Morphometric differentiation among West African populations of the rodent genus Dasymys (Muroidea, Murinae), and its taxonomic implications

17 Pages·1991·7 MB·English

by  Michael D Carleton

PROC. BIOL. SOC. WASH.
104(3), 1991, pp. 419-435
MORPHOMETRIC DIFFERENTIATION AMONG
WEST AFRICAN POPULATIONS OF THE RODENT
GENUS DASYMYS (MUROIDEA: MURINAE),
AND ITS TAXONOMIC IMPLICATIONS
Michael D. Carleton and Christine Martinez
Abstract.—CrdtmodQnXdX variation among West African samples o^Dasymys
is reassessed based on the larger museum series now available. Two morpho-
logical kinds can be discerned in this region and are referable to the locally
availableepithetsDasymysrufulusMiller(1900),describedfromMountCoffee,
Liberia, andDasymysfoxiThomas (1912), named from Panyam, Nigeria. The
degree ofcraniodental differentiation between these two phena matches that
documentedforothercongenericpairsofmuridrodentspeciesinhabitingparts
ofWestAfrica. Dasymys rufulusdiffers from D.foxibyits overall smallersize,
especiallyas observedin its smaller, narrowercranium and less robust molars.
In West Africa, samples ofD. rufulus are broadly distributed from east ofthe
Niger River, Nigeria, west to northern Sierra Leone; whereas, the known ex-
amples ofD.foxi are restricted to the Jos Plateau in Nigeria. Dasymys rufulus
and D.foxi are provisionally accorded specific status until taxonomic studies
ofDasymyspopulationsfromotherpartsofAfricaareundertakentodetermine
theirrelationships and distributionallimits. Itis doubtful thateitherform, but
especially D. rufulus, is synonymous with D. nudipes or D. incomtus.
Shaggy-furred or marsh rats ofthe genus pears very unlikely that there is more than
Dasymys are broadly ifintermittently dis- one species ofthis genus." His conviction
tributed over much ofAfrica south ofthe in this regard may have been influenced by
Sahara Desert, inhabiting a variety ofwet- Osgood's(1936:256)earlierobservationthat
landareaswiththickgroundcover. Current "Bothexternalandcranialcharactersinthis
classifications recognize a single species, genus are elusive andthe range ofvariation
Dasymys incomtus (Sundevall, 1847), throughout is comparatively slight," which
throughoutthisbroadrange(Misonne 1974, he offered in the description ofZ). incomtus
Honacki et al. 1982, Corbet & Hill 1986). griseifrons. Except for the occasional reten-
The genus has not always been viewed as tionofD. nudipesasaspecies(Roberts 1951,
monotypic, for 13 ofthe 20 species-group Crawford-Cabral 1983), subsequent classi-
taxa associated with Dasymys have been fications and regional catalogues have uni-
named as species. As of 1939, Allen listed formlyacknowledgedthe one speciesD. in-
10 species, four of them containing from comtus (for example, Ellerman et al. 1953,
two to four subspecies. Hanney 1965, Rosevear 1969, Delany 1975,
The current perception ofspecies diver- De Graff 1981, Meester et al. 1986, Hap-
sity in Dasymys originated with Ellerman's pold 1987). A notable omission in arriving
(1941) treatment of the genus. Ellerman at the current species arrangement ofshag-
(1941:121) regarded all named forms, ex- gy-furred rats has been the presentation of
cept orthos Heller (1911) which he did not datasupportive ofthesetaxonomic changes.
examine, as races of D. incomtus: "It ap- The basis forrecognition ofa single species
420 PROCEEDINGSOFTHEBIOLOGICALSOCIETYOFWASHINGTON
has hinged on Ellerman's surmise that "it ciatedskulls,wereexamined(seeTaxonom-
appears very unhkely" that more than one icSummary)fromthefollowingcollections,
exists. theiracronymslistedinparentheses: Amer-
The present study contributes distribu- icanMuseumofNaturalHistory,NewYork
tional and morphological information that City (AMNH); Museum of Comparative
bears on the status ofDasymyspopulations Zoology, Harvard University, Cambridge
inhabiting portions ofWest Africa, namely (MCZ); and the U.S. National Museum of
from Nigeria west to Sierra Leone. Com- Natural History, Smithsonian Institution,
paredtothenumberofformsdescribedfrom Washington, D.C. (USNM).
