The distribution of the water rat Dasymys (Muridae) in Africa: A review

Abstract

The genus Dasymys is widespread throughout sub-Saharan Africa. Once regarded as monotypic, we described 11 morphologically defined Dasymys species in a previous study. Dasymys is adapted for living in mesic areas, which possibly explains its disjunct distribution throughout its range. Here, we postulate that the distribution of the 11 species in Africa can be explained as reflecting the presence of water sources, which in turn define current geographical features, as well as the palaeoclimatic history of the continent. We compare the distribution of Dasymys with that of other animals, particularly other rodents, to examine whether there are similarities in occurrence across taxa with similar habitat requirements and to consider how geographical features influenced the distribution of small animals. The fossil record indicates that Dasymys originated in southern Africa, implying that the genus migrated northwards into the rest of Africa. Dasymys is divided into two morphologically defined groups: a southern group (D. capensis, D. incomtus, D. robertsii, D. shortridgei and D. nudipes) and a northern group (D. medius, D. montanus, D. griseifrons, D. longipilosus, D. foxi and the D. rufulus complex). Within the northern group, two possible radiation routes are suggested, one from Mozambique to Ethiopia and the other from Zambia to northern Angola and northwards through Central Africa to West Africa. These routes are based on the past or current presence of continuous water features in conjunction with morphological affinities observed for Dasymys species. We envisage that our model may be used to generate testable hypotheses in future investigations; for example, genetic studies could be conducted to complement the morphometric classification.

Full text

The distribution of the water rat Dasymys
(Muridae) in Africa: a review
S.K. Mullina*, N. Pillayaand P.J. Taylorb
Introduction
The African water rat, Dasymys, is widespread throughout
sub-Saharan Africa and follows a savanna distribution1–3 (Fig. 1).
Altitude does not limit its spread, as the genus occurs at altitudes
from 23 m to over 4000 m throughout Africa.4,5 It is common in
the northern parts of its distribution range,1but it is considered
rare and vulnerable in southern Africa6because of habitat loss
through desiccation and wetland destruction.7Its association
with water sources8represents an ecological constraint on
dispersal and gene flow and may account for its disjunct or
patchy distribution and the presence of relict populations.9–13 In
addition, many of the present-day Dasymys species seem to be
found typically in areas that are either geographically unique,
that receive high levels of rainfall or are watershed areas (e.g. the
Western Cape province of South Africa, the Jos Plateau in
Nigeria, Mt Cameroon, Mt Ruwenzori, and the Congo water-
shed).9,14–16 African rodent and shrew genera typically constitute
a large number of endemics compared to other animal
genera,17–19 which is generally attributed to their diet, stenotopic
nature and small size.20 Endemic hotspots have usually been
attributed to the effects of isolation and to barriers against gene
flow.21 Areas of high species richness often correspond to those
of high endemic richness.22 Several Dasymys species seem to
represent relict populations that are associated with endemic
hotspots (e.g. the Western Cape province, and the highlands or
montaneregionsofCameroon,Nigeria,UgandaandEthiopia).
Although Dasymys was considered monotypic historically
(being referred to as D. incomtus),23 Musser and Carleton24
recognized five species: D. foxi, D. incomtus, D. montanus, D. nudi-
pesand D. rufulus. Werecentlycompletedanextensiveinvestiga-
tion of Dasymys that included studies of non-geographical
variation,25 variations within Dasymys incomtus incomtus,26 as
well as an analysis of the entire genus27 and of the Dasymys
rufulus complex.27,28 We confirmed the existence of these species,
and also distinguished an additional six species (based on dis-
tinct morphological characteristics).
Of the 11 provisional, morphologically defined species
described by Mullin27 (Figs 2 and 3), seven are considered
endemic to their countries of origin: Dasymys capensis (Western
Cape province, South Africa), D. nudipes (southwestern Angola),
D. shortridgei (Okavango Delta, Botswana), D. montanus (high
altitudes on Mt Ruwenzori), D. longipilosus (high altitudes on Mt
Cameroon), D. foxi (Jos Plateau) and D. griseifrons (Lake Tana,
Ethiopia). The remaining four species are more widespread:
D. medius occurs in East Africa; D. incomtus in eastern South
Africa and southwestern Zimbabwe; D. robertsii in northern
South Africa, eastern Botswana and eastern Zimbabwe; and the
D. rufulus complex in West Africa, southern Chad, southern
Review Articles South African Journal of Science 101, March/April 2005 117
The genus Dasymys is widespread throughout sub-Saharan Africa.
