Download Africa as a Species Machine

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Approaching a Field-Study of African Wildlife
Personal note. The topic of this assignment is important to me for two reasons. First, it
expresses my conviction that tropical habitats are special—and that, to me, Africa is the most
special tropical place of all. I have tried to teach biology in North and South America, Southeast
Asia, and southern Africa. I have liked all of these places, in different ways, but the first and the
last found their way most deeply into my consciousness. South Carolina is, of course, my home;
her Lowcountry will always be the place of my heart. But if I have a second home, it is in
Africa, where the sounds and the smells and the laughter are a familiar benediction to any child
of sand and palmetto. Besides, for a natural historian, Africa has to be special. In Africa, at the
coming of the rains, frogs build nests in the trees. In Africa, termites taste like cashews. In
Africa, pythons grow to Dream-Time size. And only in Africa can we still observe what the
world of mammals was like at the height of the Pleistocene. When I have taught in Zimbabwe,
my highest priority has been to convince my students that the biodiversity of their Africa is
something truly extraordinary. But this is not a lesson for African biologists only. Rather, the
special-ness of Africa is a lesson for all biologists—indeed for all people—everywhere.
Second, when I’m a little sad—particularly on lonely evenings, when most folks can
become somewhat melancholic—I often cheer myself up by considering the evolutionary
processes that filled the world with its present biota. (Yeah, I agree that I’m a couple of standard
deviations beyond “weird.”) During these pensive moments I do not usually conceptualize
vertebrate natural history as a set of formal phylogenetic diagrams, complete with bootstrap
values and estimates of divergence times. Instead, I visualize evolution as a great dance in which
the world-music is ever-changing and the dancers swirl in intricate patterns—coming together,
dividing, disappearing, arriving, dying, giving birth. I am thankful that I have available, for my
most difficult times, this vision of Life. My dearest hope for Interim ’08 is that Africa will
conjure up this vision for you. My objective in this assignment is to explain, in very dull words,
what the African manifestation of that vision is all about.
Introduction: Illustrating Tropical Biodiversity.
The business of a biologist is to study life, and therefore every good biologist
needs to live for at least a few years in the tropics, where life is seen in so many forms.
Let me illustrate this statement with two tables. The first reports mammalian diversity in
three geographical areas where I have worked— the South Atlantic USA, Vietnam, and
Zimbabwe.1 As you can see, in all taxonomic categories, the tropical areas hold an
enormous numerical advantage.
1
The South Atlantic USA is defined as South Carolina, North Carolina, Virginia,
Maryland, and West Virginia. Although taken from field guides and other publications,
the numbers in my table should be considered approximate. (Decisions had to be made
about animals at the margins of geographic areas, about reintroduced species, about
species recently extinct, and about the splitting or lumping of species and genera.) To be
fair to landlocked Zimbabwe, neither cetaceans (whales, etc.) nor sirenians (sea cows)
were included in the counts for Vietnam or the USA.
2
Place
S.Atl. USA
Zimbabwe
Vietnam
Mammalian Diversity: the Temperate Zone versus the Tropics
Area (km2)
# of Orders
# of Families
# of Genera
394426
8
16
48
390308
13
38
123
329566
11
33
79
# of Species
74
193
118
The second table considers another vertebrate group, the squamate reptiles
(snakes, “lizards,” etc.). We’ll count varieties of these animals in South Carolina and in
my second home, Old Mutare2 (in eastern Zimbabwe).
Place
South Carolina
“Old Mutare”
Squamate Diversity: the Temperate Zone versus the Tropics
Approx. area(km2)
# of Families
# of Genera
70,000km2
7
29
7,500km2
15
61
# of Species
50
96
Note that Africa’s numerical lead in reptilian biotypes is as impressive as its
advantage in mammalian diversity. I am less familiar with birds and insects, but I’m told
that inter-continental comparisons within these groups show similar results. Indeed,
African biodiversity is impressive across a wide range of living taxa—and although the
northern continents3 surpass Africa in the diversity of some biotypes, these are
exceptions, not the rule.4 Now, before we really get started with the scientific stuff, here’s
the first sermon: I want you Interim students to go to Africa with a passion to observe
biodiversity (and I don’t mean just elephants).
