Notes on rodents of the genus Gerbillus (Mammalia: Muridae: Gerbillinae) from Morocco

L... -..J.' V OF

ILLh.oiw LIBRARY

AT UR3ANA-CHAV.PAIGN

N/mr*''^ ------- -M-.,^

/

^FIELDIANA
Zoology

Published by Field Museum of Natural History

Volume 65, No. 8 October 23, 1975

Notes On Rodents of the Genus Gerbillus
(Mammalia: Muridae: Gerbillinae) from Morocco

Douglas M. Lay

Department of anatomy

University of North Carouna at Chapel Hill

The current systematics of rodents of the genus Gerbillus from
Morocco and adjacent eireas of Algeria are based upon the works of
Cabrera (1932), Ellerman and Morrison -Scott (1951), and St. Girons
and Petter (1965). Much remains to be learned of the distribution,
ecology, evolution, and systematics of these rodents in North Africa.
Africa.

Species of Gerbillus have the plantar surface of the hindfeet
either covered with hair or naked and classification reflects this
dichotomy at either the generic or subgeneric levels (e.g., Allen,
1939; Ellerman and Morrison -Scott, 1951; Petter, 1959; Wassif et
al., 1969; Harrison, 1972; Schlitter and Setzer, 1972). Generally,
hairy -footed Gerbillus inhabit areas of sand or light sandy soils and
naked -footed Gerbillus occur on compact substrates containing
much rock. This study concerns only hairy -footed species of
Gerbillus.

During 1971 and 1972, I obtained specimens of Gerbillus from
several localities in Morocco for cytogenetic study. I visited
Morocco in 1973 and collected additional specimens, karyotypes,
and data on the environmental and physiographic characteristics of
specimen localities. Analysis of this material suggests that at least
three medium -sized species of hairy -footed Gerbillus occur in
Morocco, two of which are described as new.

MATERIALS AND METHODS

Chromosomes were analyzed from cell suspensions of femoral
bone marrow (Nadler and Lay, 1967). Karyotypes were arranged in

Library of Congress Catalog Card No.: 75- 18214

US ISSN 0015-0754 lATHAL KTWT 9M

Publication 1213 89

iiFC 2 21975
UBRABir

90 FIELDIANA: ZOOLOGY, VOLUME 65

pairs according to size and centromere position (metacentric,
submetacentric, acrocentric). The fundamental number of
chromosome arms (FN) is computed by counting biarmed
autosomes as two arms and acrocentric autosomes as one arm.

Voucher specimens of karyotyped animals and other specimens
were prepared (usually as skins and skeletons) and deposited in the
collections of the University of Michigan, Museum of Zoology
(UMZ) and Field Museum of Natural History (FMNH). Other
specimens and cytogenetic preparations remain in my personal
collection (DML), but will ultimately be deposited in the FMNH
collection. Specimens in the collections of the British Museum
(Natural History) (BM) were used in compeuisons.

The following cranial dimensions, useful in distinguishing the
taxa treated herein, were taken with dial calipers and rounded to
the nearest 0.1 mm.: greatest breadth across the posterior mastoid
bullae; greatest breadth across squamous portion of temporal
bones; the width of the anterior process of the bsisioccipital at its
articulation with the sphenoid was recorded indirectly as the least
width between the medial walls of the tympjmic bullae at the level
of the above articulation; narrowest point of the interorbital
constriction; height from base of tympanic bulla to superiormost
point of suprameatal triangle (measured with ventral parts of
tympanic bullae and incisors placed on glass slide and then
subtracting slide thickness); greatest length of skull; length of
nasals from distal tip to frontal articulation; greatest breadth
between the lateral tips of the zygomatic process of the temporal
bones. The anatomical terminology of middle ear structures follows
that of Lay (1972). Color references are based on the color guide of
Palmer (1962).

Gerbillus hoogstraali sp. nov.

