranean islands, southern Greece, and Portugal, and is one of a relatively small number of native wild mammals in Ireland. Further east, they inhabit just about every temperate, forested, region of Asia, reaching as far as Korea, northern Japan, and the coast of the Bering Sea.

About twenty different subspecies are currently recognised, but there seem to be few reliable ways of distinguishing between them. For instance, while most red squirrels are, in fact, red, a significant proportion are darker, or even near-black. These are apparently more common in the dense highland forests of the Alps and the Altai Mountains, but are not a reliable indicator of subspecies, since they are found in other populations, too.

Much of this area is covered by coniferous forests of pine, larch, and spruce, but red squirrels seem almost equally at home in mixed broadleaf forests of oak, beech, and so on. Since they don't hibernate, they do prefer there to be at least some conifers around, however, providing a reasonably steady supply of food through the winter. They are also less affected by fragmentation of their habitat than some other woodland species (although, obviously, the less woodland there is, the fewer of them there will be), unless travel between the isolated patches becomes particularly hazardous. At least in the more northerly parts of their range, cities can actually be beneficial to squirrel populations, so long as they contain a sufficient amount of wooded parkland.

Red squirrels are primarily herbivorous, with pine cones seeming to be their favourite food, although they won't turn their nose up at almost any form of plant matter in the right season, and, like many 'herbivores' also eat a small quantity of insects, bird eggs and the like when that's what they can find. Famously, squirrels bury acorns, pine cones, and other suitable food in caches where birds can't find them, coming back to them months later when there is less to be found on the trees; there is some evidence that they do this more frequently in deciduous than coniferous woodland.

While red squirrels do seem to have at least some memory for where they left their nuts, they also find such caches by sniffing them out, so that perfect recall isn't always necessary. As one might expect, several of the seeds aren't recovered before they can germinate in the following spring, so that squirrels help with dispersal of pines and other such plants. There is a delicate balance here, since it's also true that the squirrels that are best at finding caches and digging them up again end up fitter than their less fortunate brethren, and have more children as a result. Red squirrels also cache fungi, which can be a significant food source at some times of the year, hiding them in tree branches where they can dry out and be preserved.

Red squirrels are active during the day, spending almost all their time foraging for food, although they do take a mid-day nap in the summer when the longer hours of sunlight allow them to stay up late. Their eyes are, like those of most primates, adapted for daylight, and they can probably see in colour, although not as well as we can. Their other senses are also good, and, not only do they use chirping alarm calls when threatened, but they can recognise these in other species, too.

Each squirrel spends most of its time in a relatively small area, with stronger and more dominant females being particularly aggressive in defence of their territory, and only moving when there's a really good reason to do so. In deciduous woodlands, when food is scarce, there can be some relaxation of this, with different adults feeding close by one another when they do find a useful food resource... but they don't seem to be happy about it, and the usual wary tolerance sometimes breaks out into violence.

The males, on the other hand, are less territorial, moving about as necessary to maximise their chance of meeting females.

Red squirrels spend the night sleeping in spherical 'dreys', often around 30 cm (1 foot) across, constructed primarily of twigs and lined with leaves and moss on the inside. These are usually located at least 6 metres (20 feet) above the ground, close to a tree trunk, and hidden by vines if the squirrel can find any. The wall is quite thick, so that the inner chamber is only around half the diameter of the nest, and can be surprisingly warm in winter. The squirrels move between multiple nests over the course of a year, presumably to reduce the chance of being found by a predator, and, being anti-social, only share when they are sexually active, if at all.

The breeding season lasts from late winter to early summer, long enough to allow a sufficiently fit female to birth and raise two litters in a year. They attract males by leaving scent marks from facial glands and, perhaps more importantly, by mixing vaginal secretions with their urine. The mating ritual is fairly straightforward, with a gang of anything up to a dozen males chasing after a single female, and only the physically fittest getting to mate with her. Litters are of up to six young; most do not survive their first winter, but those that do become sexually mature at around a year of age, and can expect to live another couple of years beyond that in typical wild conditions.

The only other tree squirrel native to Europe is the Persian or Caucasian squirrel (Sciurus anomalus) which mostly inhabits the Middle East, but is also found in European Turkey and on the Greek island of Lesbos. But the important qualifier here is "native".

Eastern grey squirrel
In the British Isles, red squirrels are only common in Scotland and Ireland, although there are some in northern England and Wales, and few scattered populations elsewhere. They used to be everywhere, but their numbers are now declining across the board. The reason is the eastern grey squirrel (Sciurus carolinensis), known simply as the "grey squirrel" in Europe, due to the absence of the western sort.

The grey squirrel is not native to Europe, but to North America, where it inhabits almost the entire eastern half of the US, as well as southern parts of Saskatchewan, Manitoba, Ontario and Quebec. Apart from the grey colouration (and, as with red squirrels, they can sometimes be black instead), grey squirrels lack the long ear-tufts of the red species. They are also quite a bit larger on average; there is some overlap in size, but red squirrels reach a maximum of about 250g (9 oz.) and the grey kind just over 300g (11 oz.)