eastern and southern Africa, the taxonomy Preparation ofthe distribution map was
ofshaggy-furred rats is less problematic in based on locality information and geo-
West Africa, where only two forms have graphical coordinates basically as given by
been described: Dasymys rufulus Miller collectors on specimen tags. We also con-
(1900) from Mount Coffee, Liberia, and sulted the gazetteers provided by Davis &
Dasymys foxi Thomas (1912) from Pan- Misonne(1964), Rosevear(1965)andHap-
yam, on the Jos Plateau, Nigeria. Clarifi- pold (1987), to verify the plotting ofthese
cation ofpatterns ofmorphological varia- sitesandtoobtaingeographicalcoordinates
tion on a regional basis, set within the not listed by collectors.
context of local taxonomic designations, Several dimensions ofthe skin and skull
marks a necessary step toward refining the were recordedin millimeters (mm) to char-
species taxonomy of Dasymys as a whole acterizethemorphologyofthesamples. To-
and gaining insight into its historical zoo- tal length, tail length (TL), and hindfoot
geography. length (HFL) were extracted to the nearest
mm
LikeotherformsofDasymys, Miller'sru- whole from the skin tag, as was field
fulusandThomas'ybx/wereviewedas sub- weight to the nearest whole gram (gm).
species ofD. incomtus by Ellerman (1941), Length of the head and body (HBL) was
an arrangement which has persisted to the obtained by subtracting the tail from total
presentday(Rosevear 1969,Misonne 1974, length. Fourteen cranial and two dental
Happold 1987). Rosevear (1969:311) even measurementsweretakentothenearest0.01
mm
questioned the validity ofmaintaining any by means ofhand-held digital calipers
formal racial division among West African accurateto 0.02 mm. Thesemeasurements,
populations: ". the differences in size be- and their abbreviations as used herein, in-
. .
tweenthisrace [thatis, rufulus] 3.ndfoxiare clude: occipitonasal length (ONL); greatest
really very slight and it is open to some zygomatic breadth (ZB); breadth of the
doubt whether the distinction can be re- braincase (BBC); least interorbital breadth
tained." Happold(1987:132)formedasim- (lOB); length ofthe rostrum (LR); breadth
ilar impression based on his review ofNi- of the rostrum (BR); postpalatal length
gerian samples, noting that, in general, the (PPL); length of the bony palate (LBP);
smaller rufulusoccurs in southern localities length ofthe incisive foramen (LIF); length
andthelarger^bjc/innorthernones,butthat ofthe diastema (LD); breadth ofthe bony
"... the subspecific distinction may not be palate acrosstheupperfirstmolars (BM1s);
valid." These assessments deserve review breadthofthezygomaticplate(BZP);length
baseduponthelargermuseumseriesofWest oftheauditorybullae(LAB);breadthacross
African Dasymys now available. theexoccipitalcondyles(BEC);crownlength
of the maxillary toothrow (LMl-3); and
Materials and Methods width ofthe upper first molar (WMl). The
limits of most ofthese dimensions on the
Approximately 300 specimens, consist- murine skull are defined and illustrated by
ing ofconventional study skins with asso- Musser(1979).