Hohtype. — Adult female, in fluid, skull removed, FMNH, no.
114647, from 7 km. S. Taroudannt, Morocco, elevation about 116
m.; taken on July 16, 1973 by D. M. Lay. This locahty lies about 1
km. south of the Sous river.

Description. — A middle-sized species, mean and extremes of standard external
measurements of seven specimens are (in mm.): total length, 208.1 (190-221); tail
length, 113.8 (103-123); hind foot length, 27.3 (26-28); pinna length, 16.6 (16-17).
Dorsal hair color is intermediate to cinnamon and buffy brown and is clearly
delimited from the white ventral fur. A whitish postauricular spot exists, but its
prominence is reduced due to a buffy suffusion. The posterior borders of the nasal
bones at the articulations with frontal bones are either squared off or concave

o

c

a

«D

iH

rH

<N

lO

CO

o

o

im

■"^

O

o

o

o

o

o

•s

>
73

•a
s

I

3 <N

> 1

CO Q

OS «0
« CO
2 rH

0)

CO

00

^

CO

(N O

d d

i^; ^ o

O 00

CO

lO O CD
O <N rH

d d d

i6

(N

d t>;

o 00

CO i-H o

CO

d

CO
CO

<N

g

o
o

•a

o

5

CO

.2
'5
«

CO

O X

T3 2
eS O

2 J- 4»

3

O X

•f= c o •?;

a, 3 —

o *J

•o o

2: -k->

13 ^

O •=

c a, <u
•SI E *? «

o

c

0)

H

•J

n

91

Fig. 1. Dorsal and ventral views of skulls of: A, GerbiUus hoogstraali; B, G.
occiduus; C, G. hesperinus. White arrows denote lateral pterygoid processes. Blade
arrows mark the anterior end of the basioccipital at its suture with the basisphenoid.
The letter T ia on the ventral surface of the tympanic bulla. Scale units in
millimetera.

92

LAY: RODENTS OF GENUS GERBILLUS

93

%^-*f I ^k%/

Fig. 2. Lateral view of skulls and mandibles. Black dots mark the extent of the
posterior mastoid chambers of the bulla. 1, Anterior mastoid chamber, 2, posterior
superior mastoid chamber, 3, posterior inferior mastoid chamber. White arrows
denote posterior inferior end of the external line of the septum separating the
superior and inferior posterior mastoid chambers. Other symbols as in Figure L

anteriorly. The anterior end of the basioccipital bone and the interorbital constriction
are relatively narrow (table 1). Diploid number of chromosomes is 72 (fig. 3).

Measurements. — Selected measurements of the holotype are: total length 211;
tail length, 121; hind foot length, 28; ear length, 17; greatest length of skull, 29.3;
length of nasals, 10.9; breadth across temporals, 13.7; breadth between tympanic
bullae at basioccipital-sphenoid suture, 0.9; interorbital constriction, 5.5; height of
tympanic bulla and suprameatal triangle, 7.4.

Distribution. — Presently known only from type locality, but
likely occurs in suitable sandy habitat within the lower Sous valley.

Habitat-Natural History Miscellanea. — The type locality
extends approximately 7 km. in both east- west and north-south
directions and is characterized by areas of low sand dunes (3.5-4.5
m. above level ground) and intervening level ground of hard-packed
sandy clay. Fine reddish sand forms a light cover on the hard
alluvial surface. During the days of July 16 and 17, 1973 a strong
westerly wind shifted loose sand over the entire area.

The principal vegetation consisted of thorny Zizyphus shrubs
which varied in density but seldom exceeded 1.5 m. in height. Six
specimens were trapped at burrow entrances located at the bases of
Zizyphus stands. No animals were caught elsewhere, but fresh

94 nELDIANA: ZOOLOGY. VOLUME 66

tracks and diggings indicated activity throughout the area. One
large rodent, possibly Meriones grandis, was observed in the area at
dawn one morning. Shed snakeskins were very numerous in the area
but no snakes were observed during night (6 hours) or day (8 hours)
observations. One female trapped July 16 showed six placental scars
and two subadults (total length 190, 191) were trapped at the same
time, indicating a breeding period in the late spring or early
summer.