Grey squirrels were first imported into Britain in 1876 when, for some unimaginable reason, it became fashionable to have them in country estates. They spread rapidly, and were brought across to Ireland in 1913, where they had a similar effect, and currently occupy essentially the entire eastern and northern parts of the island.

Nor is Britain alone. Attempts to introduce grey squirrels to Hawaii and Australia both failed in the long run, while those brought to South Africa don't seem to like it much there, and haven't managed to spread. But they have been introduced to the western US, where they aren't native, and now have populations along the western seaboard, most notably in California, where it seems to be harming the native western grey and Douglas squirrel populations. Perhaps more significantly, grey squirrels have also been introduced into Liguria in Italy, where they are causing much the same damage as in Britain.

Wherever grey squirrels become common in Europe, the native population of red squirrels crashes. It's not that the grey squirrels physically beat up the smaller red ones; there's no real evidence that they even care whether or not the reds are around. However, they do eat essentially the same food, and are active at the same times of day, living in the exact same habitats. The forests just aren't big enough for the both of them... but they do have two other advantages in their favour.

Firstly, grey squirrels find it easier to digest acorns, so that they do better in any forest with a significant number of oak trees, while still eating everything else that the reds eat. Secondly, they seem to be slightly better at the scatter-hoarding thing, not only having a better memory for where they hid their food, but being able to find and dig up caches hidden by red squirrels, leaving them with nothing to survive on through the winter.

A partial selection of "tree" squirrels
As if that wasn't enough, grey squirrels in Britain (though not Italy) carry a poxvirus that, in a parallel to the decimation of Native Americans after the arrival of the white man, is harmless to them but deadly to the red species.

In addition to their effects on the native wildlife, grey squirrels also cause economic damage in Europe by their habit of stripping the bark from trees (presumably this is tastier than those in America, or the trees are less adapted to recovering from the damage). Introduction of grey squirrels to Britain was banned in 1937, but, by then, it was far too late to have much effect. An attempt to eradicate them from Italy in the late '90s was suspended as a result of court action just long enough for it to become impossible to complete the task.

Having gained a toehold in Europe, it's inevitable that grey squirrels will continue to spread, and that the native species will decline as a result. It's much harder to know how fast that will be, given the differences in habitat between Britain and the continental mainland and the possible success or otherwise of any efforts to protect local biodiversity. It's not even terribly easy to assess what's happening in Britain, since grey squirrel dreys look much the same as those of the reds, and the only reliable method of short-term population monitoring is, therefore, to find and analyse the hair they leave behind, which isn't cheap. (Simply looking for the squirrels will help with evaluating trends, but not for a census).

One analysis concluded that, if everything goes perfectly for the conservationists, we may be able to hold off grey squirrels from invading France and Switzerland until the latter part of the century, but that, if not, it could happen as soon as 2026.

Red and grey squirrels are similar because they are fairly closely related. They are not, however, each other's closest relatives, and it's the red squirrel that's a bit of an outlier here, since the great majority of tree squirrel species live in the Americas, and the red is one of only four species that don't.

This however, is only true for a narrow, and rather technical, definition of 'tree squirrel'. The squirrel family includes almost 300 different species by current estimates, and it's likely that more remain to be discovered, especially in the tropics. "Tree squirrels" are just one group within the larger family and by no means the only ones to live in trees - the flying squirrels being perhaps the most obvious other example.

Nonetheless, many squirrels have come down out of the trees in order to live on the ground. The various kinds of ground squirrel form a distinct cluster within the squirrel family tree, typically designated as a subfamily. The fact that they cluster together like this strongly implies that their ancestors were tree-dwelling, and that it was the other groups that didn't change - and, in any case, the oldest known fossil squirrels appear to already be adapted for climbing.

The squirrel family
Green - arboreal, Brown - terrestrial, Black - mixed
The best-known ground squirrels are likely the chipmunks, marmots, and prairie dogs, but there are many others, too. Chipmunks are almost entirely North American, although there is one species native to Russia, China, and some neighbouring countries. Like the eastern grey squirrel, this species, the Siberian chipmunk (Eutamias sibiricus), has been introduced to Europe, but, unlike its tree-dwelling counterpart, it hasn't really managed to escape much beyond urban parks on the continental mainland and hasn't made much of a nuisance of itself - although there is some concern about its ability to carry Lyme disease.

However, squirrels seem to be the sort of animal that people just like bringing into non-native habitats, because there are two other species of squirrel found in Europe that don't belong there. These are both "ornate squirrels" (Callosciurus spp.), arboreal animals native to southeast Asia, and are relatively recent arrivals in Italy and the Netherlands. They haven't yet spread far, and it's perhaps too early to say if they're likely to be a real problem... but by the time we know, it might be too late to do anything about it.

But it's not all bad news. One species of rodent recently introduced into Britain is arguably having something of a positive effect, and it is to that that I will turn next...

[Photos by "4028mdk09" and Ken Thomas, from Wikimedia Commons. Cladograms adapted from Oshida & Masuda 2000, Arslan 2009, and Crosby 2012.]

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