Why Such Richness? Africa as a “Species Machine.”
I. Listing reasons for African species-richness. If we admit that Africa’s
biodiversity is impressively high, then perhaps we should ask why this is the case. To
begin my attempt at an answer, I’ll merely list three related reasons. Then, as you
probably expect, I shall discuss each reason in some detail.
First, refugia in Africa have preserved more species than one might expect,
judging by comparison with other continents.
2
Old Mutare is the site of Africa University, the United Methodist institution at which I
have taught for several semesters. Data on squamates are taken from Branch, 1998, and
Conant & Collins, 1991. A species was considered in “Old Mutare” if I thought Branch’s
range map for the critter came within 50km of the place. If a Conant & Collins range
map colored any part of South Carolina, the referent species was credited to the State.
(Note that South Carolina has about 10 times the area of my Old Mutare 50km circle.)
3
Other tropical areas are typically about as rich as Africa—except in really big
mammals. For many taxa, South American biodiversity exceeds Africa’s (at least on a
per-area basis), and the species-richness of South and Southeast Asia is extremely
impressive.
4
As a South Carolina herpetologist, I proudly note two particular exceptions. My State is
blessed with many species of turtles and salamanders—while sub-Saharan Africa is not
particularly rich in turtles and lacks salamanders entirely.
3
Second, evolution in Africa has created more species than one might expect,
judging by comparison with other continents.5
Third, fine-grained niche-partitioning in Africa has allowed sympatric species to
be packed more densely6 than one might expect, judging by comparison with other
continents.
II. Africa as species preserver.7 I shall begin with the preservation factor
because it is super-important and is very easy to understand. As most of you know,
Cenozoic (post-dinosaur) times have witnessed the flowering of countless mammalian,
avian, and squamate8 species. Yet the vast majority of these species have become
extinct, and in most biomes the faunas that we see today give only a glimpse of the
diversity that characterized the pre-Holocene Cenozoic. So, if extinction has been the
rule, then a continent capable of retaining a larger percentage of her ancient species
would be especially rich in vertebrate biodiversity. I shall argue that Africa has been the
species-preserver par excellence because she spans a wide range of both latitudes and
altitudes and because African latitudes are approximately centered on the Equator.
A. The role of latitude in species-preservation. In a sense the theme of this essay
could be, “Why is Africa biogeographically bigger than Eurasia?”9 Eurasia, of course, is
much larger in raw, geographical area. Furthermore, both Africa and Eurasia have
approximately the same amount of latitudinal spread (about 70 degrees from north to
south in each case). However, Eurasia runs basically from the extreme Arctic to about
10oN, with most of the continent lying between the Arctic and 35oN.10 On the other hand
Africa extends between roughly 35oN and 35oS.
Now consider the effects of the Pleistocene’s global coolings, and ask what
happened in Asia and Africa during the Ice Ages. The most dramatic features of these
times were the vast expansions of continental glacial sheets, which extended southward
to about 40o—or even further in upland areas (see map below). During these Ice Ages11
much of the world—even beyond the glaciated areas—became colder and drier.
5
Historically and logically, Point B precedes Point A. However, these points are easier
to understand if discussed in reverse order.
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That is, in Africa, more species can co-occur in the same amount of area.
7
In the argument that follows I shall focus largely on mammals because, in southern
Africa, mammals are often considered to be the very essence of wildlife. Nevertheless,
the basic point holds for many non-mammalian taxa as well, and occasionally I’ll indulge
myself by discussing reptiles.
8
But here’s some trivia: North America has preserved almost all of its squamate species!
9
The importance of absolute size is so obvious that I do not need to discuss it.
10
Leaving aside India and the Malay-Indochinese Peninsula (both of which are special
cases, in many ways biogeographically similar to Africa), the southern latitude of Eurasia
is not far different from the northern extent of Africa. Incidentally, you Wofford students
should be familiar with the latitude of 35oN; that’s about where our college is located.
11
Of course there was not just a single ice age but rather a succession of glacial
expansions and interglacial periods. Indeed, some climatologists argue that nowadays we
are living within another warm time between ice ages.