Once regarded as monotypic, we described 11 morphologically
defined Dasymys species in a previous study. Dasymys is adapted
for living in mesic areas, which possibly explains its disjunct
distribution throughout its range. Here, we postulate that the distri-
bution of the 11 species in Africa can be explained as reflecting the
presence of water sources, which in turn define current geograph-
ical features, as well as the palaeoclimatic history of the continent.
We compare the distribution of Dasymys with that of other animals,
particularly other rodents, to examine whether there are similarities
in occurrence across taxa with similar habitat requirements and to
consider how geographical features influenced the distribution of
small animals. The fossil record indicates that Dasymys originated
in southern Africa, implying that the genus migrated northwards
into the rest of Africa. Dasymys is divided into two morphologically
defined groups: a southern group (D. capensis, D. incomtus,
D. robertsii, D. shortridgei and D. nudipes) and a northern group
(D. medius, D. montanus, D. griseifrons, D. longipilosus, D. foxi and
the D. rufulus complex). Within the northern group, two possible
radiation routes are suggested, one from Mozambique to Ethiopia
and the other from Zambia to northern Angola and northwards
through Central Africa to West Africa. These routes are based on
the past or current presence of continuous water features in
conjunction with morphological affinities observed for Dasymys
species. We envisage that our model may be used to generate
testable hypotheses in future investigations; for example, genetic
studies could be conducted to complement the morphometric
classification.
aEcophysiological Studies Research Group, School of Animal, Plant and Environmental
Sciences, University of the Witwatersrand, WITS 2050, South Africa.
bDurban Natural Science Museum, Mammal Department, P.O. Box 4085, Durban 4000,
South Africa.
*Author for correspondence. E-mail: skmullin@aol.com
Fig. 1. Distribution of
Dasymys
in Africa (from museum collections and literature
sources).

Sudan, Central Africa and Central East
Africa. Dasymys robertsii and D. short-
ridgei represent species novitiates,27
whereas D. capensis, D. longipilosus and
D. griseifrons result from the elevation
of already recognized subspecies of
D. incomtus.
Dasymys is chromosomally conserva-
tive, with five out of six chromosomal
forms characterized by FN = 44 and an
overall 2n range of 36–46. Karyotypic
forms include: 2n = 38, FN = 44 in the
KwaZulu-Natal province of South Af-
rica,26,29 2n= 46, FN = 44 in the Limpopo
province of South Africa,29 2n = 40,
FN=50inBurundi,30 and 2n = 36, FN =
44 in Senegal, Mali and Ivory Coast.31–33
The remaining two known chromo-
somalforms are found in Ethiopia (2n =
38, 40, FN = 44),33 but unfortunately
specimens were not available for the
morphometric analyses undertaken by
Mullin.27
Dasymys is adapted for existence in
mesic areas, as revealed by descriptions
of its morphology,8,34,35 and studies of its
behaviour36 and reproductive and
life-history biology.37 On this basis, we
postulate that the distribution of the 11
species in Africa can be explained by the
geographical availability of water
sources. In this review, we suggest that
the distribution of the species reflects
water flow patterns as well as current
geographical features (e.g. highlands
such as those found in Angola,
Cameroon, Ethiopia and Nigeria) and
palaeoclimatic data for the continent. In
addition, we suggest that the distribu-
tion of the newly proposed Dasymys
species corresponds to that of other
small mammals, mainly rodents and
insectivores, which are similarly con-
strained by water sources. Interestingly,
two rodent genera (Otomys and Aetho-
mys) contain species with similar ranges
to some of the Dasymys species even
though these genera have the ability to
inhabit a wider range of biomes than
Dasymys. This suggests that vicariance
events are important in the distribution
of small animals throughout Africa.38 In
addition to comparing Dasymys’ distri-
bution with that of Otomys and Aetho-
mys, we also examined the presence of
other rodents and small animals, in
particular those that are associated
with mesic areas (such as shrews and
amphibians).