VOLUME NUMBER
104, 3 421
Recording ofmeasurements was limited yses ofvariances were applied to the three
to animalsjudged to be adult based on the largest OTUs (G3, N4, and NIO) to assess
possessionoffullyeruptedthirdmolarsand the effect ofgender on the variation ofthe
adult pelage. "Adult" animals were further 16 craniodental variables. Principal com-
segregated into crude age classes based on ponentswereextracted from avariance-co-
upper molar wear as follows. variance matrix and computed using the
Young adult.—Tricusp arrangement of craniodental variables after transformation
first and second chevrons ofMl-2 well de- totheirnaturallogarithms. Loadingsareex-
fined, the enamel borders ofthe medial (t1 pressed as Pearson product-moment cor-
and t3) and lateral (t3 and t6) cusps deeply relation coefficients of the principal com-
constricted and contiguous or virtually so; ponents with the original cranial variables.
dentinal lakes ofchevrons thus narrow and Allanalytic procedureswere carriedout us-
M3
discontinuoustransverselyornearlyso; ing Systat (Version 4.1, 1989), a series of
erupted and slightly worn; tls on M2-3 statistical routines programmed for micro-
whollyisolatedfromenamelbordersofsec- computers.
ond chevron; transverse enamel valleys be-
tween chevrons deep and extend across the
Morphometric Results
width ofthe molar.
Full adult.—Enamel indentations of the Sexual dimorphism in cranial size is un-
M
medialandlateralcuspsonchevronsof 1- appreciablewithinpopulationso^Dasymys,
2notincontact, theirdentinbasinsbroader afindingin agreementwithbiometric stud-
andcontinuousacrossthewidthofthetooth; ies ofother African murines (for example,
enamel, lessoftenthedentin, oftlsonM2- Van der Straeten & Verheyen 1981, 1982;
3 connected to that ofthe second chevron Carleton&Robbins 1985). Amongthethree
(anterior border oft5). largest population samples analyzed, only
Oldadult.—Occlusalwearheavysuchthat sixcranialdimensionsvarysignificantlywith
thedentinofchevrons,particularlythemid- regardtogender(Table 1),withmaleslarger
dle one and t8, cross-connected at their in each instance, and each significant vari-
medial and lateral edges; enamel valleys able is unique to a particular sample. We
separating chevrons appear as islands sur- attribute the sporadic occurrence ofsignif-
rounded by dentinal basins. icant findings across the three samples to a
Although the above data were recorded combination ofType I samplingerrors and
for all specimens examined, the multivar- unequalagerepresentationamongthesexes
iateanalyseswereappliedtothe largerpop- of each sample. For example, within the
ulation samples with the highest incidence Panyam sample (NIO), males are dispro-
ofintact crania. These are: portionately represented in the adult (12 to
OTU G3.-Ghana, Pulima, n = 37. 9) and old adult (2 to 0) age groups. As
OTU II.-Ivory Coast, Kong, n = 10. discussed below, age can contribute sub-
OTU 12.—Ivory Coast, Sienso, « = 15. stantial intrapopulation variation. In view
OTU LI.—Liberia, Mount Coffee, « = 5 ofthe negligible orinconsistenteffects, par-
(type series ofrufulus). ticularly with regard to the magnitude of
OTU N4.-Nigeria, Dada, n = 24. interlocalitydifferentiation, bothsexeswere
OTU N
10.-Nigeria, Jos Plateau, Pan- included in the multivariate analyses.