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However, such climate changes were least extreme in equatorial regions,12 which
remained throughout all changes the warmest and wettest areas on Earth.
In other words, during the Ice Ages, much of northern and central Eurasia was
covered by 3-4km of ice, and most of unglaciated Eurasia experienced a climate roughly
comparable to that of today’s Siberia.13 Only in the Indian and Indo-Malayan14 margins
did Asian tropical habitats persist, and even there the tropical refugia tended to be small,
patchy, and isolated. In Africa, by contrast, the north-to-south width of the tropical
biomes did constrict, but near the Equator they always persisted. Thus, at no time was
any major African habitat-type in danger of complete disappearance.
So, here’s the take-home message: Africa’s vast latitudinal extent, centered on the
Equator, helped preserve the continent’s many, diverse, habitat-biomes. And because
Africa preserved her habitats, Africa also preserved her species.
B. The role of altitude in species-preservation. Africa’s equatorially centered
latitudinal breadth was probably the primary factor in maintaining the continent’s species
diversity. But we must also recognize the role of altitudinal variability in preserving
“islands” of habitat within a “sea” of change. As you know, both temperature- and
moisture-regimes vary with altitude, and mountain ranges create microclimates on their
slopes and in their shadows. Furthermore, African landforms vary tremendously in
altitude, ranging from below sea level to almost 6000m. Thus, during a particularly dry
time, moist environments might persist on mountaintops or along windward slopes.
Conversely, when the general climate was exceedingly wet, residual patches of xeric
Climate remained essentially unchanged at the poles, but that’s irrelevant to the present
story.
13
Many parts of southern Eurasia were cold-temperate in climate.
14
This is why Indo-Malayan Asia is, biogeographically, a “little Africa.”
12
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habitat might still be found within the rain-shadows of a mountain range.15 Given
Africa’s complex matrix of altitudes, when Pleistocene climate-changes altered most
habitats across a biome, patches of the old habitats would usually be preserved.
In other words, during the Pleistocene’s dramatic fluctuations in temperature and
rainfall, major habitat-types might disappear from vast stretches of Africa, but
topographically localized habitat-patches would preserve every African biome—at least
on a small scale. And within these microclimate “refuges,” many African species
managed to survive.
C. Summary of species preservation. Thus, as world climate shifted from warm to
cold, many elements of the rich Asian mammal fauna were pushed into extinction by
habitat loss—while African species were able to remain in habitat by adjusting their
latitudinal or altitudinal ranges. So Africa retained more of its pre-Ice-Ages mammal
fauna than did Eurasia.16
IV. Africa as species-creator. But Africa has been not only a species-preserver;
she has also been an extraordinary species-creator!17
A. The general importance of allopatric speciation. I hope that you are already
familiar with this concept from a basic evolution or high school biology course. The
basic idea is that geographic isolation can be a primary step in species-creation:
interruption of gene-flow across a parent population allows two daughter populations to
“fine tune” their adaptations for their separate environments. If the interruption persists,
the daughter populations can pursue separate evolutionary trajectories, eventually
becoming separate species. To understand allopatric speciation in Africa, we need to
know how various parent populations were split in two. This requires us to consider
again the peculiarities of African climates—and to learn how these climates changed
during the Pleistocene sequence of Ice Ages and Interglacials.
B. Allopatric Speciation: the Continent-scale Big Picture. At a fine-scale level,18
patterns of change were intricate, as I shall eventually try to explain. However, as an
15
Our conventional view of the Pleistocene, based upon the geo-history of the northern
continents, envisions climate-shifts between warm and cold. In Africa, however, the
most dramatic alterations were between wet and dry. That is why, in this section, I have
emphasized the preservation of moisture-defined habitats.
16
Of course there were many exceptions (Asian survivors, African extinctions) to this
generality. Note that arguments for Eurasia also apply to North America, which might be
considered as part of a greater Laurasian landmass. The isolated continents, South
America and Australia, are special cases, never as rich in mammals as Africa (or as
Eurasia at its height).
17 As we shall see, the phenomena of species creation and species preservation are
closely related.
18
I shall not define the term precisely, but by “fine-scale,” I mean blocks of perhaps <
10,000km2.
