Squirrel Taxonomy

Squirrels are rodents (i.e., they belong to the order Rodentia) and are conventionally placed in the Sciuridae family, along with chipmunks (Tamias and Eutamias), marmots (Marmota) and prairie dogs (Cynomys). The present state of rodent taxonomy is, however, something of a mess and there are molecular data suggesting that the current phylogenetic scheme is incorrect.

Based on jaw muscle morphology, the Rodentia (from the Latin rodere, meaning “to gnaw”) are broadly divided into five groups (or sub-orders): the Sciuromorpha (modern squirrels), Myomorpha (rats and mice), Castorimorpha (gophers), Anomaluromorpha (springhares and scaly-tailed squirrels), and the Hystricomorpha (capybara, guinea pigs, coypu, porcupine, etc.). Contained within these suborders are some 2,280 species, grouped into 34 families - in all, about 40% of all known mammal species are rodents. Within the Sciuromorpha are three families and, of these, it is the Sciuridae family (Squirrels) that we're interested in here; the Sciuridae consists of 51 extant (living) genera and around 230 species. It is the so-called “tree squirrels” (i.e., those in the Sciurus genus) that are of concern to us here—the Sciurus genus presently contains 30 species in eight sub-genera (Tenes, Sciurus, Hesperosciurus, Otosciurus, Guerlinguetus, Hadrosciurus, Syntheosciurus and Urosciurus). British squirrels represent two of the 13 species within the Sciurus subgenus.

The Sciuridae family contains a wide range of rodents, from the African pygmy squirrel (Myosciurus pumilio) that grows to about 8cm (3.5 in.) to this Malaysian black squirrel (Ratufa bicolor) that grows to about the size of a domestic cat. - Credit: tontantravel

Recent molecular evidence—from nuclear DNA and mtDNA cytochrome b data—strongly suggests, however, that the current scheme is inaccurate and requires a complete overhaul. Among other points, there is doubt as to the validity of some genera (e.g., Microsciurus) and subfamilies within the Sciuridae. More immediately relevant to us here is the subspecific taxonomy of the Red squirrel (and Grey squirrels to a lesser extent), which has been the subject of great debate.

Eurasian Red squirrels

The Eurasian Red squirrel was first described by Swedish naturalist Carl Linneaeus in the 10th volume of his Systema Naturae, published in 1758, based on a specimen from Uppsala in Sweden. Linnaeus gave this rodent the binomial name Sciurus vulgaris, meaning “common squirrel”, and the terrific variation in size and colour of Reds throughout their range has since resulted in a considerable number of subspecies being proposed. In his 1978 The Mammals of the Palaearctic Region, for example, Gordon Corbet lists 61 subspecies of this sciurid. Among these is Sciurus vulgaris leucourus, a subspecies peculiar to Britain and Ireland and first described by Scottish-born surgeon and science writer/translator Robert Kerr in his 1792 The Animal Kingdom, or the Zoological System of the Celebrated Sir Charles Linnaeus (Mammalia).

Generally, the British subspecies is considered separate from Sciurus vulgaris vulgaris, the “type” species originally described from Sweden, based on a blanching (whitening) of the tail during the summer; a phenomenon Corbet describes as a “progressive bleaching of the tail after each moult”. In his 1971 review of tree squirrel subspecies, Polish anatomist Jerzy Sidorowicz studied the craniometric features (i.e., dimensions of the skull) of 1,028 Red squirrels from throughout their range and concluded that there were only 18 valid subspecies. Sidorowicz considered—rather dubiously, it has to be said—that S. v. leucourus was a valid subspecies based on a slightly shorter average skull height than S. v. vulgaris and the lightening of the tail.

Red squirrel (Sciurus vulgaris) skins at the Museum of Natural History of Milan, illustrating some of the variation in colour that has resulted in the description of 61 subspecies. - Credit: Carlo M. Biancardi

Undoubtedly some of the proposed subspecies are valid, although precisely how many is far from unequivocal and I remain unconvinced by some of Sidorowicz's conclusions. Indeed, studies on the skeletal anatomy of Red squirrel specimens from Britain during the early 1980s found insufficient differences to justify assigning the British/Irish squirrel subspecific status. Indeed, although leucourus is still widely referenced in the literature, some biologists have suggested that the repeated imports of Red squirrels into Britain from continental Europe during the last 150 years or so has diluted any British subspecies that did exist.

ln in his article for Animal Aid, for example, John Bryant noted that 20,000 imported Reds were sold in London during 1837, many of which found their way into the wild. Similarly, in a 1999 paper to Molecular Ecology, Elizabeth Barratt and colleagues point out that Reds, mostly from England, were introduced to Scotland at least 10 times between 1772 and 1872 and that the Irish Red squirrel population was augmented with animals brought over from England several times between 1815 and 1880.

In a study published in the journal Mammal Review in 1983, Victor Lowe and A.S. Gardiner applied 14 measurements to 214 Red squirrel skulls from collections in the UK and found that just over one-third had skull dimensions matching leucourus, although interestingly these individuals had coat colours more similar to those of continental animals than that described for the British subspecies. More recent genetic work by Marie Hale and Peter Lurz at the University of Newcastle-upon-Tyne suggests there is generally no significant split between Britain's Reds and those in mainland Europe and that, if leucourus does still exist in Britain it is in the north-west of England.