yam Fish Farm, « = 43 (near type locality With the large series available from near
o^foxi). the type locality ofD. foxi, populations in
OTU T3.-Togo, Padori, « = 1 1. Nigeria have provided critical insight to
Standard descriptive statistics (mean, the morphological differentiation among
range, standard deviation) were derived for West African Dasymys. Thomas (1912) di-
the OTUs (see Appendix 1), One-way anal- agnosed D. foxi on the basis of its larger
422 PROCEEDINGSOFTHEBIOLOGICALSOCIETYOFWASHINGTON
Table L—/values from one-way anovas (effect = size,especiallyasobservedinthelengthand
sex)ofthreepopulationsamplesofWestAfricanDas- breadth ofthemolarrows. Suchdifferences
ymys. were borne out in principal component
Cranial Ghana:Pulima Nigeria:Dada Nigeria:Panyam analyses of the two largest Nigerian sam-
variables (14S,239) (83, 159) (223, 169) ples, Dada and Panyam Fish Farm, whose
ONL 0.31 1.27 3.96 scoresoverlappedhttleinmultivariatespace
ZB 0.01 2.11 2.65 (Fig. 1). The uniformly positive and high
BBC 0.43 6.17* 1.54 correlations ofmost craniodental variables
lLORB 05..5720* 00..1016 03..6730 with the first principal component suggest
BR 0.48 0.29 5.46* a general size factor (Table 2) and empha-
PPL 0.54 0.49 2.47 size the overall larger size of the Panyam
LBP 0.10 0.62 6.83* specimens. The dispersion of individuals
LIF 0.61 0.01 0.83 along the second principal component re-
LD 0.70 0.63 3.17 flects certain breadth measurements (BEC,
WLMMll-3 1.69 2.06 0.01 lOB)andsizeofthemolars(LMl-3,WMl),
0.13 0.84 0.06
BMls 0.01 0.51 0.02 whichagainreflectthebroadercraniumand
BZP 0.11 1.41 0.16 more robust molars recorded for the spec-
LAB 0.22 13.68** 0.42 imens from the vicinity ofPanyam.
BEC 0.61 2.83 11.62** Some age-related sizevariation undoubt-
P < 0.05; ** = /'< 0.01. edly contributes to the elliptical spread ob-
served for each population sample in the
bivariateplotofthefirsttwoprincipalcom-
Fig. 1. Projection ofindividual specimen scores on the first two principal components for the two largest
samples ofNigerian Dasymys, Dada (« = 23) and vicinity ofPanyam (« = 38). Ellipses correspond to 95%
confidenceenvelopesforscoresofindividualswithineach sample. SeeTable 2.
VOLUME 104, NUMBER 3 423
Panyam C2)
Old
Adult
Dada C6]
Panyam C21]
Adult
Dada C17]
Young'=* „Panyam C15]
A^ddult 1_
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
First Principal Component
Fig. 2. Boxplots offirstprincipalcomponentscores(see Fig. 1)ofDadaand Panyam samplesarrangedby
three adult age classes. Mean (vertical line), one standard error ofthe mean (closed rectangle), one standard
deviation(openrectangle),andtherange(horizonalline) are indicated.
ponents (Fig. 1). The range offactor scores results are here reported only for the Dada
viewed by age class suggests such an influ- and Panyam samples, all Nigerian speci-
ence, especially for the first principal com- mens with intact skulls were subjected to
ponent (Fig. 2). Still, a marked separation the same analyses and revealed the same
between the Dada and Panyam samples is pattern ofdual clouds ofpoints. Specimens
apparent for a given age class. The absence from the Jos Plateau (Panyam Fish Farm,
of specimens classified as young adults in 2 mi N Panyam, and Ugar Jabar), which
the Dada sample probably accounts for its represent Thomas' (1912) D. foxi, formed
widerconfidence ellipse as well as the mar- oneofthoseconstellations,whereastheoth-
ginally significant difference (P = 0.052) in ercorrespondedtospecimensfromallother
the slopes oftheir regression equations. localities in Nigeria (see Specimens Exam-
Rotation(varimaxprocedure)oftheprin- ined).
cipal components underscored the same The distinctiveness ofthe animals from
disparityin cranialbreadth (ZB, BBC, JOB, the vicinity of Panyam is still apparent in
BEC) and molar size (LMl-3, WMl) be- principal component analyses including
tween the Dada and Panyam samples, as samplesfromfourotherWestAfricancoun-
wellasintheLAB(Table 2). One-wayanal- tries (Fig. 3). Again, overall larger size—in
yses ofvariance, using locality as the cate- particular as expressed by the lOB, BEC,
WMl—
gory of effect, generally corroborated the LAB, LMl-3, and contributed to
highervariabilityofthesedimensions, with the separation ofthis Nigerian sample from
WM1,BEC,andLM1-3 exhibitingthelarg- other West African Dasymys (Table 3).
est / values, respectively (Table 2). Al- Confidence ellipses for the centroids ofthe
though the principal component and anova latterpopulations, includingthe Dada sam-
424 PROCEEDINGSOFTHEBIOLOGICALSOCIETYOFWASHINGTON
Table 2.—Resultsofprincipalcomponentanalysesandone-wayanovas(effect= locality)forDada(« = 23)
andPanyam (n = 38) samplesofDasymys(seetextandFig. 2).