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initial simplification, let us consider two ecotype-belts that, as the Pleistocene climate
fluctuated, bisected Africa at different times in different directions.19 (Please see the pair
of maps immediately below and also the explanation that follows them. Then consider
some examples.)
1. An east-west axis of rainforest expansion and contraction. During warm, moist
Interglacials, rainforest extended as a broad belt from the Atlantic all the way to the
Indian Ocean. But during glacial extremes, rainforest was limited to patches along
Africa’s west coast and in the center of the continent—plus a few tiny refugia close to the
Indian Ocean. Oversimplification: During wet times, a barrier of rainforest splits
parent-populations of arid-adapted species into two big “islands” of dryness.
2. A northeast-southwest axis of arid-land expansion and contraction. During glacial
periods, when so much water was tied up in ice sheets, a band of desert extended all the
way from from Namibia to Somalia, sundering what at other times was a vast stretch of
rainforest. During moist Interglacials, as explained above, the relevant deserts were
confined to the Somali Arid Zone in the northeast and the Namib Arid Zone in the
southwest. Oversimplification: During dry times, broad barriers of desert split
parent-populations of rainforest species into smaller “islands” of moist habitat.
3. Examples of living biotypes20 with separate populations in 2 present-day “islands” of
dryness. During our Holocene Interglacial, Africa’s rainforest presently dominates
(slightly) and more or less bisects the arid-land belt. Consider in that light the distribution
19 When
we discuss the distribution of African rainfall, we should remember that East
Africa is usually drier than one might expect, given the region’s latitude. This is because
of the “monsoon effect”: during most climatic epochs, the summer warming of the
enormous Asian landmass turns moisture-bearing east-winds northward. This robs East
Africa of much summer rain.
20Note that some of my examples are related species-pairs (or paired species-sets; Oryx
and Gazella have what I’d call “isolated sub-genera” in north and south). Most are
subspecies-pairs or genetically isolated populations without separate taxonomic status
(depending on whom you ask). The range of one biotype (the giraffe) may not be
completely bisected. If you have time, it would be instructive for you to examine range
maps for these animals and also to read accounts of their preferred habitats.
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of 8 African dry-land mammals listed below—all of which have disjunct populations in
the Somali (northern) and Namibian (southern) Arid Zones.
a. The genus Oryx (different species-sets in north & south; a northern variety is shown
below)
b. The genus Gazella (different species-sets in north & south; a northern variety is
shown below)
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c. The giraffe (one species with multiple subspecies or geographic races; the rangebisection for this species is less complete because giraffe are ecologically less strictly
confined to the arid zone; a southern example is shown below)
d. The “white” rhino (one species; 2 subspecies or geographical races; the very modern
distribution of the white rhino has been disrupted by over-harvest and restocking; a
southern example is shown below)
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e. The Grevy’s zebra and the mountain zebra (two ecologically similar species; a
Grevy’s zebra, the northern variety, is shown below)
f. The aardwolf (one species; a southern example is shown below)
g. The bat-eared fox (one species; a northern example is shown below)
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h. The black-backed jackal (one species; a southern example is shown below)
C. Allopatric Speciation: the local, or micro, level. As we have seen, on the
continental level, Africa experienced vast habitat sunderings and reunions, caused by the
alternating expansions and contractions of the two above habitat axes (the east-west wet
and northeast-southwest dry). Simultaneous with these cycles, a similar phenomenon
was occurring on smaller geographical scales throughout much of Africa. As the
continent generally dried out, “islands” of moist habitat could persist on the tops and
rain-slope sides of mountain ranges. As Africa became wetter again, these humid refugia
would expand and dominate—while arid habitats would contract to “islands” within the
rain-shadows of mountain ranges. Thus, throughout the general climatic fluctuations,
populations of some taxa could cling to their habitat-niches by adjusting their
distributions up and down the slopes of Africa’s numerous mountain ranges. In other
words, throughout all the changes, Africa retained remnant habitat islands, strips, and
pockets—all with varying degrees of isolation, all affecting the evolutionary trajectories
of Africa’s biota. And don’t forget the basic point: isolation can lead to speciation!
To illustrate the importance of small biotype
refugia, we can consider briefly the snake fauna of
southern Africa—at a slightly finer resolution.
