sand-dwellers in rocky habitats: a new species of gerbillus (mammalia: rodentia) from mali

Sand-dwellers in rocky habitats: a new species of Gerbillus(Mammalia: Rodentia) from Mali

L. Granjon1,2, V. M. Aniskin3, V. Volobouev1 and B. Sicard2

1 Laboratoire de Zoologie, MammifeÁres et Oiseaux, MuseÂum National d'Histoire Naturelle, 55 rue Buffon, F-75005 Paris, France2 Laboratoire de Mammalogie (CBEO, UMR 022), IRD (ex-ORSTOM), B.P. 2528, Bamako, Mali3 Severtsov Institut of Ecology and Evolution, Russian Academy of Sciences, 33 Leninskii Prospect, Moscow, 117071 Russia

(Accepted 22 March 2001)

Abstract

The genus Gerbillus (Mammalia: Rodentia) is characterized by an important morphological homogeneity

associated with an extensive karyotypic diversity. Here we describe a new species of Gerbillus from the

Inner Delta of the Niger River in Mali, on the basis of morphological and chromosomal data. This new

species seems to be morphologically very similar to G. campestris, another bare-footed species known from

northern Africa. Nevertheless, it can be distinguished from the latter species by a different molar pattern

and a speci®c karyotype. The three specimens on which this description is based were caught in a rocky

outcrop, and behavioural observations in captivity suggest an adaptation to life in rocky habitats. This

®nding is unusual as rodents of the genus Gerbillus are generally sand-dwellers. Moreover, this new species

enriches the gerbil community of this Sahelian region, in which at least nine species of the genus can be

found. Finally, this description highlights the importance of chromosomal studies for the characterization

of biological diversity in tropical rodents, where morphological sibling species are numerous.

Key words: rodent, systematics, chromosomes, sibling species, rocky habitat, Gerbillus

INTRODUCTION

The systematics of the genus Gerbillus is far from beingwell understood, as pointed out by Musser & Carleton(1993) in the last edition of Mammal species of the world(Wilson & Reeder, 1993). The latter authors closelyfollowed Lay (1983), who provisionally recognized 62species of Gerbillus, most of which are in need of criticaltaxonomic revision. The various synonymies proposedby previous authors were indicated, but the number ofpotentially valid species was kept high by Lay (1983) tohighlight the need for a comprehensive revision beforeaccepting their validity. Within the genus Gerbillus,various characters such as the plantar surface of thefeet, have been thought to hold systematic signi®cance(Lay, 1983). Ellerman (1940) distinguished Gerbillus andDipodillus as subgenera and included in each, taxa withhairy feet and naked feet, respectively. Petter (1975) alsoconsidered the subgenus Gerbillus as comprising thehairy-footed species, whereas the subgenus Hendeca-pleura and the genus Monodia (considered a subgenusby other authors) were characterized by naked hind feet.Lay (1983) questioned the validity of this character andothers, such as the presence or absence of an accessorytympanum, and preferred not to retain any subgenericdivisions in the genus.

Nevertheless, this characteristic of gerbil hind feetremains an important descriptive feature, and helps topartition the genus, irrespective of its effective sys-tematic signi®cance. Thirty-two provisional specieswere listed by Lay (1983) in the group of naked-footedgerbils. Among them, 13 possess an accessory tym-panum while 12 lack one and the condition of sevenspecies is unknown in this respect. Karyotypic datawere known for only seven of the 32 naked-footedspecies (G. amoenus, G. campestris, G. dasyurus,G. garamantis, G. henleyi, G. nanus and G. simoni).Chromosomal results were nevertheless already avail-able for G. pusillus (Capanna & Merani, 1981), asrecalled by Qumsiyeh & Schlitter (1991) and additionalones were subsequently provided by Volobouev,Lombard et al. (1995) on G. poecilops. Karyotypic dataare especially informative in this group, where siblingspecies are probably numerous (Lay, Agerson & Nadler,1975; Petter, 1975; Qumsiyeh & Schlitter, 1991;Granjon, Bonnet et al., 1999) and would undoubtedlyhelp to resolve most of the taxonomic problems identi-®ed by Lay (1983). Here we use chromosomal resultstogether with morphological and biometric data, todescribe a new species of Gerbillus, based on specimenscollected in the northern part of the inner delta of theNiger River in Mali. Moreover, the ecological context in

J. Zool., Lond. (2002) 256, 181±190 # 2002 The Zoological Society of London Printed in the United Kingdom DOI:10.1017/S0952836902000213

L. Granjon et al.182

Fig. 1. Map of the type-locality (b) in Mali (a) and two photographs of the type locality, showing the rocky outcrop (c) and the

precise trapping site (d) of the type specimen.

