Saturday 6 January 2024

The Army Surgeon’s White Squirrel of Siam

This article could begin as part of a quiz: What mammal of unusual coloration connects a mission to Thailand, when it was Siam, and a Scottish doctor to my parents’ garden in the early 1960s?

The answer is Finlayson’s Squirrel. Callosciurus finlaysonii is unusual for two reasons. The first is that it comes in a wide variety of colour forms in different parts of its range in south-east Asia. The second is that in one of those colour forms the colour of the pelage is entirely white.

Finlayson's Squirrel, photographed in Saraburi, Thailand
Photograph from the project Noah Website*

It was this white form that was seen and collected by George Finlayson in southern Thailand. He was the surgeon and naturalist to a less than successful trade/diplomatic mission from the East India Company to Siam and Cochin China (Vietnam) led by John Crawfurd (1783-1868) and comprising the following members of the Indian Army plus Mrs Crawfurd: Captain Frederick Dangerfield (1789-1828) was assistant head; he had established a name for himself as a surveyor and geologist; Lieutenant Walter Rutherford (1801-1856) was in command of thirty sepoys. As far as I can ascertain all members of the mission—apart from the sepoys—were Scottish or of Scottish descent.

George Finlayson was born in Thurso in 1790. He became clerk to Dr Somerville, head of the army medical staff in Scotland during the Napoleonic wars. He did so in succession to his elder brother, Donald, who Somerville had moved on to the army medical service, such were his talents. This was a time when clerks served effectively as clinical apprentices. George followed Donald into the army. Donald served in the engagements preceding, and at, the Battle of Waterloo but then came to a sad end. He was assistant-surgeon to the 33rd Regiment of Foot but disappeared as the army marched on to Paris. It was thought Donald had fallen to marauders following in the wake of the retreating French army. George took leave to search for his brother but could find no trace. He was so distraught that the army, at Dr Somerville’s suggestion, moved him to the medical staff about to leave for Ceylon (Sri Lanka). Once there he threw himself into the pursuit of natural history. In 1819, he moved to India as assistant-surgeon to the 8th Light Dragoons then stationed at Meerut.

In order to stay in India for Crawfurd’s mission, when his regiment sailed back to Britain after its service in India, Finlayson moved to a different regiment.

After the mission in south-east Asia, George Finlayson quickly succumbed to what he self-diagnosed as phthisis—tuberculosis, as it later became known. He died on the passage from Calcutta to London on the East India Company’s ship, General Hewett, in 1823.

Finlayson had written an account of the Crawfurd Mission. It was published in 1826 with a ‘Memoir of the Author’ written by Sir Stamford Raffles (1781-1826), founder of modern Singapore and of the Zoological Society of London. Raffles clearly thought very highly of Finlayson.

Finlayson had embarked on General Hewett with the specimens he had collected, including a white squirrel, destined for the museum of the East India Company which was in Leadenhall Street in London. The Keeper was another friend of Raffles, Thomas Horsfield (1773-1859). There Horsfield described the squirrel and named it after Finlayson as Sciurus finlaysonii. He began his description with a quotation from Buffon:

Sc. lacteus dorso flavescente, oculis vibrissis palmis plantisque nigris, cauda pilis nigris raris interspersa.

Ecureuil blanc de Siam, Buff. Hist. Nat. VII. p. 256.

Which translates as: a squirrel, milky with a yellow back; vibrissi, palms, plantar surfaces of feet black, a hairy tail interspersed with rare blacks.

Horsfield continued:

This species is dedicated to the memory of Dr. George Finlayson, (of His Majesty's 59th [2nd Nottinghamshire] Regiment [of Foot],) the naturalist, who accompanied John Crawfurd, Esq. in his mission to Siam and Cochin-China. His health was in a precarious state, from the effects of an Indian climate; and we have to lament that he did not live to return to his native country.

This species has hitherto been mentioned by Buffon alone [in 1789], from the following concise notice in P. Tachard's Travels. “Nous y (at Lonpeen, a village situated in the extensive forests of Siam) vimes aussi des Ecureuils, qui ont le poil parfaitement blanc et la peau tres-noire."—Second Voyage du P. Tachard, Paris, 1689, p. 249.

