What on earth was the Bald-headed Chimpanzee?

When I was looking through the numbers of individuals and of species of primate that arrived at London Zoo between 1883 and 1895, I came across an animal I had never heard of. It was listed as the Bald-headed Chimpanzee, Anthropopithecus calvus Du Chaillu; Habitat Gaboon, West Africa. Reference was given to three papers in Proceedings of the Zoological Society and to one in Nature. The Zoo had purchased two of these animals, one in 1883, the other in 1888.

I knew that only two species of Pan (which replaced Anthropopithecus) have been recognised for many years and that since the Bonobo does not occur in Gabon (Gaboon) the Bald-headed Chimpanzee must have been lumped into Pan troglodytes, the Chimpanzee. But why did the Zoo think it had specimens of a different species?

I found the answer in the paper in Nature written by Philip Lutley Sclater (1829-1913), the Society’s Secretary. After discussing other possible species of chimpanzee described by earlier authors, he wrote: 

In 1860 the well-known traveller Mr. P. B. Du Chaillu gave his account of the anthropoid apes of the Gaboon to the Boston Society of Natural History (see Proceedings of that Society, vol. vii. p. 296). Mr. Du Chaillu described, as a new species of chimpanzee, Troglodytes calvus, “with the head entirely bald to the level of the middle of the ears behind” and “having large ears” while he identified the N'tchego of Dr. Franquet as being nothing but the adult chimpanzee (T. niger). In a second communication to the same Society [op. cit. p. 358), he described another new species of chimpanzee, with a black face, but the forehead not bald, which he called Troglodytes kooloo-kamba, from its peculiar cry. 

     In 1861 the late Dr. J. E. Gray examined Mr. Du Chaillu's specimens of apes, and came to the conclusion that both his supposed new species were only varieties of the common chimpanzee (see P.Z.S., 1861, p. 273}. Such also, as was stated by Dr. Gray, was my own opinion at that time, and I have remained in a doubtful state of mind on the subject until a recent period. But the acquisition of the fine, female specimen of chimpanzee, generally known by the name of “Sally,” by the Zoological Society in 1883, caused me to change my views very materially. There can be no doubt that this animal, when compared with specimens of the ordinary chimpanzee, presents very essential points of distinction. The uniform black face and nearly naked forehead, which is only covered with very short black hairs, together with the large size of the ears, render “Sally” conspicuously different from the many specimens of the common chimpanzee (at least thirty in number) that the Society has previously re­ceived. I was at first inclined to believe that “Sally” might be referable to the Troglodytes tschego of Duvernoy. But nothing is said, in M. Duvernoy’s description, of the bald forehead; and the small ears attributed to the N'tchego, are directly contrary to this hypothesis, as in “Sally” these organs are exceedingly large and prominent. On the whole, I was inclined to believe that “Sally” might belong to the Troglodytes calvus of Du Chaillu, and she was accordingly entered in the Register of the Society’s Menagerie as the Bald-headed Chimpanzee (Anthropopithecus calvus), which is cer­tainly a very appropriate name, even if it be not technically correct. 

     In the beginning of December we purchased of Mr. Cross, of Liverpool (from whom we had also ob­tained “Sally”) a second specimen of the Bald-headed Chimpanzee, likewise a female, which, although much smaller in size, closely resembles “Sally” in every other respect.

This is the sketch of ‘Sally’ from Sclater’s article in Nature.

The story of the Bald-headed Chimpanzee was taken up by the media throughout the world; Scientific American ran an article in 1889 based on Sclater’s report. The Times (20 December 1888) reported enthusiastically on the arrival of the second specimen that ‘has attracted great attention from naturalists, as being the only example of this distinct form of Chimpanzee known in captivity, and also on account of its remarkable intelligence’.

The reader will have gathered that the criteria for describing and naming a new species were, even with the risk of judging processes and thoughts in the past through 21st-century eyes, not rigorous. It is not then surprising to find that the Bald-headed Chimpanzee came to be lumped, along with other proposed species, into what is simply the Chimpanzee. After a number of changes in taxonomy from a plethora of authors, that species is now known as Pan troglodytes. The chimpanzees from Gabon fall into the Central Chimpanzee subspecies, P. t. troglodytes—the 'Tschego' referred to by Sclater.

The first character Sclater argued was different in the two female chimpanzees were the nearly naked forehead. I see in Handbook of the Mammals of the World that female Chimpanzees ‘in particular, have a tendency to go bald on the crown’. That character, varying between individuals and with age therefore goes by the board. The second character was the size of the ears. I can find nothing in the literature on variation in the size of the ears. However, between childhood and adulthood a clear relative reduction in the size of the ears compared with the head—an example of negative allometry—can be seen. Are there further changes, as in human ears, with age such that the ears become larger? Or is there simply a lot of variation in size and conformation just as, again, in the human population. I suspect, with no quantitative evidence, that this is the case. There is a photograph here of a number of Chimpanzees together and the degree of variation between them in terms of the shape of size of the ears can be seen.

Not only images of ‘Sally’ survive. I found this depiction, described by Finch & Co of London as a ‘rare sculpture’ in bronzed finished plaster by Rowland Ward (1848-1912) better known, perhaps, as a taxidermist.

'Sally'
from Finch & Co website

Sclater PL. (as P.L.S.). 1889. The bald-headed chimpanzee. Nature 39, 254-255.