Palaeoclimates and faunal distributions in Africa
It is widely accepted that climate fluctuations in the Pleisto-
cene influenced current geographical features and affected the
distributions of animals and plants by potentially causing
disjunct distribution patterns, particularly among species that
depend on perennial water sources or disperse along water
routes.14,40,43,47 It therefore follows that the speciation processes/
delineation in Dasymys largely reflect the legacy of palaeo-
climates and current geographical features, in addition to
118 South African Journal of Science 101, March/April 2005 Review Articles
Fig. 2. Distribution of 11 morphologically defined species of
Dasymys
described by Mullin.27 ●=
rufulus
complex
(including
D.r ufulus
from West Africa and material from the CAR, Cameroon, northern Angola, Zambia, Tanzania,
Malawi and Mozambique); ✻=
D.foxi,
❖=
D.griseifrons
; ? = unidentified);33 +=
D.longipilosus
;s=
D.medius
;×=
D. montanus
;u=
D. nudipes
;r=
D. shortridgei
;q=
D. robertsii
;■=
D. incomtus
;¯=
D. capensis.
Fig. 3. A phenogram based on cranial measurements, showing the relationship between the 11
Dasymys
species
recognized by Mullin.27 Correlation coefficient = 0.95.

stochastic processes, such as drift. The African environment
during the Pleistocene was characterized by fluctuating temper-
atures and rainfall levels.39,40 Pluvials and interpluvials affected
thetopographyindiverseways,especiallyintermsofthe expan-
sion and contractions of forests, deserts, savannas and water
sources.41–44 At least three pluvials are recorded45 when tempera-
tures were 5°C and 8°C lower than today.46 The interpluvials
marked a drier period, with lakes, riverine networks and basins
either retreating or drying-up.48 In addition, forest biomes
retracted, and deserts and montane grasslands expanded.40,49,50
Changes in habitat due to climate fluctuations have been
used to explain disjunct distributions, isolated species and the
occurrence of endemics among diverse animals including
butterflies,14 small mammals,51,52 avifauna,53 herpetofauna,54
large mammals43 and fish.44 Climate changes have had a large in-
fluence on the distribution of rodents, particularly since climate
defines biotypes and rodents are primary consumers.38,52,55 From
the above information, we postulate that the present spread of
Dasymysreflects the effects of palaeoclimates on water courses in
Africa.
Possible dispersion routes for
Dasymys
It was originally thought that the Murinae had an Asian origin
and radiated outwards to the rest of the African continent from
East Africa.56–58 However, fossil evidence suggests that some
murids (e.g. Acomys, Aethomys and Otomys ) had a southern
African origin whence they radiated northward.55,59–63 The oldest
records of Dasymys date back to the Pleistocene (1.7–1 Myr) from
Makapansgat in Limpopo,55 which is likely to be the area of
origin of this genus (Fig. 4). Details of the 11 proposed Dasymys
species are discussed in further detail below, in order of their
proposed distribution throughout Africa, beginning with the
species currently found in Limpopo, D. robertsii. Each section
also describes biogeographical features associated with each
region that could have hindered or facilitated the animal’s
dispersal as well as identifying species that have similar distribu-
tions to Dasymys. Figure 4 illustrates the distributions of each of
the Dasymys species, with the numbers 1 and 2 on the map indi-
cating important separations in the morphological dataset into a
northern and southern Dasymys group.27
Dasymys robertsii
(Dro in Fig. 4)
Dasymys robertsii is found in eastern Botswana (known only
from Kasane), eastern/northeastern Zimbabwe and the
Limpopo (2n = 46, NF = 44),29 Mpumalanga and Gauteng
provincesofSouth Africa.Thisdistributionroughlycorresponds
to the Limpopo watershed area described by Revenga et al.16
Zimbabwean Dasymys is similar to Dasymys from South Africa,
but also has a strong morphological similarity to the genus in
Zambia. The Zambezi River, which separates Zimbabwe from
Zambia, has been implicated as a barrier for butterflies and the
mole-rat genera Cryptomys and Heliophobius.14,64 Aridification of
the Limpopo basin, which separates the endemic-rich highlands
of Zimbabwe and Limpopo, may have caused the disjunct
distribution of certain amphibian, reptilian and butterfly species
between Zimbabwe and the Limpopo province.14,65–67 The
morphological differences between Zimbabwean and South
African Dasymys suggest that the Limpopo basin has acted as a
barrier to Dasymys as well.