Unrotated Rotated
Cranialvariables PCI PCII PCI PCII /(locality)'
ONL
0.92 0.29 0.88 0.34 37.0
ZB 0.90 0.04 0.68 0.52 54.8
BBC 0.79 -0.21 0.45 0.65 59.8
JOB 0.52 -0.53 0.02 0.73 37.5
LR 0.79 0.44 0.84 0.14 15.8
BR 0.74 0.29 0.81 0.24 9.9
PPL 0.88 0.29 0.86 0.32 29.3
LBP 0.59 0.11 0.40 0.27 12.2
LIF 0.71 0.29 0.76 0.22 14.9
LD 0.81 0.47 0.89 0.13 12.6
LMl-3 0.76 -0.46 0.25 0.83 119.7
WMl 0.75 -0.61 0.15 0.94 273.7
.
BMls 0.84 0.04 0.51 0.47 27.7
BZP 0.77 0.25 0.41 0.24 20.2
LAB 0.79 -0.27 0.47 0.71 76.0
BEC 0.77 -0.43 0.35 0.82 194.5
Eigenvalue 0.027 0.006 0.016 0.014
%
Variance 60.4 14.0 34.1 31.2
All/values significantatP < 0.01.
0.0
PC
I
Fig. 3. Projectionofgroup centroids onthe first two principal components forsix largest samples ofWest
AfricanDasymys(seeTable3; samplecodesandsizesaregiveninMaterialsandMethods).Ellipsescorrespond
to95%confidencelimitsofagroup'scentroid. EnclosedstarrepresentstheholotypeofDasymysrufulusMiller
(1900).
VOLUME 104, NUMBER 3 425
pie, were broadly congruent and circum- Table 3.—Results ofprincipal component analysis
scribed the data point that represents the usingsixlargestOTUsofDasymys{n = 130)(seetext
type specimen (USNM 83844) of Miller's and Fig. 3).
(1900)D. rufulus. Althoughwe didnotper- Cranialvariables PCI PCII PCIII
sonallyexaminethetypematerialofD.foxi,
the cranial measurements of the holotype ONL 0.96 -0.18 0.09
(BMNH 12.4.3.15 from Panyam) supplied ZBBBC 00..7869 00..0139 00..0156
by Dr. Erik Van der Straeten clearly depict lOB 0.30 0.58 0.16
arobustskullthatfitswiththeUSNMseries LR 0.87 -0.33 0.08
from the Jos Plateau. BR 0.81 -0.24 0.13
PPL 0.92 -0.09 0.05
LBP 0.68 -0.14 -0.17
Discussion LIF 0.80 -0.15 0.22
LD 0.91 -0.27 0.07
Several ofthe commonnames applied to LMl-3 0.61 0.60 0.17
WMl
Dasymys—namely, "marsh," "swamp," or 0.51 0.74 0.16
BMls 0.87 0.05 -0.01
"water" rats—underscore the regular asso-
BZP 0.78 0.16 -0.59
ciation ofthese rodents with various kinds LAB 0.71 0.41 0.12
of wetland environments. Typical habitat BEC 0.68 0.37 0.16
references include stream and river banks, Eigenvalue 0.033 0.005 0.003
marshyareaswithreedbeds, swampyedges %Variance 63.5 10.4 5.9
along rivers, lush grass covering moist
ground, matted semi-aquatic grasses and
sedges,wetterpartsoffloodplains,andhigh- variation in habitat and habit reflects pop-
land bogs (Rosevear 1969, Smithers 1971, ulational or specific differences.
