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Spatial and temporal patterns in Arvicanthis niloticus(Desmarest, 1822) as revealed by radio-tracking
Anke Hoffmann1,3,*, Katja Eckhoff2,3 and Hans Klingel2,3
1Museum fur Naturkunde, Humboldt-Universitat zu Berlin, Institut fur Systematische Zoologie, Invalidenstr. 43, 10115 Berlin, Germany,2Zoologisches Institut, Technische Universitat Braunschweig, 38092 Braunschweig, Germany and 3Uganda Institute of Ecology, QENP, PO Box
3530, Kampala, Uganda
Abstract
Arvicanthis niloticus was radio-tracked in the grasslands
of the Queen Elizabeth National Park in Uganda. Home
range sizes calculated by Ranges V� using the Minimum
Convex Polygon Method (at 95%) were on average 5.5
times larger in the bushland–grassland mosaic than
those in the Abutilon guineense–Ocimum suave bushland.
An inverse relation between home range size and
population density was found. In both habitats the spe-
cies was highly active during daylight hours but differed
in activity patterns.
Key words: activity, Arvicanthis niloticus, habitat utilization,
home range, radio-tracking, rodent
Resume
L’Arvicanthis niloticus fut suivi par radio-emetteur dans la
prairie du Parc National de Queen Elizabeth en Ouganda.
Les tailles des domaines vitaux calcules avec Ranges V�en
se servant de la methode de polygones convexe minimum
(a 95%) furent 5,5 fois plus grand en moyenne dans le
terrain en mosaıque de brousse et prairie que celles dans la
brousse d’Abutilon guineense–Ocimum suave. Une relation
inversee entre la taille des domaines vitaux et la densite de
population fut observee. Dans les deux habitats les especes
furent tres actives pendant le jour mais differa dans la
configuration d’activite.
Introduction
Arvicanthis niloticus (Desmarest, 1822)1 is a common
savanna rodent of tropical Africa (Rosevear, 1969; King-
don, 1974; Delany, 1986; Musser & Carleton, 1993). The
species is a typical inhabitant of grasslands, but also appears
in bushland and agriculture (Delany & Neal, 1966, Neal
1967, Kingdon, 1974). Detailed ecological information on
spatial and temporal patterns of Arvicanthis sp. is limited
(e.g. Delany & Neal, 1966; Delany & Kansiimeruhanga,
1970; Delany & Roberts, 1978; Delany & Monro, 1985;
Senzota, 1990) and has been mainly determined using
Capture-Mark-Release (CMR) methods. Activity patterns
were studied in the laboratory by Katona & Smale (1997). In
this study radio-tracking was used to elucidate home range
size, home range utilization and activity pattern of A. nilo-
ticus in its natural habitat. The data presented here are part
of larger studies of the ecology of small mammal populations
in Uganda (Eckhoff, 1998; Hoffmann, 1999).
Materials and methods
Study area
The investigation took place in the Queen Elizabeth National
Park (00�15¢S, 30�00¢E), in south-west Uganda. From 1995
to 1997 the ecology of small mammals in different grassland
communities was studied using CMR methods throughout
the year, and radio telemetry for some species and for shorter
periods (Hoffmann, 1999; Hoffmann & Klingel, 2001a,b).
For the telemetry experiment of A. niloticus, we selected two
plots of 1 ha each in the northern part of the park, about
5 km apart and with different vegetation.
Plot 1 was in the bushland–grassland mosaic of the
Crater Region with six to eight Capparis tomentosa–
Euphorbia candelabrum bush patches of 10–20 m in
1Arvicanthis nairobae (J.A. Allen, 1909) is considered synony-
mus, however the possibility of two sympatric and phenotypically
identical species can not be excluded.
*Correspondence: E-mail: [email protected]
� 2006 The Authors
72 Journal Compilation � 2006 East African Wild Life Society, Afr. J. Ecol., 44, 72–76
diameter, per hectare, within the Bothriochloa–Themeda–
Chloris grassland (grass height 70 cm). In plot 1, 6% of the
area was covered with those bushes. Plot 2 was also in the
bushland–grassland mosaic, but had more bushes (15%
coverage) within the Cynodon–Bothriochloa grassland and
was dominated by the shrubs Abutilon guineense and Oci-
mum suave. Plot 1 was more frequently visited, during the
study, by big game e.g. elephants (Loxodonta africana) and
buffalos (Syncerus caffer) than plot 2. On both plots
Odontotermes sp. termite mounds were found at the rate of
15 ha)1. Annual grass fires occurred only in plot 1.