Today about 25 essentially South African snake
species have relict populations far to the north, in
Zimbabwe’s Eastern Highlands. The highly disjunct
distribution of the rinkals cobra, Hemachatus
haemachatus (right), is a particularly striking (but
not atypical) example.
D. Allopatric Speciation, Summary: Africa as a species-creator. In other words, with
each Pleistocene climate shift, some African species expanded their ranges, some
contracted their ranges—and some were forced into or released from genetically isolated
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refugia. In a sense, the Pleistocene alterations turned Africa into a gigantic speciation
machine.21 The intricacies of this process can be illustrated by considering a single
species-complex, and that is the next task that I shall address. (What follows is complex
and particular to one Tribe of hoofed animals. Biology majors interested in mammals
should definitely read on, but my discussion of duikers may get a bit sticky for other
folks. Also, duikers are not a hugely important component of the mammalian faunas in
the areas we are likely to visit; only one species is found in Namibia.)
V. Africa’s duikers: A more particular example of species-creation and
preservation—and an introduction to the concept of species-packing.
A. Introduction: what are duikers and where do they live? Duikers are small forestantelopes. Ecologically all are concentrate-selectors, which means that they choose
dietary items carefully to maximize concentration of nutrients. Most duiker species in
most habitats obtain the majority of their calories by eating fallen fruit, but all
supplement this diet (particularly for protein) with leaves. The basic duiker body form is
rounded, with proportionally massive hindquarters that are higher than the animal’s
shoulders (below, center); heads are large and are usually held low. Duikers exhibit little
sexual dimorphism; in most species both genders have small, short horns.22 Duikers are
usually considered to be a separate Tribe of the Subfamily Antilopinae, within the Family
Bovidae. They are given this separate taxonomic status in large part because of the
structure of their preorbital glands. These consist of several secreting tissue-layers
encapsulated into a single organ by connective tissue. Each preorbital gland opens into a
hairless slit (illustrated below, right). One slit is on each side of the nose, beneath and
forward of the eyes.23 In most bovids, (simpler) preorbital glands are rubbed against
leaves and twigs to leave smell-markers that designate territory, thus warning other
members of the species against trespassing. This is true for duikers too. In these small
antelopes, however, the preorbital glands have an equally important function in
cementing pair bonds (see “Sociality,” below).24
Duikers are split into two genera (the common duiker belongs to a monospecific
genus) including about 16 species.25 As a group they are found in almost all sub-Saharan
habitats except deserts and the most open grasslands. However, the epicenter of their
current distribution is West Africa and the equatorial rainforest.
21 Of
course this was true, to some degree, for every substantial piece of real estate
(including many aquatic habitats) on Earth. I have argued, however, that for terrestrial
vertebrates, the species-birth/species-death ratio was generally higher in Africa than
elsewhere.
22 In a few varieties only the males have horns.
23The preorbital glands account for the cheek swelling that is particularly noticeable in
males.
24 Apparently all duikers are monogamous.
25 Fifteen species comprise the genus Cephalophus; the common duiker is placed in the
monotypic genus Sylvicapra. Some modern authorities also separate the blue duiker into
its own genus.
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B. Appreciating the general biological importance of duikers. Natural historians find
duikers fascinating in their own right. Furthermore, to the theoretical biologist, these
antelope are particularly important for two reasons: First, they partition their habitat more
intricately than any other “large” terrestrial animal I know about. (For example, 6 duiker
species live sympatrically in the rainforest of Gabon.) A study of duikers’ partitioning
mechanisms could be important for general ecology, and it is clearly relevant to our
eventual interest in African species packing.26 Second, the duiker-Tribe’s distribution
provides insight into Pleistocene speciation processes—creation and preservation—in
sub-Saharan Africa. Here the basic ideas involve (a) waves of cross-continent duiker
colonization during Pleistocene wet periods and (b) stranding of relict duiker populations
in separate refugia during dry periods. During such times of fragmentation, allopatric
speciation could at least begin.
C. Background in duiker natural history. Before we can fully appreciate hypotheses
about the speciation of duikers, we need to learn something about the biology of these
little antelopes.