Sheppe 1972, Kingdon 1974). In addition In West Africa, the pattern of morpho-
to preferring generally wet conditions, the logical variation supports the existence of
presenceofdensegroundcoverappearscrit- two separate species, to which the closest
ical to theirecological occurrence in afford- regional epithets Dasymys rufulus Miller
ing protected runways and nesting sites. (1900) and Dasymys foxi Thomas (1912)
Their diet is thought to consist principally apply. The marked discrimination between
ofgreen vegetable matter, yet some insects these two sets of populations clearly dis-
remains have been identified in stomach putes the notions that the traits of rufulus
contents(Hanney 1965,Smithers 1971,Coe andfoxi are insufficiently marked for even
1975). Shaggy-furred rats appear to be subspecific recognition or that they inter-
mostly terrestrial, although semi-aquatic grade imperceptibly in size along a north-
habits have been noted, and their activity southcline (Rosevear 1969, Happold 1987).
has been described as principally nocturnal Instead,thecraniodentaldifferentiationob-
but to a limited extent diurnal (Thomas served between populations ofeach resem-
1912, Roberts 1951, Smithers 1971, De bles that demonstrated for other pairs of
Graaff 1981). congeneric species inhabiting portions of
Knowledge ofthelifehistoryandecology West Africa—for example those of Hybo-
ofDasymys, however, remains sketchy and mys, Lemniscomys, Malacomys, and Prao-
stems largely from trapping experiences re- mys (Van der Straeten & Verheyen 1978,
latedinfaunalaccountsandfromcollectors' 1979, 1981, 1982; Carleton & Robbins
notations on specimen tags. Thorough field 1985).
studiesonthebiologyofthisgenushaveyet How these two species in West Africa re-
to be conducted, a situation which leaves late to forms of Dasymys described from
unanswered whether some ofthe apparent central, eastern, and southern Africa will
426 PROCEEDINGSOFTHEBIOLOGICALSOCIETYOFWASHINGTON
^-
4
Fig.4. ViewsofthecraniumandmandibleofanadultmaleDasymysfoxi(USNM403644)fromthevicinity
ofPanyam,Nigeria. Approximately x1.75.
require careful revisionary attention to the Africa). His enumeration ofthe differences
entire genus, which is beyond the scope of between D. rufulus and D. incomtus is apt
our paper. The primary descriptive htera- and emphasized the generally smaller size
ture on Dasymys lends few clues. and more rufous pelage ofthe former rel-
Miller (1900) contrasted his new species ative to the larger skull, robust molars, and
Dasymys rufulusto D. bentleyaeand D. in- dark-toned pelage ofthe latter. These mor-
comtus, but had at hand for examination phological contrasts are apparent in more
only a single specimen ofthe latter (prob- recently collected examples ofD. incomtus
USNM
ably 1 1884fromPortNatal, South availableto us from South AfricaandZim-
VOLUME 104, NUMBER 3 427
Fig. 5. Views ofthecranium andmandibleofanadultmaleDasymysrufulus(USNM 83901) from Moimt
Coffee, Liberia. Approximately xl.75.
babwe. The eastern distributional limits of as a close relative ofD. foxi, based princi-
rufulusareuncertain;examplesthatwehave pally on the former's smaller molar rows,
seen from Cameroon (AMNH specimens and instead focussed his comparisons to D.
from Tongo and Ndokayo) are referable to medius from central Africa.
rufulus. Our principal component analyses cor-
In his description of Dasymys foxi, roborateThomas'(1912)emphasisofmolar
Thomas (1912) had available for exami- size as a cardinal trait for distinguishing D.