The following description is extracted from Dr. Finlayson's manuscripts:—“The head and body yellowish white; the head round; the cheeks full; the nose large ; the ears large, plain, not tufted; the iris dark brown; the whiskers long and black; the tail bushy, interspersed with black hairs, and tufted; palms of the feet black. This is an elegant, lively, and active species of Squirrel, almost perfectly white. The body is about seven inches in length, and the tail is equal in length to the body. The eyes are black and lively; and the animal, though white, has not the leucæthiopic habit common in the animals of Siam. It frequents large trees, feeds on their bark and fruit, and is generally seen upon a tall species of Aleurites. One of the specimens was shot by Lieut. Rutherford, on the Islands called Sichang, in the Gulf of Siam."

The India Museum closed in 1879 and the specimens transferred to what is now the Natural History Museum in South Kensington. And there the type specimen—the one shot by Lieutenant Rutherford on the island now known as Koh Si Chang—can be found, catalogued under its original scientific name of Sciurus finlaysonii. In his journal, Finlayson noted that on the group of islands that includes Koh Si Chang:

The [squirrel] is rare, about eight inches in length; an active, lively, and handsome animal.

On the morning of the 13th [August 1822) we landed on the principal island, in pursuit of white squirrels. 

Moving to the 21st century, Finlayson’s Squirrel is now known to vary widely in the coloration of its fur. It varies geographically (the basis of the erection of numerous subspecies, 16 of which are currently recognised). It varies within populations and it varies according to season. Coloration across its range (Burma/Myanmar, Laos, Cambodia, Vietnam and centred in Thailand) varies widely including, as well as white, black, red and agouti. Intergrades between these forms have also been seen where populations meet. An example of variation within a population is in C. f. bocourti, common in Bangkok parks. Individuals may be dark on the dorsal surfaces of the body and forelimbs or entirely white. Variation with season has been described in a population that is maroon-red at one time of year and white at another. Charles Francis in his book published in 2008 made the point that there is a need for more observations in the field and for molecular genetic studies. A key question, of course, is whether all the Finlayson’s Squirrels currently labelled as such are indeed of a single species and whether other forms currently included in different species should be moved to C. finlaysonii. Possible reasons for the great variation in colour and pattern do not seem to have been explored and one can only speculate on the selective forces at play.

George Finlayson realised that white squirrels were present in southern Thailand as well as on the island of Si Chang. It just happens that the formal description of the species was from one shot on Koh Si Chang.

The variations in colour have been shown in several books and papers:

From Francis 2008 - see below

From Wilson, Lacher & Mittermeier 2016 - see below

The coloration of the species varies so much that the name Variable Squirrel has been used and indeed adopted by IUCN for its Red List. Why they should do that when everybody worth knowing has always called it Finlayson’s Squirrel I do not know.

Distribution of Finlayson's Squirrel
Adapted from the IUCN Red List

Horsfield’s use of -ii for the genitive of Finlayson in its specific name of finlaysonii is not the preferred form. Most, and certainly modern usage, would be finlaysoni, the single -i. As the latter it has often appeared in scientific papers and books including Walker’s Mammals of the World in at least some of the editions published since 1964.  However the rules are such that it is also incorrect to change it to the single -i. C. finlaysonii is the original spelling and therefore retained.

Unlike the vast majority of animals that I have kept over the past 65 years I cannot recall how I obtained a female Finlayson’s Squirrel in 1960 or 1961. Had I known then what I know now I would not have done so. Variegated Squirrel Bornavirus 1 (VSBV-1) was first reported in 2015 following reports of the three human deaths in Germany between 2011 and 2013 from of a previously unknown encephalitis. The novel virus was isolated from tissues of a Variegated Squirrel, Sciurus variegatoides, native to Central America, owned by one of those who had died. The virus has since been found in other captive squirrels, including C. finlaysonii and Prevost’s Squirrel (Callosciurus prevostii), in collections in Germany, the Netherlands and Croatia. Those who died were breeders of squirrels, were known to each other and belonged to a squirrel breeding association where they met regularly. Another fatal case occurred in a zoo keeper in Germany, with the virus being found in a Prevost’s Squirrel in the collection. The whole history of the appearance and evolution of the virus has been investigated by studying the molecular epidemiology, together with the activities of the individual keepers and the movement of animals between breeders and from the breeders to private keepers and zoos. Current evidence indicates that a Prevost’s Squirrel was responsible for the primary introduction of the virus.