1. Ecological strategy. Some duiker species are strict specialists, and others can
persist under a wider variety of ecological conditions. However, as suggested above, all
species in most habitats (and some species in all habitats) obtain most of their calories by
eating fallen fruit. This diet is supplemented by leaves, which are usually selected
carefully for their protein/fiber ratio. Most duikers eat a few fungi for calories. Probably
all species eat some animal matter (mostly insects or carrion), and on rare occasions some
species have been observed hunting small vertebrates such as birds or mice. (This
suggests that the basic diet is not over-rich in protein.)
Duiker mouths have a wide gape, and few forest fruits are too big to be taken in
and chewed. Saw-like cheek teeth enable duikers to chew roots and bark; the animals
26Remember,
we said Africa was so rich in mammalian biodiversity for three reasons.
Thus far we have been discussing species-preservation and species-creation. Before our
discussion of African biodiversity is complete, we must also tackle the phenomenon of
species-packing. And can you imagine six species of little hoofed animals living in one
North American forest?
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also occasionally dig for special foods with their feet or even their noses. The precise
nutritional reasons for digging and bark-chewing are not well understood.
2. Sociality. Let us first consider pairing and territoriality. All duiker species are
basically monogamous.27 Pairs (mated, it appears, for life) defend small territories
against same-sex conspecifics. Duikers generally live in low-visibility conditions, and
their territories are apparently designated largely by chemical markers. Most of this
marking is done by the male. Pair bonding is also chemically facilitated. In all species
that have been studied, mated males and females spend a lot of time standing head to
head, pressing their preorbital glands together. It is not known exactly how this cements
pair bonding. (I suspect that gland-pressing mixes both animals’ scents so that
“signposts” put out by the male also carry information about the female pair-member.
Perhaps it’s almost as if the male is signaling, “P.S., I’m married.”) Although territorial
boundaries appear to be marked largely by secretions from preorbital glands, duikers also
leave scent-information from their pedal glands as well as in feces and urine. Probably,
then, entire duiker territories are densely packed with chemical information—and for
duiker family members within territories, the lasting matrix of smells may substitute for
continuous visual contact (which, given the nature of duiker habitat, must be rare).
In all duiker species that have been studied, breeding appears to be nonseasonal—even in strongly seasonal habitats. Neonate birth weight is about 10% of
mother’s weight. Gestation periods are not well known for most species; lengths between
3 and 7 months have been reported. As far as I know, duikers do not commonly
reproduce more than once per year.
3. A few notes on habitat partitioning. A general principle of ecology suggests
that very similar species living in the same area should evolve partitioning-mechanisms
to minimize inter-specific competition. That is, the various species should use different
parts of the habitat or use the same parts in different ways or at different times. This is
what I have called habitat partitioning, and duikers appear to be very good at it, though
the full details of their success are incompletely known. Here are a few clues that
biologists have recently begun to uncover.
Even within habitat that appears uniform to the human observer, different animal
species may restrict their activities largely to tiny patches of microhabitat. For duikers of
similar size,28 the main dimension of such partitioning seems to be based on soil
moisture. Some species select very wet areas; others prefer microhabitats that are
somewhat drier. Such partitioning along a moisture gradient may reflect (or reinforce, or
even cause) dietary specializations. Also, when duiker species exploit very similar
microhabitats, some species are typically day-active while others are generally nocturnal.
These two hypotheses about habitat-partitioning are interesting to me, but their
importance is far from clear, and further research is needed. On the other hand, biologists
are more certain about the importance of duiker size.
27
Generally rare among mammals, monogamy is particularly rare among artiodactyls.
See the following paragraph for information on how duikers select their microhabitat
according to species-size, which appears to be the most important factor in the Tribe’s
habitat-partitioning.