Specimens examined. - Total of 10: 114567 FMNH (type);
117632, 122760-2 UM; 4572-3, 4575-7 DML. Karyotypes are
available for three of these specimens.

Remarks. — I take great pleasure in naming this species for Dr.
Harry Hoogstraal in recognition of his signiHcant contributions to
knowledge of the fauna and medical zoology of North African
mammals.

Comparisons. — Pertinent comparisons are provided in the
following species account.

G«rbiUu8 occiduus sp. nov.

Holotype. - Adult female, skin and skeleton, FMNH no. 114648
from Aoreora (80 km. WSW Goulimine), Morocco, elevation about
43 m.; taken on January 16-17, 1971 by L. Robbins and J. Gruwell.

Description. — A middle-sized species, mean and extremes of standard external
measurements of eight specimens are total length 207.1 (202-213); tail length. 111.0
(103-119); length hind foot, 29.1 (28-30); length pinna, 13.9 (12-16). Doraal hair color
is intermediate to cinnamon and huffy brown and is clearly delimited from the white
ventral fur. A prominent white postauricular spot exists. The posterior border of the
nasal bones is wedge-shaped or truncated (fig. 1). The anterior end of the
basioccipital bone and the interorbital constriction are relatively wide (fig. 1, table 1).
Diploid number of chromosomes is 40 (fig. 4).

Measurements. — Selected measurements of the holotype are: total length, 209;
tail length. 111; hindfoot length, 29; pinna length, 15; greatest length of skull, 30.7;
length of nasals, 12.7; breadth across temporals, 14.2; breadth between tympanic
bullae at basioccipital -sphenoid suture, 1.5; interorbital constriction, 6.0; hei^t of
tympanic bulla and suprameatal triangle, 7.6.

Distribution. — Presently known from only the type locality.

Habitat-Natural History Miscellanea. - The type locality is an
area of sparsely vegetated sand dunes but no other information is
available (E.T. Hopper, in litt.).

Two pairs of animals trapped wild on January 16-17, 1971 bred
beginning in June 1971. One female gave birth to ten litters of the
following sizes (dates in parentheses): 3 (June 14), 4 (July 14), 3

LAY: RODENTS OF GENUS GERBILLUS 95

(Aug. 22), 5 (Sept. 22), 5 (Nov. 2), 3 (Dec. 10), 5 (Feb. 5, 1973), 1
(Aug. 6, 1972). Another female produced a single litter of four on
June 15, 1971. Mean litter size is 3.45. At least four of the litters of
the first female totalling 16 progeny, contained six partial albinos:
the fur was entirely white but the eyes were black. The ratio of 16
wild: 6 pfutial albinos suggests that both parents were heterozygous
for a simple mendehan recessive locus for partial albinism. All other
progeny had wild type coat color. Causes for the failure of these
albinos to breed were not determined.

Specimens examined. — Total of 18; FMNH(type) 114648;
UMZ 122765-6, 122773-86; DML 4392. Karyotypes are available for
nine of these.

Remarks. — The name occiduus is Latin for western.

Comparisons. — G. hoogstraali, G. occiduus, and G. hesperinus
Cabrera, 1906 are distinguished by the following cytogenetic,
cranial, and pelage characteristics.

Karyotypes of the 8p>ecies differ significantly. G. hoogstraali
has a diploid complement (2N) of 72, and FN = 80. The karyotype
contains six submetacentric and 64 acrocentric autosomes. The sex
chromosomes are a large submetacentric and a medium -sized
metacentric (fig. 3). G. occiduus shows a 2N = 40 and FN = 76.
The karyotype shows eight metacentric and 30 submetacentric
autosomes. The sex chromosomes are medium -sized metacentrics
(fig. 4). G. hesperinus Cabrera possesses 2N = 58 and FN = 76. The
karyotype reveals 20 submetacentric and 36 acrocentric autosomes.
The sex chromosomes are large and medium -sized metacentrics (fig.
5).