Dasymys robertsii was formerly recognized as D. incomtus and,
indeed, both species were considered as the same subspecies,
D. incomtus incomtus,68 until morphological and genetic data
wereusedtoseparatethemintodistinctspecies26,27,29 (Fig.3).Very
few rodent species appear to have a similar distribution to either
D.robertsiiorD. incomtus and instead have a wider range, usually
encompassing South Africa, Zimbabwe and Botswana and
sometimes including Mozambique and Namibia (e.g. Otomys
irroratus, Aethomys chrysophilus, A. ineptus, Mastomys coucha,
Cryptomys h. natalensis and C. h. hottentotus).69–72 Since none of
these taxa has a similar distribution to either D. incomtus or D.
robertsii, we suggest that Dasymys was previously more wide-
spread in this region, and its separation into two species is the re-
sult of habitat fragmentation, or perhaps a reflection of Dasymys’
restricted habitat requirements.
Denys55 mentioned a general association between rodent
specimens from the Limpopo and Mpumalanga provinces (i.e.
the former Transvaal) and the Western Cape. The Orange River
was once connected to the Okavango and Limpopo rivers,
which would have possibly provided a distribution route for
mesic species between northern and southwestern South Africa.
In addition, this region was supposedly wetter in the Pleisto-
cene, and the Karoo was previously a barrier for Dasymys.48
However, Mullin27 reported that the Western Cape Dasymys was
more similar to specimens from KwaZulu-Natal, which does not
support a dispersal route via the Karoo.
Dasymys incomtus
(Di)
Dasymys incomtus sensu stricto occurs in KwaZulu-Natal (2n =
38, NF = 44),26,29 with the type specimen for the genus described
from Port Natal (= Durban), South Africa. Northern KwaZulu-
Natal, characterized by Tongaland–Pondoland regional coastal
mosiac vegetation,74 has been associated with endemic
amphibia,75 rodents and shrews.10 The Drakensberg mountain
range in Mpumalanga physically separates populations of
Dasymys between Limpopo and KwaZulu-Natal, which appear
to have been isolated from each other for a lengthy time, based
on morphometric and genetic differences.26,29 As the distribution
of Dasymys does not appear to be affected by altitude and a
Review Articles South African Journal of Science 101, March/April 2005 119
Fig. 4. Possible dispersion of
Dasymys
based on morphological data provided by
Mullin.27 1indicates a morphological split between ‘southern’ and ‘northern’
Dasymys
;2signifies a separation between East African/Ethiopian and
Central/West African
Dasymys.
Karyotyped species include:
Dro,
2n = 48, FN = 44;
Di,
2n = 38, 46; FN = 44;
Dru,
2n = 36, FN = 44 in the Ivory Coast, Mali and
Senegal;
Dme,
2n = 40, FN = 50.

possible dispersal route between KwaZulu-Natal and Limpopo
is via Mpumalanga populations on the eastern side of the
Drakensberg, it is not clear why there is such a strong distinction
between Dasymys incomtus and Dasymys robertsii.
Mullin27 indicated that D. incomtus has a potentially disjunct
range, occurring in southeast Zimbabwe in the eastern High-
lands (Mt Selinda and Mazoe). Mazoe is the type locality for the
subspecies D. i. fuscus, which appears to represent a valid
synonym of D. incomtus.27 This disjunct distribution still needs to
be confirmed by sampling more localities in Zimbabwe. The
possibility that the Dasymys individuals found here represent a
relict population of D. incomtus that is not morphologically
distinct from D. incomtus should not be discounted. The Mt
Selinda area is home to at least one rodent endemic, Aethomys
silindensis.76 Aethomys silindensis (which used to be considered a
subspecies of A. chrysophilus) is presently encompassed by
A. nyikae, which occurs elsewhere in Zimbabwe, as well as
A. chrysophilus, which is widespread in southern Africa. Similar
to Aethomys, Dasymys i. fuscus is surrounded by D. robertsii
elsewhere in Zimbabwe, which raises uncertainties about its
status as a valid subspecies of D. incomtus. To clarify this situa-
tion, any future genetic studies should compare Mt Selinda
specimens with D. incomtus and D. robertsii elsewhere.