(b)

(c)

(d)

(a)

which these specimens were found was exceptional inthis group of usually sand-dwelling species. Hence, these®ndings not only add to our knowledge of the rodentdiversity in this region, but also enable us to addressquestions related to community ecology in this genus.

DESCRIPTION

Gerbillus rupicola nov. sp.

Holotype

CG 1999.184 (MuseÂum National d'Histoire Naturelle,Paris, France), adult female, collected on 2 January1999 by B. Sicard and S. Ag Atteynine; skin with skulland mandible, in good condition; liver tissue preservedin ethanol, and cells and tissues cryopreserved (number1998.64 in the cell and tissue collection), from which thekaryotype was prepared.

Type locality

Emnal'here, right bank of the main course of the NigerRiver, 12 km east Mopti, Mali (14828'26''N;4805'16''W). The exact place of capture was at mid-slopeof a rocky outcrop situated c. 100 m westwards from thevillage, itself located close to the main road leading toSeÂvareÂ (Fig. 1). Herbaceous cover (mainly Cenchrusbi¯orus, Aristida kertingii, Crotalaria spp., Cassia tora,Pergularia dania, Celosia trygina and Borreria spp.) isabundant up to mid-slope. Scattered shrubs and smalltrees include Borassus aethiopium and Balanites aegypt-iaca at the bottom, and several species adapted to rockyenvironments on the slope (Combretum micranthum,C. glutinosum, C. nigricans, Acacia ataxantha andDiospyros mespiliformis) with Cola cordifolia directlyestablished on the rock outcrops.

Paratypes

CG 1999.182 and CG 1999.183, both adult femalesdeposited as skin with skull and mandible specimens atthe MuseÂum National d'Histoire Naturelle, Paris,

Fig. 2. Skins of the holotype and two paratypes of Gerbillus rupicola nov. sp.

183A new species of Gerbil from Mali

France; collected on 2 January 1999 by B. Sicard andS. Ag Atteynine at Emnal'here, within 5 m of the placeof capture of the holotype; liver tissue of both specimenspreserved in ethanol; karyotype of CG 1999.183 pre-pared from bone marrow.

Distribution

Known only from the type locality.

Etymology

The name refers to the rocky habitat where these speci-mens were caught, an exceptional one for gerbils.

Diagnosis

A large-sized gerbil, with a long tail terminated by a tuftof dark, long hairs, and soles of the hind feet naked(Fig. 2). Dorsal colour of pelage tawny-rufous, with thebase of hairs grey (more marked in specimen 1999.183).Ventral colour of pelage white at the base, with supraor-bital and postauricular white spots. A small tuft ofwhite hairs also present just above the anterior part ofthe eyes.

Skull (Fig. 3) with a relatively broad braincase,narrowing gradually to the interorbital constriction;nasals long and narrow; zygomatic plate forming a rightangle with the anterior part of the zygomatic arch, whenviewed from above (or below); bullae relatively small,with posterior margin of their mastoid portion notextending beyond level of supraoccipital; auditorymeatus without a well-marked swelling; accessory tym-panum absent.

Upper incisors with single groove; molar rows rela-tively long; M1 with an anterior cusp wellindividualized; transverse lamination of the followingcusps present in M1, M2 and M2; early con¯uency of thethree anterior cusps of M1; M3 small but showing aposterior accessory cusp (Fig. 4).

Karyotype consisting of 2n = 52 chromosomes, with16 acrocentric pairs (fundamental number of arms,NF = 72; Fig. 5).

Measurements

Standard external and cranial measurements are givenin Table 1 for the holotype and the two paratypes.

Ecology and behaviour

Behavioural observations made during the period ofcaptivity of the three specimens revealed abilities forclimbing similar to those previously described inAcomys sp., another rupicolous rodent living in rockyinselbergs (Sicard & Tranier, 1996). The gerbil is ableto climb on smooth surfaces as it can `hug' the

surfaces. This is the result of a particular adaptation ofthe articulation of limbs on the pectoral and pelvicgirdles that allows them to move in a parasagittalplane. The gerbils are able to climb narrow structures(i.e. stems) by tightly gripping them as a result of amovement of pronation that allows the hands and feetto oppose one another. All these behaviours suggestoriginal adaptations of the species to life in rockyenvironments.