A Red squirrel (Sciurus vulgaris) in its winter coat. - Credit: Marc Baldwin

A lack of comparable genetic material from museum collections meant their analysis was unable to confirm the presence of leucourus genetically. They did, however, find gene combinations (haplotypes) in squirrels from north-west England that were found nowhere else in Europe. Squirrels in the north-east, by contrast, were closely related to Scandinavian individuals. This ties in well with their analysis of 92 squirrel pelts, which found bleached tails were much more common in western England, with only 8% of pelts having the dark tails associated with continental stock; 54% and 45% of pelts from northern and eastern England, respectively, had dark tails. In their 2003 paper to the Journal of Zoology, Hale and Lurz wrote:

Both genetic and morphological studies suggested that populations in the western region of Cumbria are quite different to populations where introductions of European S. vulgaris are known to have occurred. The tail colour data also suggest that Cumbria is most likely to contain S. vulgaris populations possessing characteristics similar to S. v. leucourus.

In a paper to Conservation Genetics during the following year, Hale, Lurz and Kirsten Wolff reached much the same conclusion. Their data suggest the Scandinavian haplotype had rapidly become the most dominant in north-eastern Britain, despite only appearing in 1966, and that the Reds from Dorset had Swedish ancestry, implying this population was also heavily supplemented, if not entirely introduced, from the continent:

There seems little doubt that the majority of the extant populations of British S. vulgaris are of continental European subspecies ancestry.

Most modern references follow Wilson and Reeder's 2005 Mammal Species of the World and list 23 “recognised” subspecies, but there is considerable debate over the validity of these too.

Eastern Grey squirrel

The Grey squirrel was first described in 1788 by German naturalist Johann Friedrich Gmelin in the 13th volume of his Caroli a Linne Systema naturae based on a 'gray, white and rusty mixed' specimen from Carolina in the eastern US. Its subspecifics are much less extensive than the Red and I am only aware of six proposed subspecies, only one of which, Sciurus carolinensis carolinensis, is found in the UK. Nonetheless, as with the Red squirrel, the number of truly valid subspecies remains to be established, with Wilson and Reeder listing only five.

While the subspecific taxonomy of the Grey squirrel may be less complex, there is an interesting subgeneric peculiarity. In his 1880 Catalogue des Mammiferes Vivants et Fossiles [Catalogue of the Living and Fossil Mammals], the late French naturalist Edouard Louis Touessart proposed that some members of the tree squirrels (i.e., Sciurus) should be grouped together into the subgenus Neosciurus (literally, “new squirrels”, but probably referring to New World squirrels). Touessart didn't justify his reasoning for this grouping and over subsequent re-workings of squirrel taxonomy this subgenus was discarded, until the mid-twentieth century.

A Grey squirrel (Sciurus carolinensis) showing typical fur colouration. - Credit: Marc Baldwin

In A Field Guide to the Mammals of Britain and Europe, published in 1953, Frederik Hendrik van den Brink not only resurrected Neosciurus, but elevated it to generic status and allocated it monotypically to the Grey squirrel. In other words, rather than following the previous schemes, which place the Grey squirrel with all the other living tree squirrels as Sciurus carolinensis, van den Brink considered it sufficiently different to warrant being placed in its own private genus, as Neosciurus carolinensis. Unfortunately, quite why van den Brink considered this justifiable is something that he probably took to his grave—he doesn't state his reasoning in the book, and I'm not aware of any evidence to support his application of Neosciurus, even at a subgeneric level.

Although no thoroughly satisfactory scheme has yet been put forth to group the rodents, for the purposes of this summary I will follow the scheme laid out by Malcolm McKenna and Susan Bell in their Classification of Mammals Above the Species Level. Ergo, that which follows is the most widely accepted and best-supported taxonomic scheme for Red and Grey squirrels.

Red Squirrel (Sciurus vulgaris) Grey Squirrel (Sciurus carolinensis)
Kingdom: Animalia (Animals) Kingdom: Animalia (Animals)
Phylum: Chordata (basic ‘backbone’) Phylum: Chordata (basic ‘backbone’)
Class: Mammalia (Mammals) Class: Mammalia (Mammals)
Order: Rodentia (Rodents) Order: Rodentia (Rodents)
Sub-order: Sciuromorpha (squirrel-like) Sub-order: Sciuromorpha (Squirrel-like)
Family: Sciuridae (Squirrels) Family: Sciuridae (Squirrels)
Genus: Sciurus (Tree squirrels) Genus: Sciurus (Tree squirrels)
Sub-genus: Sciurus * Sub-genus: Sciurus *
Species: vulgaris (from the Latin vulgare, meaning “common”) Species: carolinensis (having a range including North and South Carolina, USA)

* Sciurus is the Latin masculine noun for “squirrel” and stems from the Greek skia, meaning 'shadow' and oura, meaning 'tail' (as in one who 'sits in shadow of its own tail'). For more details on how we classify organisms, see my Taxonomy page.