Dasymys capensis
(Dc)
The Western Cape province has a distinct vegetation type
(fynbos) and mediterranean climate and is an area rich in
endemic or relict species of frogs, fish and rodents.9,63,73,77,78 The
region is isolated by the mountains to the north and east as well
as by the arid regions to the north. Dasymys capensis represents
either a distinct or an isolated relict population as it is restricted
to the Western Cape.9,27 As mentioned above, D. capensis is more
similarto D. incomtus than to D. robertsii, based on morphological
characteristics (Fig. 3), indicating a coastal distribution pattern
between the Western Cape and KwaZulu-Natal for these
species. This pattern appears to support the hypothesis that
invokes a link between the lowlands of the Western Cape and
Ethiopia, via an east coast grassland corridor that was once
inundated with floodplains.9,14,74
Dasymys shortridgei
(Ds)
The Okavango Delta Dasymys specimens (from the extreme
northeast of Namibia and known only from one northern
Botswana locality, Moremi) were previously classified as either
D. i. nudipes or D. nudipes.68,79 More recently, some proposed that
the delta material represented a distinct subspecies of D. nudipes,
separate from the nominate D. nudipes occurring in southwest-
ern Angola.13,80–81 Mullin27 confirmed that this taxon represented
a distinct species and suggested the provisional name of
Dasymys shortridgei for specimens from the Okavango Delta in
honour of G.C. Shortridge, who was responsible for the large
collections from this area.
Dasymys shortridgei appears to be restricted to the delta and the
eastern Caprivi Strip along with another rodent species,
Mastomys shortridgei.70 Dasymys specimens from western
Zimbabwe have been associated with specimens from
Botswana,82 and as D. shortridgei is more similar to southern
African material than to individuals from Central Africa,27
Dasymys most likely migrated from Zimbabwe and/or Limpopo
into the Okavango Delta. A faunal link between the delta/east-
ern Caprivi Strip and Zimbabwe/South Africa rather than with
Central Africa is supported by the distributions of other rodents,
such as Cryptomys h. damarensis and Aethomys chrysophilus,69,71
which have similar, but wider, distributions than D. shortridgei,
D. incomtus and D. robertsii.
Dasymys nudipes
(Dn)
Other than the nominate D. incomtus, D. nudipes was one of the
first Dasymys species to be recognized, based on its distinctive
large size and the unique presence of a sixth planar pad.13,81 This
animal is one of three endemic rodent species isolated on the
southwestern Angolan plateau; the two others are Otomys
anchietae and Praomys angolensis.13,72 Although a narrow escarp-
ment runs along the west coast of Angola, forming a link with
the Congo in the north,13,41 D. nudipes does not have an affiliation
with the Dasymys found in northern Angola, which instead is
part of the rufulus complex (see below).27 One possible dispersal
route for Dasymys into southern Angola was from the Okavango
Delta via tributaries from the Zambezian (Katanga) plateau in
the east that led into northeastern Namibia;83 another possibility
was via a historical link between the Limpopo and the
Okavango, Cunene and Zambezi–Luangwa rivers.44
Dasymys rufulus
(Dru)
The number 1 in Fig. 4 marks a morphometric separation
between southern species (D. capensis, D. incomtus, D. nudipes,
D. robertsii and D. shortridgei) and the others (D. foxi,
D. griseifrons, D. longipilosus, D. medius, D. montanus and D.
rufulus). Number 2 on the figure marks the second clear morpho-
logical separation between East African and Ethiopian Dasymys,
on the one hand, and Central East, Central and West African
forms on the other.27The easter n limit of the D. rufulus complex is
in Mozambique and Tanzania and extends to northern Angola,
northwards into Central Africa [Democratic Republic of Congo
(DRC), Congo, Central African Republic (CAR), Cameroon] and
West Africa (Nigeria to Senegal) and includes Dasymys from
southern Chad and Sudan. It is not uncommon for small
mammals to extend across large areas between East, Central and
West Africa; in the case of Dasymys, West and East African
Dasymys share a strong morphological similarity (Fig. 3), but we
were able to separate them into distinct species.27 Faunal links
are also well established between East Africa, Zambia and
Angola in respect of rodent and amphibian species.54,84–88 Many
species have also been documented as having ranges that
encompass northern Angola, Central and West Africa.89–92
Mullin27 proposed five subgroups within the rufulus complex,
although the nature of the relationship between them was
unclear (e.g. species or subspecies). The five subgroups are
considered below.