Karyotype

Chromosome preparations for female CG 1999.83±183were obtained from bone marrow cells by the standardcolchicine method, whereas those of female CG 1999.184were made from ®broblast cultures established after askin biopsy. The chromosomes of both specimens werestudied by G-banding (GTG: Seabright, 1971) andC-banding (CBG: Sumner, 1972). In addition, replica-tion banding (RBG) was carried out for female CG1999.184 (Viegas-PeÂquignot & Dutrillaux, 1978; ISCN,1995). At least 20 metaphases were analysed for eachspecimen.

The karyotype of both females studied contains eightpairs of meta- to submetacentrics, two pairs of sub-telocentrics and 16 pairs of acrocentrics, giving a diploidnumber (2n) of 52 and a number of chromosome arms(NF) of 72 (Fig. 5). RBG allowed unambiguous identi®-cation of all autosomal pairs as well as the Xchromosomes. In addition to one late replicating Xchromosome, there is an additional pair of clearlyheteromorphous middle-sized metacentrics which is latereplicating (no. 7; Fig. 6). Such a heteromorphous pairthat is late replicating and C-band negative is character-istic of many species of Gerbillinae studied by repli-cation banding (Viegas-PeÂquignot, Petit et al., 1986;Volobouev, Lombard et al., 1995; Volobouev &Granjon, 1996).

The C-banding technique revealed large blocks ofcentromeric heterochromatin on all autosomal pairs,many of which were heteromorphous. This hetero-morphism is most likely related to the molecularcomposition of the centromeric and other heterochro-matic regions as has been observed in other Gerbillusspecies (Viegas-PeÂquignot, Derbin et al., 1982; Viegas-PeÂquignot, Benazzou et al., 1984; Volobouev, Vogt etal., 1995). The short arms of the subtelocentric chromo-somes (pairs 8 and 9) are entirely heterochromatic.Additionally, the prominent distal parts of both arms ofthe X chromosomes are also heterochromatic, althoughtheir staining intensity is weaker than that of thecentromeric region (Fig. 7).

DISCUSSION

Description and comparisons

Among the naked-footed gerbil species listed by Lay



Fig. 3. Dorsal, ventral and lateral views of the skull of the holotype of G. rupicola nov. sp.

(1983) as potentially valid, the majority are currentlyknown only from the Middle East and/or from easternAfrica. The species whose geographic distribution in-cludes the western part of northern Africa (Maghreb,western Sahara and Sahel) are G. campestris, G. gara-mantis, G. henleyi, G. jamesi, G. maghrebi, G. nanus,G. simoni, and possibly G. amoenus (Osborn & Helmy,1980). Gerbillus henleyi is a small species (total length< 155 mm, see Maddalena et al., 1988; Kowalski &

Rzebik-Kowalska, 1991), whereas G. nanus can reach alarger size: total length < 205 mm in Algeria (Kowalski& Rzebik-Kowalska, 1991); < 200 mm in Mauritania(F. Colas, pers. obs.); head and body length < 105 mm,and tail length < 145 mm in Morocco (Saint Girons &Petter, 1965; Aulagnier & TheÂvenot, 1986). Both arecharacterized by a diploid number of 52 chromosomes,with 38±42 acrocentric chromosomes (summarized inVolobouev, Lombard et al., 1995). Gerbillus simoni


Fig. 4. Detail of the maxillary molar row and skull in dorsal view of Gerbillus rupicola nov. sp. (CG 1999. 184, left), compared

with those of G. campestris (CG 1997. 396, right).

(= Dipodillus simoni) is also a small species: total length< 190 mm (Petter, 1975; Osborn & Helmy, 1980),characterized by a short tail, and a karyotype with2n = 60 chromosomes, including 50±52 acrocentricchromosomes (Wassif, Lutfy & Wassif, 1969). Gerbillusjamesi was included in G. campestris by Corbet (1978),as was G. maghrebi by Lay (1983). However, the onlyknown specimen of G. jamesi, which may well be a sub-adult G. nanus or G. amoenus based on features andmeasurements of the skull (D. A. Schlitter, pers.comm.), is small: total length 184 mm (Harrison, 1967).On the other hand, G. maghrebi is characterized by arelatively short, untufted tail, 103±110 mm long (Schlit-ter & Setzer, 1972) when compared with G. campestris(see Table 1). Gerbillus garamantis may represent aNorth African form or a vicariant species of G. nanus(Lay, 1983; Kowalski & Rzebik-Kowalska, 1991) andwas found by Matthey (1954) to have 2n = 54 chromo-somes. Gerbillus amoenus is a small species (averagehead and body length 81 mm, tail 106 mm according toOsborn & Helmy, 1980) the diploid number of which is2n = 52 chromosomes, with 42/43 acrocentrics (Wassif etal., 1969). Finally G. campestris, probably the speciesmost similar to G. rupicola on external morphology, ischaracterized by a karyotype with 2n = 56±58 chromo-somes, including 40±44 acrocentric chromosomes

(Jordan, Davis & Baccar, 1974; Lay, Agerson & Nadler,1975; Vistorin & Gamperl, 1978; see Table 2 forcomments).