The diagnostic cranial features of G. hesperinus that dis-
tinguish it from both G. hoogstraali and G. occiduus are: less
voluminous auditory bullae with smaller tympjuiic and posterior
mastoid chambers; short and constricted posterior palatine fora-
minae; narrow lateral pterygoid processes, which produce smaller
fossae for the medial pterygoid muscles; small posterior inferior
mastoid chamber (about one-half as voluminous as in either of the
other two species) (figs. 1, 2).

The crania of G. hoogstraali and G. occiduus can always be
identified by the anterior process of the basioccipital that is broad
in the latter and narrow in the former and the least interorbital
breadth which varies in the same manner (fig. 1, table 1). This
difference in basioccipital width is also constant between subadult

It ft* •■

it II II II il II

II A» II II II II

1% II II II II 11

41 II II II II II

II II »A i« A4 It

.. .. J,

X Y

Fig. 3. Karyotype of G. hoogstraali with 2N »■ 72. It is not possible to
distinguish between the X and Y chromosomes until female specimens are studied.

li U X^ ly IK XX

XV lA IK hh %A "ht

X Y

XX

X X

PlC. 4. Karyotype of G. occiduus with 2N - 40.

96

LAY: RODENTS OF GENUS GERBILLUS 97

u u u 1^ u n

U %% C^ h< ««

Jin if\ HA n hh 4A
M III M A/) a U

M 19 Af Ad Ift

K

X Y

Fig. 5. Karyotype of G. hesperinus with 2N = 58. It is not possible to distinguish
between the X and Y chromosomes until female specimens are studied.

specimens of both species. The nasal bones (at the articulation with
the frontal bones) are truncated into a posteriorly directed wedge in
G. occiduus but are broader and squared off or are concave
anteriorly in G. hoogstraali (fig. 1). Beading of the supraoribital
ridge is usually more prominent in G. occiduus particularly along
the posterior third of the ridge (figs. 1, 2). The external orifices of
both hypoglossal canals are readily visible when the inferior surface
of the skull is viewed from the anterior in G. occiduus, but the
laterally broadened occipital condyles obscure these canals in G.
hoogstraali. The portion of the tympanic bullae anterior to the
external auditory meatus is more inflated in all dimensions in G.
occiduus than in G. hoogstraali (figs. 1, 2). Further, these two
species differ significantly in at le£ist five cranial dimensions (table
1).

These species differ in a number of pelage features. A narrow
but distinct dark eye ring characterizes G. occiduus but is less
prominent than that in G. hoogstraali, in which a distinct ring of

M FIELDIANA: ZOOLOGY, VOLUME 65

dark hairs surrounds the eye. The eye ring of G. hesperinus is very
similar to that of G. occiduus, but lacks the clear white band of hair
between the eye and base of pinna that characterizes the latter. The
pinnae of G. occiduus show a slight, dark pigmentation around the
distal free edge while the distal one-half to two-thirds are darkly
pigmented in G. hoogstraali and G. hesperinus. IHnna length of G.
occiduus averages shorter than in the other two species. Mean
pinna length, range, and sample sizes are: G. hoogstraali (16.3, 14-
17, n « 9); G. hesperinus (16.1, 14-19, n = 12); G. occiduus (13.8,
12-16, n = 8).

Prominent black terminal rings on the fresh dorsal pelage of G.
hoogstraali and hesperinus give this region a much darker
appearance than in G. occiduus. Further, the subterminal rings are
more cinnamon colored in the former two species; the latter tends
toward buffy brown.

Other Specimens Examined. — G. hesperinus, total of 14 from
Morocco: 5 km. NE-5 km S Essouira ( = Mogador), UM 122755-57,
22.5.30.24-31, (BM) 7.6.17.3, DML 4582; Cape Tefelney, (BM)
66.6067.