1.
Mozambique, northern Malawi, southern Tanzania (Dro1)
Several relict populations of rodents exist in Malawi, Mozam-
bique and Tanzania; genetic and morphometric studies have
identified numerous new species and species complexes in the
Acomys, Cryptomys, Otomys and Praomys genera.63–64,72,93–94 As
indicated above, Dasymys specimens from this region have a
similar skull size and shape to D. rufulus from West Africa, and
areseparatefrom material from East Africa,withthe exceptionof
specimens from only one of the southern Tanzanian localities
examined, Mlali.27 This area is part of the mesic coastal grassland
corridor described by Davis,9Carcasson14 and Werger;74 this
corridor appears to have been important in Dasymys’ dispersal
throughout Africa. This region has also been associated with
new descriptions of species and comprises several subzones
based on vegetation types, indicating a high level of faunal
separation in this area.94–96 We mention this point to suggest that
Dasymys in this area might be more segregated than Mullin’s
morphological evidence27 indicated and propose that future
studies of the genus should include a focus on material from this
region.
120 South African Journal of Science 101, March/April 2005 Review Articles

2.
Zambia and southern Malawi (Dro2)
Zambian and southern Malawian specimens appear to have a
similar skull size and shape to northern Malawian, Mozambican
and Tanzanian material, but also appear to be distinct at either a
subspecies or perhaps even a species level. This area is encom-
passed by the Zambezi to the north, the Limpopo River to the
south,and the Shire to the east. The distribution of this subgroup
of Dasymys coincides with the limits of the Zambezi watershed
area described by Revenga et al.16 Moore and Larkin44 stated that
because the Zambezi was linked with the Limpopo, Dasymys
could have migrated from southern Africa northwards and
potentially migrated within Zambia. In addition, Moore and
Larkin44 suggested that the Zambezi was formerly joined with
the Shire (in southern Malawi). This could have provided a
dispersalroute for Dasymys and could explain the morphometric
similarities between specimens from Zambia and southern
Malawi.27
Zambian rodents have been the focus of systematic studies
recently and several genetically diverse forms of Cryptomys were
found64,97 as well as a new species of Lemniscomys.98 A small relict
highland forest important for butterflies has been identified,14
and it has been suggested that the high altitude of the Katanga
plateau in Zambia could facilitate speciation in rodents.83 The
Katanga plateau, which is characterized by numerous drainage
rivers, extends into eastern and northern Angola via a former
continuous forest system.14
3.
Central Africa (Dro3)
Northeastern DRC is an important region for Dasymys as speci-
mens from there are morphologically separate from the other
Central African Dasymys and more similar to material from East
Africa. Dasymys individuals from northeastern DRC are there-
fore included in the discussion of their East African relatives (be-
low). Dasymys from the lowland forests of Central Africa,
including lowland Cameroon, northern Angola, Congo, CAR
and the DRC, form the third subgroup in the rufulus complex.
The Congo watershed is extensive throughout the DRC, and ex-
tends into all of the areas where Dasymys from this subgroup are
located (i.e. northern Angola, eastern Cameroon, eastern Congo
and southern CAR).16 Various fauna are thought to be restricted
to this area because the Congo and Lualaba rivers and the Congo
(or Zaire) Basin act as barriers to dispersal,9,91,99 indicating the
possibility that Dasymys might be confined to this region, and
potentially explaining the distributional limits of the subgroup
of D. rufulus.
4, 5.
