Mammals in the forests of eastern africa - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 239

populaions may have progressed further towards
specialised forest-living and moisture dependence
than some easten populaions.

sideraions. hen isolate subspecies of forest
mammals from far easten Africa are examined
they fall into three very disinct categories: mon-
tane, lowland and island.
Seventeen subspecies of mammals have been
described from the islands of Zanzibar, Pemba
and Mafia. An equal number have been described
from the mountain massifs of eastern and south-
westen Tanzania. The only clearly deined dis-
crete focus for lowland forest mammals on the
mainland is found in the narrow enclaves of forest
that run up from Mozambique to the Kenya coast,
from which some eight subspecies have been
named. All share the common characterisics of
physical and ecological isolaion. Major rivers
along the easten seaboard are known to define
the boundaries of some savanna mammal dis-
tributions, notably the Rufiji. It remains to be seen
whether forest mammals have also been limited
by rivers or by Pleistocene oceanic incursions up
the Ruiji estuary. hen the detailed composiion
of subspeciaing mammals is examined it is clear
that broadly the same groups are involved, namely
cercopithecid primates, insecivores, squirrels,
viverrids, carnivores and duikers. In the
impoverished communiies to which they belong,
each of these groups seems to fill a single, rather
broader niche than their westen congeners and,
in most instances, they are relaively numerous (or
have been unil recently), even if only in very
restricted localities.
Several species from the Kenya coastal enclave
have markedly different forms in Tanzania,
notably Rhyncoyon ini chysopygus, Colobus
badius uo mitratus and Bdeogale crassicaua
omnivora. It is possible that this communiy once
had some direct westerly connecion across what
is now arid and once volcanically acive land.
It has been postulated elsewhere (Kingdon,
l 982a) that the overall evoluionary trend in the
main line 'red' duiker lineage has been a phylo-
geneic increase in body size. On this criterion
Ader's duiker, at 6.5-12 kg, is the most conserva-
ive species in the enire red duiker radiaion. It
may therefore be significant that we find it
'stranded' on two marginal forest outliers,
Zanzibar and the Kenya coastal enclave.
Cphalphus spadix is another duiker restricted

Island or land bridge?

There has been much theoretical discussion as to
whether Tanzanian montane biota belong to
oceanic island analogues or to broken land bridge
islands (Diamond & Hamilton, 1980; Diamond,
1981; Stuart, 1981). Recruitment from non-
forest sources has had scant discussion. As far as
mammals are concened, we have tried to show
that all three elements seem to be present in the
mammalian fauna of easten Tanzania. Fur-
thermore, intrinsically 'land bridge' type species
(such as Beogale and Cicetomys) can be idenified
in the fauna of actual islands such as Zanzibar
(Pakenham, 1979).
It could be argued that phyletically ancient
groups such as the golden moles and elephant
shrews are more likely to belong to land bridge
fauna, while recently evolved, versaile species are
more likely to be oceanic island type colonists.
This, however, takes little account of widely dif-
fe ring rates of speciaion within mammalian taxa.
Some isolated species might have been able to
maintain their basic genotype over millions of
years while others difereniate very readily in
response to local peculiarities of climate.
Someimes both elements may be present in the
same species. For example, Rhyncoyon is known
to be a very conservaive genus. Its anatomical
peculiariies and ecological niche date back to the
Oligocene. Its colouring, on the other hand, is
highly labile and variable, so the criteria for dis-
tinguishing endemic subspecies can ulimately
tum on our procliviy to seize upon the more
obvious and superficial aspects in an array of
museum skins.

Forest endemics of restricted
disribuion in easten Africa

Several important mammalian endemics have a
much more restricted disribuion than the spe-
cies listed above, but discussion of subspeciaion
in local populaions involves rather different con-