Discussion. — The ranges of G. hesperinus, G. hoogstraali, and
G. occiduus are allopatric, each is apparently limited to a relatively
small area of sand, and all lie near or along the Atlantic coast west
of the High Atlas and Antiatlas mountains.

The High Atlas extend to the ocean north of Agadir and
separate the sandy lowlands inhabited by G. hesperinus and G.
hoogstraali. Similarly, an extension of the Antiatlas reaches the sea
south of Tiznit forming a barrier between the ranges of G.
hoogstraali and G. occiduus.

To my knowledge, hairy-footed Gerbillus are known to occur
only in and around places of abundant, loose sand. Thus, the
availability of loose sand seemingly constitutes an ecological
constraint on the distribution of these gerbils and particularly the
three above-mentioned species. The intervening mountain ranges
are formidable barriers. Not only are they almost exclusively rock,
but the higher regions support(ed) extensive forests, which could
also form a significant barrier to dispersal.

If the distributions of these three species are to be interpreted
on the basis of the present geomorphology and ecology of
southwestern Morocco, one could conclude that G. hesperinus and
G. hoogstraali dispersed across two and one mountain range,

LAY: RODENTS OF GENUS GERBILLUS 99

respectively, to reach their present ranges. Another explanation
seems more likely.

The crustal orogeny that produced the present elevations of the
High Atlas and Antiatlas mountains began in the late Miocene and
continued into the middle Pliocene (Choubert, 1945). The Sous
valley, a broad syncline, was formed during the initial uplift of the
High and Antiatlas ranges.

Pliocene and Early Pleistocene eustatic sea levels were high
(200-300 m. above present MSL), but fluctuated, and most or all of
the areas now inhabited by G. hesperinus, G. hoogstraali, and G.
occidims were inundated (Choubert, 1950). Sea levels for the period
between 1,000,000 and 400,000 BP are insufficiently known, but
were considerably higher than at present during most of this time
(Lecointre, 1952); but, levels during the period from the latter date
to the present are known with considerable accuracy (Fairbridge,
1961; Kassler, 1973). Mean sea level varied between 20 and 100 m.
above the present level from 400,000 until about 120,000 BP, the
beginning of the Riss-Illinoian glacial. The Riss and Wurm glacial,
interstadial and interglacial sea level changes are well documented
and averaged much lower than present levels (Fairbridge, 1961;
Kassler, 1973). The Riss marine regression exposed a corridor along
the present continental shelf that at maximum was 9 - 23 km. wide
between Aoreora and Essouira. The corridor was open for about
20,000 years and closed during the Monastrian (Sangamon)
interglacial. Bottom profiles and contour maps reveal that the
continental shelf off Morocco slopes gently seaward from the coast
to the 100 m. contour (Heezen et al., 1959). The Wurm regression
reopened this corridor for a period of more than 40,000 years, and
maximum exposure and width (12-30 km.) occurred about 20-17,000
BP. By 6000 BP, the Flandrian transgression reached present sea
levels (Fairbridge, 1961; Kassler, 1973).

On the basis of the known geological record it is unlikely that
G. hesperinus or G. hoogstraali could have spread into south-
western Morocco during the Pliocene or early Pleisotcene, assuming
that either/both existed then. There is a distinct possibiUty that
hairy-footed Gerbillus dispersed northward from the Spanish
Sahara region during either or both the Riss and Wurm low stands
or possibly at some as yet unknown earlier Pleistocene low stand(s).

The discovery by Tchemov (1968) of fossils of the extant G.
dasyrus in deposits dated as ± 125,0(X) BP suggests that other

100 FIELDIANA: ZOOLOGY. VOLUME 66

modem species such as G. hesperinus and G. hoogstraali could have
existed then. In any event, the Flandrian transgression onto the
continental shelf seems likely to have isolated the latter two species
or their ancestors about 6000 BP.