Prevost’s Squirrels, a beautifully marked species, were commonly available from animal dealers in the 20th century. I have seen them for sale recently in UK and several zoos breed them. I do hope they are all tested, since the fate for those keepers who have caught VSBV-1 has been dire. Fortunately, there have been no cases of human-human infection reported. I do wonder if there have been unreported cases of encephalitis of unknown aetiology from keepers of other species of Callosciurus. For example, In Hong Kong, Pallas’s Squirrel (C. erythraeus) was a popular pet, kept in small cages in small flats and, therefore, in close proximity to the human inhabitants.

Since the discovery of VSBV-1 there has been considerable work in Europe to follow-up these findings with surveys of wild squirrels, zoo workers and the wider human population.

Returning to my Finlayson’s Squirrel of the early 1960s, it was an unsatisfactory addition for the simple reason is that it only appeared from its sleeping box at first light for a short run around its capacious outdoor cage and to eat. Except on rare occasions it stayed in its box for the rest of the day, summer and winter, for years. Only much later did I realise how it could survive on one meal a day. In the wild they are known to eat fruit, seeds, bark, buds, flowers and sap, all of low energy density. I gave the sort of mixture suitable for rabbits or rats: biscuit, wheat, maize, oats, sunflower seed and the like plus fruit like apples and vitamin supplements. All the former have 4-5 times the energy density of tropical fruits. It did not need to emerge for more food and it never or hardly ever did so even in the depths of winter. By contrast, I have seen photographs of tame Finlayson’s Squirrels reared in captivity.

All I remember from around that time is that the Finlayson’s Squirrels were advertised as white. Only much later did I find that not all Finlayson’s Squirrels are white. My guess is that importers wanted white squirrels and that whoever was exporting them from Thailand sorted them accordingly. 

As far as I can recall Finlayson’s Squirrels were only imported into Britain for a relatively short period from around 1960. However, they have now become naturalised as a result of releases/escapes in Singapore, in two areas of Italy from the 1980s, and the Philippines. Genetic evidence suggests they are also present amongst the introduced squirrels of Japan. In short, there is great concern in those regions that they have become a problem as an ‘invasive species’.


Cadar D, Allendorf V, Schulze V, Ulrich RG, Schlottau K, Ebinger A, Hoffmann B, Hoffmann D, Rubbenstroth D, Ismer G, Kibbey C, Marthaler A, Rissland J, Leypoldt F, Stangel M, Schmidt-Chanasit J, Conraths FJ, Beer M, Homeier-Bachmann T, Tappe D. 2021. Introduction and spread of variegated squirrel bornavirus 1 (VSBV-1) between exotic squirrels and spill-over infections to humans in Germany. Emerging Microbes & Infections 10 doi: 10.1080/22221751.2021.1902752 

Finlayson G. 1826. The Mission to Siam, and Hué the Capital of Cochin China in the Years 1821-2. London: John Murray.

Francis CM. 2008. A Guide to the Mammals of Southeast Asia. Princeton: Princeton University Press.

Horsfield T. 1824. Zoological Researches in Java and the Neighbouring Islands. London.

Mazzamuto MV, Wauters LA, Koprowski JL. 2021. Exotic pet trade as a cause of biological Invasions: the case of tree squirrels of the genus Callosciurus. Biolog 10, 1046. doi:10.3390/ biology10101046 

Public Health England. 2019. Human Animal Infections and Risk Surveillance group. Qualitative assessment of the risk that variegated squirrel bornavirus 1 presents to the UK population.

Wilson DE, Lacher TE, Mittermeier RA. 2016. Handbook of Mammals of the World. Volume 6. Lagomorphs and Rodents I. Barcelona: Lynx Edicions.

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