West Africa (Dro4, 5)
The denomination ‘West Africa’ generally refers to the area
from Senegal to Nigeria, including the southern parts of Mali
and Burkina Faso. Although West African Dasymys are repre-
sented by a stable karyotype (2n = 36, FN = 44 in Senegal, Mali
and Ivory Coast),31–33 Mullin27 suggested that West African speci-
mens were divided into two separate morphological groups. It
should be noted, however, that it was not possible, using
morphometric data, to determine what level this separate group
represented (i.e. subspecies or species). Dasymys rufulus is not
confined to West Africa, confirming the ideas of Musser and
Carleton.24 In addition, there appears to be some degree of
inter-locality separation within D. rufulus, suggesting the pres-
ence of subspecies or possibly of cryptic species.
The Dahomey Gap ranges between southeastern Ghana and
southern Togo and Benin and has been implicated in causing
disjunct distributions in species of duikers, primates, rodents,
fish and frogs.91,100,101 Other barriers implicated in the disruption
of faunal ranges include the Niger, Senaga, Senegal, Volta and
Zaire rivers.91,101,102 However, the two D. rufulus groups did not
appear to correspond with either the Dahomey Gap borders or
to major rivers in West Africa. One group, which includes the
type specimen for D. rufulus (from Mt Coffee, Liberia), ranged
from Senegal to western/southwestern Ivory Coast, while the
second group was found in northern Ivory Coast to western
Nigeria. This indicates that the Ivory Coast, an area that has
already been associated with endemic rodent species,91,103,104
might also be important for Dasymys, perhaps representing a
contact zone between two West African groups.
Misonne105 indicatedthatWest African forest species have been
largely isolated from other forested areas since the early Miocene,
except for an apparent faunal connection with Ethiopia. Ethiopian
Dasymys are distinct from the West African Dasymys rufulus in
terms of both chromosome number and morphological charac-
teristics, indicating that Dasymys from these two areas have been
isolatedfor a long time. However,Dasymys from southernSudan
and Chad does appear to have a close link with D. rufulus,
suggesting that Dasymys dispersed along a corridor there, per-
haps via the Sudanese savanna. West African forests are not
isolated from Central African forests, as they appear to share
species (see below). Although some rodent species from Nigeria
and Central Africa have close ties with East African forms (e.g.
Mastomys, Otomys, Praomys),89,93,106 this is not the case with
Dasymys; one reason may be the poor collection records from
Central Africa.107
Dasymys longipilosus
(Dl)
Dasymyslongipilosus,a species characterized by relatively small
cranial features,27 is known only from high altitudes on Mt
Cameroon, although this might be a reflection of poor sampling
in the region90 rather than of a high degree of specificity. The
Dasymys here probably represent a relict population, given that
other Cameroonian Dasymys appear to be part of a D. rufulus
complex, and especially since it has been suggested that the
uniqueness of the Cameroon mountains may have facilitated
speciation in several different animal species.108 The highland
region of Cameroon, particularly the area encompassing the
Oku, Cameroon and Lefo mountains, is associated with a large
number of endemics (e.g. Dasymys, Otomys, Crocidura, Praomys,
Hybomys, Lophuromys), restricted either to a specific mountain or
to neighbouring mountains.72,90,93,109,110
Dasymys foxi
(Df)
Dasymys foxi is restricted to the Jos Plateau in Nigeria, an area
occupied by several different endemic rodent species (e.g.
Cryptomys foxi, Praomys viator).90,111 This region is separated from
the rest of Nigeria by its higher altitude, which reaches over
1200 m in places. In addition, the Jos Plateau receives more rain-
fallthan the surrounding area,112 perhaps linking D. foxi with this
region. Dasymys foxi is surrounded by D. rufulus, which occurs
throughout West Africa.24 The wider skull, larger molars and
generally larger size of D. foxi and the slightly redder colour of
D. rufulus separate the two groups at a species level,111 which is
supported by the morphometric studies of Mullin:27 D. foxi is
significantly larger than D. rufulus with respect to upper
toothrow length and greatest skull height.
