Thursday 24 April 2014

The origin and diversification of Bushbabies.

Bushbabies (Galagidae) are African nocturnal Primates related to Lorises (Lorisidae). Like the Lorises they are solitary nocturnal foragers living in the tree canopy, but while Lorises are slow, deliberate animals, the Bushbabies often move rapidly through the forest, and are capable of leaping from tree-to-tree like Monkeys.  The group does not have a fantastic fossil record (common for small, forest dwelling animals), though two stem group Bushbabies (i.e. animals more closely related to modern Bushbabies than to modern Lorises) are known from the Late Eocene of Fayum in Egypt and crown group Bushbabies (animals that are considered to be more closely related to some modern Bushbabies than others) appear in the fossil record from the Late Miocene onwards.

The Greater Brown Bushbaby, Otolemur crassicaudatus. Wikipedia.

The classification of modern Bushbabies is also somewhat uncertain; the group was traditionally split into two genera (Euoticus and Galago) and five species, though in recent years three more genera have been erected (Otolemur, Galagoides, and Sciurocheirus) and the group divided into as many as twenty species (the exact number varying due to a lack of consensus among primatologists). Bushbabies have proved difficult creatures to study in the wild, as they are small, nocturnal animals living high up in dense forests. They are also morphologically conservative, i.e. different and distantly related species retain essentially the same bodyplan as this suits their lifestyle well. They also lack the distinctive colouration and markings seen in Monkeys and Apes, as nocturnal animals they apparently rely on scent and vocalization to differentiate one-another. Recent studies have attempted to use genetic data to classify the group, but this has also proved problematic, with studies of different gene groups leading to different classifications.

In a paper published in the journal BMC Evolutionary Biology on 2 April 2014, Luca Pozzi of the Center for the Study of Human Origins at New York University, the New York Consortium in Evolutionary Primatology and the Behavioral Ecology and Sociobiology Unit at the German Primate Center, Todd Disotell also of the Center for the Study of Human Origins at New York University and the the New York Consortium in Evolutionary Primatology and Judith Masters of the African Primate Initiative for Ecology and Speciation at the Department of Zoology and Entomology at the University of Fort Hare describe the results of a new genetic study of Bushbaby phylogeny, which looks at 27 different sets of genes, with the aim of untangling the current confusion about relationships within the group.

Firstly Pozzi et al. found that the Galagidae is a valid taxonomic unit, i.e. all the Bushbabies were more closely related to one-another than they were to anything else. However they did discover some evidence that they were more closely related to Asian Lorises than to African ones. If this is correct (and the evidence is not yet conclusive) then the Lorisidae is a paraphyletic group, and the two Loris groups would need to be placed in different families.

Within the Family Galagidae the genus Euoticus (Needle-clawed Bushbabies) was found to be the sister taxon to all other members of the group, i.e. this genus is thought to have split away from the others earliest, possibly as early as 33 million years ago, shortly after the Eocene-Oligocene boundary. 

The genus Galagoides (Pygmy Bushbabies) was found to be paraphyletic; the West African members of the genus split away from the other Bushbabies early in the group, around 20 million years ago in the Middle Miocene, but the East African members of the genus were found to be closely related to (but still separate from) the genus Galago, from which they split around 14 million years ago. Since the generic name Galagoides was first used to describe West African species, Pozzi et al. believe a new genus should be erected to describe the East African Pygmy Bushbabies. This will be done formally in a separate paper with Judith Masters as the lead author.

Family tree for the Galagidae with estimated divergence dates. Pozzi et al. (2014).

The Early Miocene is associated with a period of cooling in Eurasia and North America, and increased aridity in Africa, with many forest species shrinking into refugia in West and Central Africa. The presence of the earliest fossil Bushbabies in Egypt, and the splitting of the oldest two groups of modern Bushbabies from the rest of the group (Euoticus and western members of Galagoides) suggests that this pattern was also seen in the Galagidae, though as with many other African forest species no fossils are available from this time, as the forests of the West Central African Early Miocene have produced no known major fossil deposits (forests are generally considered poor environments for the preservation of fossils). Two fossil Bushbabies are known from the Middle Miocene of East Africa, Komba and Progalago, though the relationships of these are uncertain, and they are not thought to be ancestral to any modern Bushbabies.

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