nation examples of D. rufulus, as well as foxi from D. rufulus. The distinction rests
specimens ofDasymys from other regions notonlywiththegreaterlengthofthetooth-
ofAfrica contained in the British Museum rows in D. foxi but also with their greater
(Natural History). With only passing men- width (Tables 2, 3; Appendix 1). Breadth
tion,hedismissedMiller's(1900)D. rufulus measurements across the zygoma and
428 PROCEEDINGSOFTHEBIOLOGICALSOCIETY OFWASHINGTON
braincasecontribute substantiallytothero- sent a variety of taxa including incomtus
bust appearance ofthe skull ofD. foxi ver- proper. Only examples ofD. nudipes seem
sus that ofZ). rufulus (Figs. 4, 5), in general to typically lack a sixth pad (Hill & Carter
relativelymoresothanlengthvariables(Ta- 1941, Crawford-Cabral 1983), but we have
ble 2), although specimens of D. foxi do examined too few specimens ofthis taxon
average larger in most cranial dimensions tojudge the constancy ofthe trait. In other
(Appendix 1). Field weights and external featuresaswell—morebrightlycoloreddor-
measurements ofthe skin similarly reflect sal and ventral pelage, large size (hind foot
the larger size of D. foxi, but overlap in lengthtypically38to42mm), andtheusual
ranges is appreciable (Appendix 1). Exam- presence ofa tiny t3 on the upper second
ples ofD.foxiandD. rufulusare practically molar—nudipes represents a well-marked
inseparable on the basis of pelage colora- form that probably warrants specific status
tion:bothexhibitagrayishtoreddishbrown (also see Crawford-Cabral 1983 and Craw-
dorsumandalightto darkgrayventerwith ford-Cabral & Pacheco 1989).
varying concentrations of a creamy-buff" Available data on chromosomal varia-
wash. Red tones seem more predominant tioninthegenusDasymysislimitedtostan-
in the dorsum ofD. rufulus and gray in D. dard karyotypic preparations. Matthey
foxi, but this impression is undermined by (1958)reportedadiploidnumberof38,with
the range ofchromatic variation observed fourpairsofbiarmedautosomes, forasam-
within series ofeach form. ple (number of individuals unknown) of
We have not yet examined series of D. Dasymys from Ivory Coast. Later, Tranier
medius to form an opinion on its degree of & Gautun (1979) obtained a similar figure
similarity ofD.foxi. Inexternal andcranial forIvory CoastDasymys, 2N = 36 with six
size, however, specimens ofD.foxi closely pairs of metacentrics and submetacentrics
resemblethose ofZ>. incomtus, as known to based on two males. Both records are pre-
us from several localities in South Africa sumablyreferabletoD. rufulus,butthevari-
and Zimbabwe. The agreement is particu- ation implied by these studies should be
larly strong in their possession of heavily reconfirmed using larger samples of fresh
constructed,longtoothrows.LikeD. rufulus material. Theonekaryotypicexamplefrom
thepelagecolorofD.foxicontrastswiththe eastern Africa, a single male from Burundi,
darker, grayish-black upper parts typical of exhibited a 2N = 40, with six pairs of
D. incomtus. Moreover, the texture of its biarmedautosomes(Maddalenaetal. 1989).
fur seems finer relative to both D.foxi and The authors provisionally identified their
D. rufulus. The taxonomic significance of specimen as D. incomtus but stressed the
thesepelagedifferencesmustawaitrigorous lackofchromosomaldatabasedontrue in-
evaluation ofvariation using large series of comtus from South Africa. Although these
D. incomtus and determination ofits rela- studiessuggestthepotentialvalueofkaryo-
tionship with forms described from eastern typic information in Dasymys systematics,
and central Africa. the evidence is, as yet, too fragmentary to
Miller(1900)erredinreportingfiveplan- offer insight on species boundaries.
tar pads as characteristic of D. rufulus. A Unlike some other rodent species, sam-
sixth pad, the hypothenar, is present and, ples ofDasymys from West Africa are not
althoughsmall, isclearlydiscemable onthe restricted to a particular faunal region or
dried hind feet of most specimens com- biome. Instead their geographic distribu-
A
prisinghis type series. total ofsix plantar tion spans several major biotic realms, in-
pads also appears characterisitic ofD. foxi cluding (per Delany & Happold 1979) low-
and of most other Dasymys from central, landrainforest, Guineasavanna,andSudan
eastern, and southern Africa, which repre- savanna (Fig. 6). Moist living conditions