The kipunji, and new light on the evolution of drills, mandrills and baboons
In the previous post I discussed the Highland mangabey Lophocebus kipunji, a new monkey whose discovery is an interesting and controversial story. Though initially described only from photos, and not from a museum-accessioned specimen, a sub-adult male was found dead in a trap in August 2005, and is currently accessioned at the Field Museum of Natural History, Chicago. Study of its DNA sequence data has provided new information on the affinities and evolution of the Highland mangabey, and on mangabeys and their relatives as a whole, and this is what we’re going to look at here.
Mangabeys are an entirely African assemblage of cercopithecid monkeys, traditionally grouped together in the genus Cercocebus Geoffroy Saint-Hilaire, 1812. They’re all superficially alike, being long-tailed and long-limbed, and with moderately long muzzles and large incisors. However, molecular studies have consistently found mangabeys to be diphyletic, with the six terrestrial mangabey species forming a clade with drills and mandrills (and with macaques too in some studies), and the two arboreal mangabeys forming a clade with baboons and geladas.
The two kinds of mangabeys also differ from one another in many morphological details. Consequently, it has been widely agreed that the two arboreal species should be split from Cercocebus and given their own genus, Lophocebus (originally coined by Palmer in 1903 to replace Semnocebus Gray, 1870. The latter needed replacing as it was preoccupied by Semnocebus Lesson, 1840, a name now regarded as a junior synonym of Avahi Jourdan, 1834). I like Jonathan Kingdon’s use of the term ‘drill-mangabey’ for the terrestrial Cercocebus species, and of ‘baboon-mangabey’ for the arboreal Lophocebus species, and I’ll use them from hereon. Incidentally, drill-mangabeys are sometimes called eyelid monkeys because of their white upper eyelids, and baboon-mangabeys are sometimes called black mangabeys, for the obvious reason. This division of the mangabeys has been mostly accepted by mammalogists, but not universally so (McKenna & Bell 1997 still treat all mangabeys as the single genus Cercocebus, for example).
If correct, this diphyletic take on mangabey affinities would mean that geladas, baboons, drills and mandrills do not form a clade of ‘dog-faced cercopithecids’ as conventionally thought.
Apparently good morphological support for the non-monophyly of mangabeys came from Fleagle & McGraw’s (1999) study. They found that drill-mangabeys and drills and mandrills shared numerous features that aren’t present in baboon-mangabeys and baboons. In the humerus, drill-mangabeys, drills and mandrills share a notably broad deltoid plane, a proximally extended supinator crest, a broad flange for the brachialis, and a narrow olecranon process with a deep lateral ridge, and there are also characters in the radius and ulna that unite these monkeys to the exclusion of their close relatives. Drill-mangabeys, drills and mandrills are also united by particularly large, rounded posterior premolars, a robust ilium, a reduced gluteal tuberosity on the femur, sharp borders to the margins of the patellar groove, and other characters.
So that’s a pretty impressive list of characters, but note that they’re all associated with the terrestrial foraging style that these monkeys employ. Drill-mangabeys, drills and mandrills search manually through rotten wood and leaf litter, consuming hard nuts and seeds, and audibly cracking them with their large teeth. Furthermore, outgroup comparison (with macaques, for example) indicates that some of these characters are primitive for the cercopithecid clade that includes these species (Papionina). If we only had this morphological data, the possible monophyly of a drill-mangabey + drill/mandrill clade would be suspicious (Fleagle & McGraw 1999). But of course we don’t just have this morphological data, we also have the molecular data discussed above. So things are looking pretty robust.
And this is where the new data from that Chicago specimen of the Highland mangabey comes in. Its genetic sequences independently confirm the relationships indicated by previous molecular studies, and by Fleagle & McGraw’s morphological characters. Though initially described as a third species of baboon-mangabey (Jones et al. 2005), the Highland mangabey’s DNA show instead that it is closer to baboons that it is to baboon-mangabeys, yet it lacks the characters that unite all baboons proper. It therefore needs to be recognized as a new genus, and accordingly it’s now known as Rungwecebus kipunji Davenport et al., 2006. These authors proposed that members of Rungwecebus should now be referred to as kipunjis, and not as mangabeys anymore. Within Papionina, Rungwecebus and Papio form a clade, and the Rungwecebus + Papio clade forms a trichotomy with baboon-mangabeys and geladas. The sister-group to this kipunji/baboon/gelada/baboon-mangabey clade is the drill-mangabey + drill/mandrill clade (Davenport et al. 2006).
