XENOTHRIX - A MYSTERY MONKEY FROM JAMAICA

Prince Bernhard's titi Callicebus bernhardi – a relative of Xenothrix? (Dr Marc van Roosmalen)

Down through the centuries, several remarkable, unique species of mammal have become extinct on various West Indian islands in the Caribbean. In two ShukerNature blog posts on this subject, I shall be examining certain controversial examples that conceivably survived into much more recent times than officially accepted – and may even be still alive today. And I begin, via this first blog post, with a truly mysterious monkey.

Today, some monkey species inhabit Jamaica, but none of them is native; they are all South American or African species that have eventually established themselves following the escape or release of pets or other captive specimens here during the 18th Century or later.

However, there is at least one enigmatic report of monkeys existing on Jamaica prior to this time scale. In Hans Sloane's two-volume tome, A Voyage to the Islands Madera, Barbados, Nieves, S. Christophers and Jamaica (the two volumes were published in 1707 and 1725 respectively), when documenting the fauna of Jamaica he included a brief but tantalising mention of monkeys "found wild in this island". What could these have been?

On 17 January 1919, a remarkable discovery was made that may have provided the long-awaited answer to that question. That was the day when palaeontologist Harold Anthony from New York's American Museum of Natural History disinterred a mandibular (lower jaw) fragment and femur of a monkey in the yellow limestone detritus of Long Mile Cave in Jamaica's Cockpit Country. As these were discovered not too far away from some human remains, Anthony wondered whether they were from an introduced monkey specimen (the pet of a seafarer, perhaps?).

After they were collected, these bones remained undescribed and forgotten for many years, until, in 1952, two graduate students, Karl F. Koopman and Ernest E. Williams, discovered them in a drawer at the American Museum of Natural History. And when finally examined, they surprised everyone, because they combined features from several different types of New World monkey. The mandible's dental formula, for instance, differed from that of all New World monkeys except for the marmosets and tamarins, but its size was much bigger than the mandible of these latter species. Further studies emphasised similarities between the Jamaican monkey and South America's titis and douroucoulis (night monkeys). Consequently, when formally described, this anomalous species was placed within a new genus, all to itself, and was christened Xenothrix mcgregori.

A pair of douroucoulis or night monkeys (birdphotosDOTcom/Wikipedia)

During the 1920s, Anthony uncovered some additional material from this species, including post-cranial remains such as an os coxae (a bone from the pelvic girdle), and two tibiae. As for the femur from 1919, this was not included in the 1952 description of Xenothrix mcgregori, remaining unstudied until 1991. When its specific form was closely assessed, however, scientists concluded that Xenothrix mcgregori habitually moved and climbed in a slow quadrupedal manner, while hanging upside-down from branches (and even feeding in this inverted position), closely analogous therefore to a tree sloth, and thus very different from any living species of New World monkey.

Further remains, including part of the lower face from one specimen, were unearthed by various expeditions to Jamaican caves between 1994 and 1996, and these supported the notion that X. mcgregori was most closely related to the titis, although one researcher believes it to be a Jamaican species of douroucouli. Moreover, the precise nature of its dentition indicated that it was primarily frugivorous (a fruit-eater), and estimates of its size have proposed that it probably weighed 2-4 kg.

In short, X. mcgregori was clearly a very distinct, valid species in its own right, and certainly not merely based upon specimens of introduced, non-native species. But when did it die out? A partial skull and palate of X. mcgregori found in Lloyd's Cave near Jackson's Bay, Jamaica, were discovered in surface debris together with remains of various domestic animals and also introduced black rats Rattus rattus, and just like those they were unmineralised and unencrusted. This suggested that X. mcgregori was still alive at the time when Western explorers such as Christopher Columbus first reached the West Indies (i.e. the late 15th Century). But could it have survived even later? The Sloane reference in the early 18th Century to monkeys in Jamaica provides one intriguing indication of this, but there is another, even more baffling piece of evidence to consider too.

1860s engraving of the mystifying poto

During the 1860s, a strange creature referred to in the publicity literature as "a poto from the mountains of Jamaica" was exhibited in London. However, the accompanying engraving of this animal depicts a creature wholly unlike any species known to exist in Jamaica today. Consequently, some researchers have speculated that it may have been a living specimen of X. mcgregori. However, to my eyes it looks nothing like any type of monkey – on the contrary, what it does look very like is a kinkajou Potos flavus. However, even if it was a kinkajou, that only adds to the confusion, because this small raccoon-related species of carnivore is not native to Jamaica either – only to mainland Central and South America. The same is also true of its superficially similar relatives, the olingos. Moreover, it is nothing if not worthy of note that the name 'poto' is very similar to 'Potos', the kinkajou's taxonomic genus.

And as a further, final twist to this already much-tangled tale: when the original femur of X. mcgregori that had been obtained back in 1919 by Anthony was finally examined during the 1990s, it was closely likened not to that of any species of monkey, but instead to that of...the kinkajou!

1849 colour engraving of a kinkajou

Incidentally, two other endemic Antillean monkeys, both of which were related to X. mcgregori, have been described from disinterred remains. These are the Hispaniolan monkey Antillothrix bernensis and the Cuban monkey Paralouatta varonai. The latter died out during the late Pleistocene epoch (and a second Paralouatta species died out even earlier, during the Miocene). The Hispaniolan monkey, conversely, whose remains have been found in the Dominican Republic, may have persisted until the 16th Century, its demise precipitated by the island's settlement in 1492 following Columbus's arrival there. In July 2009, a skull of this vanished species was discovered by Walter Pickel while diving in one of Hispaniola's underwater caves.

Reconstruction of the Cuban monkey Paralouatta varonai (© American Museum of Natural History)