When compared to all these species, G. rupicola doesshow signi®cant differences that warrant its speci®cstatus. It is clearly larger than G. henleyi, G. simoni,G. amoenus, G. nanus and G. garamantis, from which italso differs from a chromosomal point of view (2n = 52in G. rupicola vs 2n = 54 in G. garamantis and 2n = 60in G. simoni, 16 pairs of acrocentric autosomes inG. rupicola vs 19 to 21 pairs in G. henleyi, G. nanus andG. amoenus; see Table 2). An attempt to compare R-and G-banding patterns of G. rupicola (Figs 6 & 8) withthose available for other Gerbillus species sharing thesame or similar diploid numbers, namely G. campestris(Vistorin & Gamperl, 1978), G. hesperinus (Viegas-PeÂquignot, Bennazou et al., 1984); G. nanus, G. henleyiand G. poecilops (Volobouev, Lombard et al., 1995)revealed few if any clear homologous banding patterns,which strongly supports the high speci®city of itskaryotype.

Gerbillus rupicola is also much larger than G. jamesi,and shows clear differences in tooth pattern (molartubercles alternate in G. maghrebi, not in G. rupicola)


Fig. 5. Karyotype of Gerbillus rupicola (CG 1999.183) after

standard Giemsa staining.Fig. 6. Karyotype of Gerbillus rupicola (CG 1999.184) after

RBG-banding. Note that one X chromosome and pair 7 are

late replicating.

and tail characteristics when compared with G. magh-rebi, the tail of which is relatively short and lacks aterminal tuft (Schlitter & Setzer, 1972). Finally, it isclearly different in tooth pattern and skull shape(Fig. 4), as well as in karyotype from G. campestris,from which it is similar in external morphology andskull biometry and with which it may coexist. Thisoverall biometrical similarity between G. rupicola andG. campestris is illustrated by the fact that all theclassical external and cranial measurements taken onthe three specimens of G. rupicola fall within the rangeof measurements obtained from a sample of G. campes-tris from various localities in Algeria and Morocco(Table 1). Relative to the same features in G. campestris,G. rupicola tends to have a long tail, a long maxillarytoothrow and short tympanic bullae.

Biogeography and ecology

The speci®c richness, distribution, and ecology ofgerbils south of the Sahara are much less well knownthan in northern Africa (see for instance Petter, 1961;Saint Girons & Petter, 1965; Ranck, 1968; Jordan et al.,

1974; Lay, 1975; Osborn & Helmy, 1980; Aulagnier &TheÂvenot, 1986; Kowalski & Rzebik-Kowalska, 1991).Nevertheless, recent investigations in the Sahelian striphave yielded new data, speci®cally in Mauritania(Granjon, Cosson et al., 1997; Granjon, Bonnet et al.,1999; Ba, Mathiot et al., in press), Senegal (Duplantier,Granjon & Ba, 1991; Ba, Granjon et al., 2000), Mali(Dobigny et al., in press; B. Sicard, pers. comm.),Burkina Faso (Sicard, 1992) and Niger (G. Dobigny,pers. comm.). The gerbil community in this region hasproved to be diversi®ed, as seven species have beenunambiguously identi®ed (G. cf. nancillus, G. henleyi,G. nanus, G. gerbillus, G. nigeriae, G. campestris, G. tara-buli). Another large-sized, hairy-footed species with 38chromosomes has also been found in northern Mali,and provisionally called G. cf. pyramidum (Dobigny etal., in press). With the addition of G. rupicola, there arenine species present in this Sahelian zone. Note that allof them can be found in a small, central area of thisWest African Sahelian region, located between south-eastern Mauritania, northern Mali, and western Niger,encompassing both sides of the Niger River loop (seeFig. 1).

