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Darwin Correspondence Project

From J. E. Gray   2 April 1871


My dear Darwin

The two toed sloth and the black crested the toed Sloth Brady torquatus are Both Sexes alike   But the females Ai (Arctopithecus Gray) are covered with uniform fur varing in length and colour in the different Species   The Male have a path of short shining very soft hair between shoulders1   In three out of the four species in the B.M this patch is large and of a bright orange yellow color with a central black streak and black spots on the edge2   In the other (A flaccidus) the patch is small and pure white with a short black central streak3

The female of A. griseus4 has a small pure white tuft of very soft hair like a powder puff on each side of the back over the loins almos hid from view by the longer hairs of the fur   I have not found these tuft in the females of the other species   Buffon who knew the male of one species called the Ai a dos brulee which Lesson has named Acheus ustus.5 Teminck thought the spot was produced by the long hair being worn off6

Wagler in 1813 suggested they were the males and lately I have been able to verify the fact7

With kindest regards | Ever yours sincerely | J E Gray

I did not learn to write with my left hand until I was more than 70 so excuse its badness8


No letter from CD to Gray concerning sloths has been found. Two-toed sloths are in the genus Choloepus (family Megalonychidae); three-toed sloths in the genus Bradypus (family Bradypodidae). Bradypus torquatus, the maned sloth, is also known as the ‘aï’. Gray’s reference to Arctopithecus cannot be identified because the name is now nomen nudum, that is, does not have any type specimen attached to it. For more on the synonymy of Bradypus and Arctopithecus see Gardner 2007–, 1: 158–64. Females of Bradypus torquatus have different coats from the males (Lara-Ruiz and Chiarello 2005, pp. 66–8).
Gray refers to specimens in the British Museum. CD cites this description in Descent 2d ed., p. 534.
Arctopithecus flaccidus is now Bradypus tridactylus (the pale-throated sloth).
Arctopithecus griseus is now Bradypus variegatus (the brown-throated sloth).
Gray refers to George Louis Leclerc, comte de Buffon, and René Primevère Lesson. For ‘Dos brûlé’, see Buffon 1749–1804, 13: 62. For Acheus ustus (now Bradypus variegatus brasiliensis), see Lesson 1840, p. 271.
Gray refers to Coenraad Jacob Temminck and Temminck 1820, p. 212.
Gray wrote ‘1813’ in error and refers, in fact, to Georg Johann Wagler and Wagler 1831, p. 607. Wagler actually referred to Bradypus cuculliger (now B. tridactylus) and B. ai (now B. variegatus brasiliensis). He described the females as being distinguished from the males by the possession of dark spots.
Gray’s use of his left hand was probably due to the paralytic stroke he suffered in 1869, from which he never fully recovered (ODNB).


Buffon, George Louis Leclerc, comte de, et al. 1749–1804. Histoire naturelle, générale et particulière, avec la description du cabinet du roy. 44 vols. Paris: Imprimerie royale.

Descent 2d ed.: The descent of man, and selection in relation to sex. By Charles Darwin. 2d edition. London: John Murray. 1874.

Lesson, René Primevère. 1840. Species des mammifères bimanes et quadrumanes; suivi d’un mémoire sur les oryctéropes. Paris and London: J.-B. Baillière.

ODNB: Oxford dictionary of national biography: from the earliest times to the year 2000. (Revised edition.) Edited by H. C. G. Matthew and Brian Harrison. 60 vols. and index. Oxford: Oxford University Press. 2004.

Temminck, Coenraad Jacob. 1820. Sur le genre Bradype et description d’une espèce encore peu connue. Annales générales des sciences physiques 6: 204–18.

Wagler, Johann Georg. 1831. Mittheilungen über die Gattungen der Sippe Bradypus. Isis (1831): 604–12.


Sexual differences in sloths. J. G. Wagler article on sloths [Isis 24 (1831): 604–12].

Letter details

Letter no.
John Edward Gray
Charles Robert Darwin
Sent from
Source of text
DAR 88: 95–6
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 7652,” accessed on 4 August 2020,

Also published in The Correspondence of Charles Darwin, vol. 19