28
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In forests where duikers of different sizes co-occur, primary microhabitat is
selected according to the adult body-size of each duiker species.29 Here is how that
works. Although duikers are anatomically adapted to push their way through thick
habitat, cross-branches of dense understory vegetation will nevertheless impede a
duiker’s movement. So each duiker species selects microhabitat characterized by the
“lowest” vegetation that the species can easily negotiate. The smallest duiker species can
go almost anywhere, but they stay mostly in the low stuff to avoid competition with
larger members of the Tribe. And, as you would expect, progressively larger species
inhabit progressively “higher” understory.30
E. Duiker species accounts. In order to flesh out our understanding of duiker
biology (and before we turn finally to the topic of duiker speciation) we should
learn a bit more duiker natural history. In this context I shall write about two very
different species, the blue duiker (Cephalophus monticola) and the common
duiker (Sylvicapra grimmia).31 In my opinion, these are the two most different
members of the Tribe; when you know how they operate, you will have a good
handle on what duikers can and cannot do. Again, some of this material relates to
a species that we will not see. Therefore, the casual reader (if she or he is
perchance still reading…) might wish to skip to the boxed topic immediately
above Section D, which concerns the more general topic of duiker speciation.
Cephalophus monticola, the blue duiker. These antelopes are small creatures.
Males average about 35.5cm high, with mass of about 4.6kg. As is usual among duikers,
the females are slightly larger, averaging about 36.2cm high, with mass of around 5.4kg.
As the common name implies, this species’ fur often has a bluish cast, but under some
light conditions the color appears slate-gray or grayish-brown. Short horns (about 5cm)
are usually borne by both sexes but are sometimes absent in females.
Blue duikers are diurnal, open-forest fruit-eaters ranging throughout moist central
Africa and into relict forest habitats to the east and south. Males and females live in
monogamous pairs on tiny territories (typically about 2.5-4ha). Territorial boundaries are
marked largely by the male, mostly using preorbital gland secretions. A pair’s entire
territory is probably mapped by a matrix of chemical signals (from preorbital glands,
from pedal glands, from urine and feces).
Pair bonds in C. monticola are established after an intricate courtship involving
aural, visual, and especially chemical signals. Pair solidarity between mated animals is
periodically reinforced when the female and male press their preorbital glands together.
Whenever possible, a pair of blue duikers likes to maintain immediate contact. They do
29
Of course all duikers are small antelopes, and size differences important to habitatpartitioning would probably not be noticed by a casual observer. (Also, in my
experience, all duikers are highly secretive, and a merely “casual observer” would be
unlikely to observe any duikers at all.)
30 Some research also suggests that vegetation “passage-heights” in African rainforest are
distributed in discrete steps, which duiker species-heights (also discrete) have evolved to
match.
31 Note that these two species are depicted above.
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this by making short, soft moans (probably inaudible at more than 10m) and also by
visually signaling with up-down flicks of their tails (which are white underneath).
Gestation in C. monticola is probably about 4 months. Young are weaned after an
additional 2.5-3 months. First-year mortality in one study was estimated at 34%. Annual
adult mortality is about 10%; female mortality may be slightly higher. Young blue
duikers usually out-migrate as yearlings, but unlike many other antelopes, blue duiker
parents make little apparent effort to chase their offspring away. Females usually
disperse first (at 1-1.5 years; males tend to depart their parents’ territories at about 1.9
years). Nearly anything will eat a blue duiker, but major predators include pythons and
crowned eagles. The photograph below shows an intrepid white hunter who has faced
Death and managed to slay the fearsome blue duiker.
Sylvicapra grimmia, the common duiker. Compared to forest duikers (such as C.
monticola), this antelope has longer legs, a flatter back, and straighter horns, which occur
in males only. The color is red to gray. Sizes have been recorded as follows:
Males
Females
Height (Zambia)
Mass (Zambia)
Mass (Botswana)
50cm
12.9kg
18.7kg
52cm
13.7kg
20.7kg
The common duiker is found throughout sub-Saharan Africa except in
rainforest.32 Almost its only habitat requirement is for patches of very dense cover, so
the common duiker can live right up to the edge of villages, and in many African habitats
it is the last antelope to become extinct.
Despite the common duiker’s near-ubiquitous distribution, it has not been well
studied. The animal is highly adaptable, eating fruits, leaves (perhaps more than most
other duiker species), and flowers. In areas where tsetse-control programs attempt to kill
32
If we keep our eyes open, we should be likely to see this little antelope in some brushy
part of Namibia.
