A Note on the Affinities of the Ebo Forest Gorilla
The discovery of an important new and hitherto unsuspected population
of gorillas was announced in 2004, in Ebo Forest, Cameron (Morgan 2004),
which is at about 4°30' N, 10°30' E. Biogeographically, its importance
is that it lies 100 km to the north of the Sanaga River, which forms a
boundary between sister species or subspecies within other species. For
example, the moustached monkey (Cercopithecus cephus) occurs to
the south of it, and is replaced by the red-eared monkey (Cercopithecus
erythrotis) to the north; it forms a boundary between different species
in three genera of small nocturnal primates (angwantibo - Arctocebus;
needle-clawed bushbaby - Euoticus; Allen's bushbaby - Sciurocheirus);
the mandrill (Mandrillus sphinx) lives to the south of it, and
the drill (Mandrillus leucophaeus) lives to the north, although
according to Grubb (1973) drills are also known from immediately south
of the river's mouth. In cases of more widespread species, the Sanaga
often forms a barrier between subspecies - including, supposedly, the
chimpanzee. Finally, two primate species are actually restricted to the
region north of the Sanaga: Preuss's red colobus (Piliocolobus preussi)
and Preuss's guenon (Cercopithecus - or Allochrocebus -
preussi).
The Sanaga River separates the Ebo gorillas from the known distribution
of the western lowland gorilla (Gorilla gorilla gorilla); the other
subspecies of western gorilla, the Cross River gorilla (Gorilla gorilla
diehli), is also found north of the Sanaga, but 250 km further north,
spanning the Cameroon-Nigeria border, and separated from the Ebo Forest
by Mt Cameroon and the Bamenda Highlands.
Morgan et al. (2003) described and provided some measurements of a skull
which they had found in a nearby village. They noted that in two measurements,
basion-inion and palate lengths, it seemed to resemble G. g. gorilla
rather than G. g. diehli.
To test the affinities of the skull from Ebo, I added the measurements
given by Morgan et al. (2003) to an extensive data file of gorilla skull
measurements, and used Discriminant Analysis (SPSS version 12.0.2) to
compare the skull with samples of the three geographically closest populations:
Cross River (19 skulls); Cameroon Coast (Bipindi and Edea districts, 24
skulls); and Cameroon Plateau (Batouri and Yaounde districts, 25 skulls).
The first of these samples represents Gorilla gorilla diehli; the
other two represent G. g. gorilla.
Discriminant Analysis is a method which uses all available measurements
simultaneously to find the relative amounts of difference between three
or more samples; in this instance, there being three samples, the program
calculated two Discriminant Functions. Having found the differences between
the three samples, I entered the measurements of the Ebo skull as an unknown,
to see which of the three samples it would most resemble.
High scores on the first Discriminant Function mainly indicate skulls
with a relatively long basion-inion length and long palate compared to
the general length and breadth measurements. High scores on the second
Function indicate skulls with a relatively long braincase and long face.
The first Function accounts for 71% of the total variation in the sample;
the second, for the remaining 29%.
As the diagram shows, the three samples do not separate very well, although,
as might be expected, the Cross River skulls are more distinct from the
Plateau and Coast samples than these two are from each other. The position
of the Ebo skull is very interesting: it falls well away from both Cross
River and Coast samples, and at the outer edge of the Plateau sample.
What the Ebo gorilla is certainly not is a derivative of a coastal population
isolated by a southerly displacement of the river's mouth, still less
is it a southeastern range extension of Gorilla gorilla diehli.
It could be a recently isolated population of Plateau gorillas; but, considering
its peripheral position, it is much more likely to be a population that
does not fit into any of the known "demes" of western gorillas
- a relict, that is to say, of a formerly more widespread population living
north of the Sanaga. Indeed, Morgan et al. (2003) drew attention to two
other gorilla populations apparently surviving north of the river.
In this context, it is relevant that the researchers observed drills,
not mandrills, and Cercopithecus erythrotis, not C. cephus,
in Ebo (Morgan, 2004): recall that these are species typical of the north
bank of the Sanaga. If the Ebo Forest were an isolated fragment of the
Cameroon Plateau forests, cut off, say, by a course change of the Sanaga,
one would have expected mandrills and C. cephus.
There seems to be one further respect in which the Ebo gorillas are unusual:
there are "good numbers" of red colobus in the same forest.
This makes the forest of very particular interest: in most areas where
gorillas are present, red colobus are absent, and vice versa. Red colobus
are, for example, absent from almost the entire range of the western gorilla,
and from the lowland portions of the distribution of Gorilla beringei
graueri, between the Maiko and Lowa Rivers. Gorillas in turn are absent
from Korup, where red colobus are numerous, and from the eastern Democratic
Republic of the Congo, broadly speaking north of the Maiko and south of
the Lowa. The main regions where red colobus and gorillas are sympatric
would appear to be the Ngotto Forest (in the Central African Republic)
and the mountain forests west of the Western Rift Lakes.
For quite a number of reasons, therefore, the Ebo Forest gorillas would
seem to be a unique and significant population, which should be protected
as soon as possible.
Colin P. Groves
References
Grubb, P. (1973) Distribution, divergence and speciation of the drill
and mandrill. Folia Primat. 20, 161-177 Morgan, B. (2004) The gorillas
of the Ebo forest, Cameroon. Gorilla Journal 28, 12-14 Morgan, B. J.,
Wild, C. & Ekobo, A. (2003) Newly discovered gorilla population in the
Ebo Forest, Littoral Province, Cameroon. Int. J. Primat. 24, 1129-1137
Prof. Colin P. Groves wrote his PhD
thesis on gorilla osteology and taxonomy. After working at American and
British universities, he emigrated to Australia in 1974. He now teaches
primatology and human evolution at the Australian National University,
and does research on various animals.