Additional study of these rodents is necessary to clarify the
details of their origin and evolution. Fortunately, what is known of
their distribution and affinities offers encouraging prospects for
estimating rates of differentiation and speciation within Gerbillus.

ACKNOWLEDGEMENTS

I thank E. T. Hooper, H. Setzer, and L. Robbins for specimens.
Alan Feduccia kindly criticized the manuscript. Kirk Agerson
rendered valuable technical assistance. Funds for part of this
research were provided by William and Janice Street and the
Medical Foundation of North Carolina.

REFERENCES

Allen, G.M.

1939. A checklist of African mammals. Bull. Mus. Comp. Zool., 83, 763 pp.

Cabrera, A.

1932. Los mamiferos de Marruecos. Trab. Mus. Nac. Cien. Nat., Ser. Zool. no. 57,
361pp.

Choubert, G.

1945. Note preliminaire sur le Pontien au Maroc (Essai de synthese orogenique du
Moroc Atlasique). Bull. Soc. Geol. France, ser. 5, 16, pp. 677-764.

1950. La limite du Pliocene et du Quatemaire au Maroc. Proc. Int. Geol. Cong.,
Part IX, section H, pp. 11-18.

Ellerman, J.R. and T.C.S. Morrison-Scott

1951. Checklist of Palaearctic and Indian mammals. Brit. Mus. Nat. Hist., London,
810 pp.

Fairbridge, R.W.

1961. Eustatic changes in sea level, pp. 99-185 in Ahrens, L.H., F. Press, K.
Rankama, and S.K. Runcorn, eds.. Physics and Chemistry of the Earth, vol. 4,
New York.

Harrison, D.L.

1972. The mammals of Arabia, vol. 3. E. Benn Ltd. London, pp. 385-670.

LAY: RODENTS OF GENUS GERBILLUS 101

Heezen, B.C., M. Tharp, and M. Ewing
1959. The floors of the oceans. Geol. Soc. America, Spec. Paper, 66, 122 pp.

Kassler, p.
1973. The structural and geomorphic evolution of the Persian Gulf, pp. 11-32. in
Purser, B.H., ed., The Persian Gulf, New York.

Lay, D.M.

1972. The anatomy, physiology, functional significance and evolution of specialized
hearing organs of Gerbilline rodents. Jour. MorphoL, no. 138, pp. 41-120.

Lecointre, G.
1952. Recherches sur le Neogene et le Quatemaire marins de la cote Atlantique du
Maroc, Part 1 Stratigraphy. Notes Memoires Service Geol. Maroc, no. 99, 198
pp., 8 pi.

Nadler, C.F. and D.M. Lay

1967. Chromosomes of some species of Meriones. Z. Saugetierk, no. 32, pp. 285-291.

Palmer, Ralph S.

1962. Handbook of North American Birds, Vol. 1, Yale Univ. Press, New Haven,
567 pp.

Petter, F.

1959. Evolution du dessin de la surface d'usure des molaires des gerbillides.
Mammalia, no, 23, pp. 304-315.

Saint Girons, M. and F. Petter

1965. Les rongeurs du Maroc. Trav. Inst. Sci. Cherifien., Zool. Ser., no. 31, 58 pp.

Schlitter, D.A. and H.W. Setzer

1972. A new species of short-tailed gerbil (Dipodillus) from Morocco (Mammalia:
Cricetidae: Gerbillinae). Proc. Biol. Soc. Washington, no. 84, pp. 385-392.

TCHERNOV, E.

1968. Succession of rodent faunas during the upper Pleistocene of Israel. Berlin.

Wassif, K., R. G. Lutfy, and S. Wassif

1969. Morphological, cytological and taxonomical studies of the rodent genera
Gerbilbis and Dipodillus from Egypt. Proc. Egypt Acad. Sci., no. 22, pp. 77-97.