At a more local scale, G. rupicola has been found in


Table 1. Standard external and cranial measurements of the holotype and two paratypes of G. rupicola nov. sp., compared to asample of G. campestris from Algeria and Morocco (specimens in MNHN, Paris). Data given for G. campestris are minimum ±mean ± maximum, and sample size for each

Holotype Paratype Paratype G. campestrisCG1999.184 CG1999.183 CG1999.182

Sex Female Female FemaleWeight 33 39 39Head and body length 106 102 100 82±94±110, 30Tail length + hair tuft length 142 + 12 ± 136 + 10 110±131±145, 27Hind foot length + claw length 26 + 1 27 + 1 27 + 1 25±27±30, 32Ear length 16.5 14.5 15 12±15±17, 32Greatest length of skull 30.6 29.5 29.9 28.0±29.9±33.4, 34Condylobasal length 27.1 26.5 26.4 24.6±26.7±30.2, 34Zygomatic breadth 15.9 15.6 15.9 14.7±15.7±17.2, 31Least interorbital breadth 5.5 5.4 5.3 4.7±5.5±6.0, 34Maxillary diastema length 7.5 7.4 7.3 6.9±7.5±8.9, 34Length of anterior palatal foramina 5.2 5.5 5.5 4.6±5.1±6.3, 32Maxillary toothrow length 4.1 4.1 4.2 3.6±3.9±4.2, 33Braincase breadth 13.6 13.9 14.1 13.2±13.8±14.5, 34Tympanic bulla length 7.8 7.9 7.8 7.8±8.2±9.0, 34

Table 2. Summary of chromosomal data available for naked-footed gerbils present in north-western Africa. 2n = diploid numberof chromosomes; M/SM = number of biarmed chromosomes; A = number of acrocentric chromosomes; the morphology of the Xand Y sex chromosomes is given when known. M/SM and A include sex chromosomes only when they were not identi®ed (*). InG. campestris, two pairs of subtelocentric chromosomes have been counted as acrocentric by some authors (Wassif et al., 1968;Lay et al., 1975), resulting in higher A (up to 48) and lower M/SM (down to 8). They are considered biarmed here

2n M/SM A X Y Reference

G. amoenus* 52 9±10 42±43 ± ± Wassif et al. (1969)G. campestris 56±58 12±14 40±44 M/SM M/SM In Qumsiyeh & Schlitter (1991)G. garamantis 54 12? 40? SM SM Matthey (1954)G. henleyi 52 8±12 38±42 SM A Lay et al. (1975); Volobouev et al. (1995)G. nanus 52 8±12 38±42 M/SM A In Qumsiyeh & Schlitter (1991); Volobouev et al. (1995)G. rupicola 52 18 32 M This studyG. simoni* 60 8±10 50±52 ± ± Wassif et al. (1969)

close proximity to G. tarabuli, G. nigeriae and G. henleyi(B. Sicard, pers. comm.), which raises questions relatedto the community ecology in this guild. However, thehabitat where the three specimens of G. rupicola werecaught, i.e. a rocky hill, makes this species one of the fewexceptions in this genus in terms of habitat preferences.Indeed, species of Gerbillus are typical sand-dwellers,with naked-footed species more associated with induratesoils. The association in Emnal'here of G. rupicola withAcomys sp., which are typical rock-dwellers, suggests asurprising adaptation to this habitat that deservesfurther studies. Similar accounts concern the presence ofG. lowei or G. campestris venustus in various jebels ofnorthern Sudan (Happold, 1969). The latter species wasdescribed as co-existing with Acomys cahirinus cineraceusin crevices and grassy patches on some jebels (Happold,1967). Gerbillus campestris is also often mentioned asbeing associated with rocky habitats (Rosevear, 1969).Finally, the Middle Eastern species G. dasyurus is con-sidered an inhabitant of the rocky steppe desert, but isalso able to live in moist, more vegetated biotopes(Harrison & Bates, 1991). As for G. rupicola, furthersampling should aim at more precisely determining itscurrent distribution, and check how close is its associa-tion with rocky habitats.

Acknowledgements

We are grateful to S. Ag Atteynine for help in ®eldwork, R. Cornette for work on the type specimens,J. Cuisin for collection-related assistance, C. Houssinand G. Dobigny for help in preparing the illustrations.We also thank J. Britton-Davidian for checking theEnglish, M. Tranier for additional information on thetype specimens and for his remarks on a ®rst draft ofthis paper, as well as D. Schlitter who also commentedon a previous version, and sent us very useful infor-mation on gerbils.

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sand-dwellers in rocky habitats: a new species of gerbillus (mammalia: rodentia) from mali

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