Dasymys medius
(Dme)
East Africa (eastern DRC, Kenya, Uganda, Rwanda and Bu-
rundi) is represented by D. medius regionally, whereas D. mon-
tanus is endemic to high altitudes on Mt Ruwenzori. Dasymys
medius appears to represent a homogeneous species,27 despite
Review Articles South African Journal of Science 101, March/April 2005 121

the high number of previously recognized subspecies present in
the area (helukus, nigridius, orthos and savannus). Denys and Jae-
ger58 suggestedthatrodents in this area typically have a slow rate
of morphological evolution; it seems likely that the Rift Valley is
important in the evolution of East African mammals. The one
East African population from Burundi30 that has been
karyotypedis different from the other Dasymys forms not only in
terms of its diploid number (2n = 40), but also because it repre-
sents the only known Dasymys karyotype with a unique funda-
mental number (FN = 50; all other Dasymys forms karyotyped
have FN = 44).33
In terms of skull morphology, D. medius is more similar to D.
rufulus than to southern African Dasymys, although specimens
from Burundi are characterized by X and Y chromosomes
similar to those from KwaZulu-Natal, and eastern and south-
western Ethiopia,33 indicating a close link between specimens
from these regions. As stated previously, this region is important
for Dasymys possibly because it was once part of a mesic eastern
coastal grassland corridor that encompassed the lowlands
between the Western Cape province and Ethiopia that were
once inundated with floodplains.9,14,74 In addition, the Nile
watershed extends from Egypt through Sudan to link Uganda,
Burundi, Rwanda, northern Tanzania, southwestern Kenya and
the eastern DRC;16 this approximately outlines the distribution
limits of D. medius, with the exception of Egypt (where Dasymys
does not occur) and Sudan.
Dasymys montanus
(Dmo)
Like D. longipilosus, D. montanus also appears to represent a
relict montane species. The latter is found only at high altitudes
in the Ruwenzori Mountains of Uganda and is encompassed by
D. medius in the surrounding lowland areas. Dasymys montanus
has the characteristic dark pelage and small features associated
with montane forms.1,2 The mountains of East Africa (e.g. Mts
Ruwenzori, Elgon, Kilimanjaro) have previously been described
as the Galápagos of Africa113 in reference to the large number of
endemic species and the level of speciation within genera found
there.
Dasymys griseifrons
(Dg)
Ethiopia probably has the second highest number of endemic
fauna after Cameroon, and includes country-endemic species
within the following shrew and rodent genera: Arvicanthis,
Crocidura, Dasymys, Lophuromys, Otomys, Praomys and
Stenocephalemys.33,114–119 The biological importance of this area for
small mammals is emphasized by the presence of multiple
endemics within the same genus, including Crocidura,114
Lophuromys120 and Arvicanthis.121 Two chromosomal forms of
Dasymys (Dasymys cf. incomtus: 2n = 40, NF = 44 from Bale
Mountains and 2n = 38, NF = 44 from Harenna Forest) and one
distinct morphological form (D. griseifrons known only from
lakes Tana and Jigga) occur in Ethiopia.27,92 These three forms are
all distinct from the nominate incomtus material from South
Africa.27,33 Future studies, both morphological and genetic, need
to be done to establish the taxonomic status of Dasymys in
Ethiopia.
Ethiopia is characterized by a large plateau with an average
elevation of 2200 m and is covered with forests and grasslands.116
Yalden et al. indicated that the high level of endemism in this
region is partly due to the Ethiopian Rift, although it is not
considered a barrier to small animals as at least one rodent
species,Lophuromyschryspus, is present on both sides of the rift.120
The arid environment of southern Sudan and northern Kenya is
thought to support restricted mammalian distributions.116
Conclusion
Geographical and palaeoclimatic factors have shaped the taxo-
nomic history of Dasymys. The existence of at least 11 Dasymys
species (not including the two Ethiopian chromosomal forms
identified by Volobouev et al.33) is not surprising, considering the
habitat restriction of the genus in conjunction with the distribu-
tions of other fauna (especially other rodent genera), particularly
endemic hotspots and the areas of high rainfall. The disjunct
distribution of Dasymys appears to coincide with water bodies
and well-defined geographical features, a phenomenon that
appears not only to explain its current distribution but also
speciation within the genus. We have attempted here to generate
a model to understand and support the morphometric distinctions
within Dasymys. Future genetic studies should be conducted to
explore random processes such as genetic drift among the 11
Dasymys species.
Received 28 January 2003. Accepted 15 July 2004.
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