16
off all wild ungulates, common duikers may actually increase in numbers—probably
because the removal of major grass-eaters allows cover to grow extremely dense (this
makes common duikers difficult for people and other enemies to find).
Mammalogists assume that the common duiker is monogamous and territorial and
that its preorbital glands play a significant role in courtship and pair-maintenance.
Breeding is apparently year-round, even in highly seasonal South Africa. During
courtship the male may follow the female for several days, making various signals and
eventually licking her genitals before copulating. Gestation has been variously estimated
at 3 months, 5.7 months, and 7months.33 One captive pair bore 11 young over about 8
years; mean inter-birth interval was 259 days. The single offspring is quite precocial; it
can be up and running within 24 hours after birth. Common duikers are preyed upon by
medium-sized mammalian carnivores. Pythons also take a great many. And here is
another intrepid human hunter, this time with a common duiker.
The main take-home point of my species-natural-history discussion is that different types
of duikers might respond in slightly different ways to habitat-alterations caused by the
fluctuating climates of the Pleistocene. If Africa was a speciation-machine, then duikers
comprised excellent raw material upon which the “machine” could operate. This
generalization is discussed at length below.
D. Duiker radiations and speciation. As I have suggested above, duiker speciation
was associated with Africa’s Pleistocene shifts between wet and dry times.34 During
interpluvial35 periods various duiker populations were isolated in moist refugia—in which
they began to follow separate evolutionary trajectories, adapting more precisely to local
conditions. When rainy climates returned and the moist forests expanded once more,
these isolated duiker populations were brought back into contact within a more or less
uniform habitat. Within this wider, wetter, and friendlier world, the best-adapted duiker
33 These
estimates cannot all be correct. Do you begin to understand that biologists do
not know a great deal about this abundant animal?
34 These are often called, respectively, pluvial and interpluvial periods. Why don’t you
make those your vocabulary words for the day?
35Literally, the word means between the rains. I know I should have used the term “dry
periods,” but I really like the way interpluvial sounds. I just can’t help it.
17
forms spread most widely—and often displaced the less-adapted forms, driving them to
extinction by competitive exclusion. Then, when the African world dried out yet
again….
Metaphorically one could consider the pluvial duiker expansions as ocean
waves— waves that would recede as the world dried, leaving isolated populations
stranded on distant “shores.” When the next wet-times duiker “wave” arrived, it could
wipe away (by genetic swamping) the strandings that had been left by the previous wave.
However, if subsequent waves did not reach quite so far, then some ancient strandingspecies would remain in isolation for a very long time.
Some population-strandings from the Pleistocene persist today as relict-species.
Thus Cephalophus adersi (of Zanzibar and Kenya’s isolated coastal forests) is a northern
relict derived from a very ancient radiation of red duikers: no subsequent “waves” ever
reached the habitat-fragments in which it survives. Probably C. natalensis is a relict of a
later “wave” that left this duiker stranded in the south. Fossils suggest that both of these
ancient duiker types once had other populations more widespread throughout the
heartland of Africa’s moist-forest block—but these populations were completely replaced
by overflowing “waves” of duikers belonging to the C. callipygus biotype.36
V. Conclusion. I want to thank you for reading this essay. During an
actual visit, only the dullest heart could find African wildlife less than exciting,
personally, at the visceral level. I also want you to understand that naturalhistory-type observations, which you could make in Africa, can also have serious
intellectual important. So, I have tried to give you some background information
that I have found relevant in my own African work. I am sorry that I have been
so wordy—and that, at times, I have meditates on esoteric examples more
meaningful to me than to you. But please don’t forget the basic points, that
Africa has been a species-creator, a species-preserver, and a species-packer.
Namibia, of course, has played an important role in this Dance of evolution: it has
been the unfailing refuge of aridity, the deep and ancient home of species
requiring a particular sort of southern dryness. I have never been to this place,
and I am so, so excited that you all are allowing me to explore it with you.
36
Our old friend, the blue duiker (C. monticola), was an exceptional case. This animal is
the smallest and probably one of the most ancient members of the Tribe—and yet it is
also one of the most widely spread members. I believe it became so extremely and so
perfectly specialized (to such a widespread habitat) that none of the “waves” washing
over it included an effective competitor.