Given that we now have dog-faced monkeys variously scattered about a phylogenetic tree that also includes shorter-faced baboon-mangabeys, drill-mangabeys and the kipunji, you should now be wondering about the polarity and evolution of the long muzzle and large body size seen in mandrills, baboons and geladas. Is the long muzzle and large size primitive for this clade, or have these features evolved convergently two or even three times? At the moment, we can’t be sure: if the long muzzle evolved at the base of Papionina, three reversals to the short-snouted condition must have occurred, but if basal members of Papionina were short-snouted, you need either two or three independent acquisitions of the long-snouted condition. Neither scenario is clearly more parsimonious than the other.
Kingdon (1997) argued that the skull morphology of the drill-mangabeys indicates that they are dwarfed, short-faced descendants of large drill-like ancestors. Do fossils help? They might, as there are assorted fossil papionins, some of which (like Pliopapio) are long-snouted, others of which (like Parapapio) are relatively short-snouted (Frost 2001, Leakey et al. 2003). Unfortunately the phylogenetic affinities of these fossil taxa remain contentious.
What’s nice about this whole mangabey-kipunji-baboon subject is that it illustrates a point I’ve been planning to make for a while: namely, that fossils are sometimes all but useless in determining evolutionary relationships, and we most certainly don’t require them in order to uncover phylogenetic patterns. Contrary to what lay-people (and creationists) seem to think, you do not need fossils in order to uncover and reveal the reality of evolution, and even if fossils didn’t exist, scientific logic would lead us to conclude that organisms changed over time.
Don’t get me wrong, fossils are great (err, I am a palaeontologist if I remember correctly), and they can certainly elucidate and inform us about evolution and past diversity, but it’s living organisms, not dead ones, that provide the best evidence for evolutionary change.
I was hoping to cover in this post the rise and fall of the geladas, giant fossil baboons, swamp monkeys, talapoins, and the diversity of macaques. Another time I guess.
Sorry about the toys, it seemed like a good idea at the time. But how many zoologists do you know with both toy baboons and mandrills in their collection?
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Refs - -
Davenport, T. R. B., Stanley, W. T., Sargis, E. J., De Luca, D. W., Mpunga, N. E., Machaga, S. J. & Olson, L. E. 2006. A new genus of African monkey, Rungwecebus: morphology, ecology, and molecular phylogenetics. Sciencexpress 10.1126/science.1125631
Fleagle, J. G. & McGraw, W. S. 1999. Skeletal and dental morphology supports diphyletic origin of baboons and mandrills. Proceedings of the National Academy of Science 96, 1157-1161.
Frost, S. R. 2001. New early Pliocene Cercopithecidae (Mammalia: Primates) from Aramis, Middle Awash Valley, Ethiopia. American Museum Novitates 3350, 1-36.
Jones, T., Ehardt, C. L., Butynski, T. M., Davenport, T. R. B., Mpunga, N. E., Machaga, S. J. & De Luca, D. W. 2005. The Highland mangabey Lophocebus kipunji: a new species of African monkey. Science 308, 1161-1164.
Kingdon, J. 1997. The Kingdon Field Guide to African Mammals. Academic Press (San Diego), pp. 464.
Leakey, M. G., Teaford, M. F. & Ward, C. V. 2003. Cercopithecidae from Lothagam. In Leakey, M. G. & Harris, J. M. (eds) Lothagam: the Dawn of Humanity in Eastern Africa. Columbia University Press (New York), pp. 201-248.
McKenna, M. C. & Bell, S. K. 1997. Classification of Mammals: Above the Species Level. Columbia University Press, New York.