During the trapping period (1995–1997) the density of
A. niloticus ranged from 4 to 14 individuals ha)1 in plot 1
and from 22 to 66 individuals ha)1 in plot 2 (Hoffmann,
1999). Radio-tracking was done for 2 weeks each in the
dry season in August 1996 (plot 1) and in September
1996 (plot 2); then density was 14 individuals ha)1 in plot
1 and 42 individuals ha)1 in plot 2.
Selection of individuals
We radio-collared individuals which had been resident on
the plot for at least two consecutive trapping sessions, i.e.
for a minimum of 6 weeks. As both home range sizes and
activity patterns were to be investigated, it was necessary to
take the telemetry fixes at short intervals. As observations
were made at 30 min intervals by only one investigator, the
number of simultaneously radio-tracked individuals had to
be limited to five animals per plot and tracking period.
Telemetry
We used radio-collars with TW-4 transmitters (Biotrack�,
Wareham, U.K.) of about 2.5 g, which is 2–5% of the
individuals’ body weights, and within the commonly
recommended value of <10% (Kenward, 1987). The col-
lars were fitted in the laboratory. Subsequently the animals
were released at their original trap sites.
Locating the animals
Generally the signal could be detected at up to 90 m. The
position of each individual was determined using the
homing-in-technique (White & Garrott, 1990) in a
5 · 5 m grid with marker posts. The animals were located
within and outside the 1 ha trapping grid. They were
radio-tracked in each period for up to 7 days, starting
usually at dawn and ending at dusk. To check for possible
nocturnal activity radio-tracking was done on two nights
in plot 1. For plot 2 nocturnal fixes were not taken because
of repeated encounters with lions.
Analysis
Size and shape of the home ranges were calculated using
Ranges V� (Kenward & Hodder, 1995) and the Minimum
Convex Polygon Method (MCPM). The latter was found to
be adequate as it allows for comparison with results of
previous studies. To minimize the influence of ‘occasional
excursions’ (Burt, 1943), the MCPM home range sizes
were also calculated disregarding 5% of the outermost fixes
(Table 1). Fixes were taken at 30 min intervals. An indi-
vidual was considered resting if at least two consecutive
fixes indicated the same location. Resting bouts during the
activity phase were disregarded.
Results
Eight individuals were radio-tracked, providing a total of
1426 fixes; fixes taken for each individual ranged from 28
to 230, mean 178 (Table 1). All selected adult individuals
in both plots were sexually active, two females selected in
plot 1 were subadult and sexually inactive (F1, F3).
Spatial patterns
Three females (F1, F2, F3) and one male (M1) were radio-
tracked successfully in plot 1. Home ranges (at 95%) were
Table 1 Home range sizes of A. niloticus in plot 1 and plot 2 as
revealed by radio-tracking
Individuals Period
Tracking
Fixes
(n)
Home ranges
[ha]
Days
(n)
Nights
(n) 100% 95%
plot 1
F 1 08/96 7 2 230 1.16 1.03
F 2 08/96 7 2 230 1.10 1.07
F 3 08/96 7 2 230 0.95 0.92
M 1 08/96 6 2 204 1.04 0.98
plot 2
F 4 09/96 1 – 28 0.11 0.11
F 5 09/96 6 – 168 0.27 0.17
M 2 09/96 7 – 168 0.48 0.25
M 3 09/96 6 – 168 0.40 0.20
F, Female; M, Male; F1 and F3, subadult; others, adult.
Spatial and temporal patterns in Arvicanthis niloticus 73
� 2006 The Authors
Journal Compilation � 2006 East African Wild Life Society, Afr. J. Ecol., 44, 72–76
from 0.92 to 1.07 ha (Table 1). In plot 2 two females (F4,
F5) and two males (M2, M3) were radio-tracked success-
fully. On average the home ranges (at 95%) were one fifth
of those of plot 1, ranging from 0.11 to 0.25 ha (Table 1).
In plot 2 the home ranges of the males were larger than
those of the females.
Home range utilization
During the activity period the animals in plot 1 spent
their time almost exclusively (99% of 542 fixes) in the
grassland. Bush patches cover 6% of the area and this
habitat was clearly under-utilized by A. niloticus. The
patterns of utilization of the four individuals did not
differ significantly (v2 ¼ 2.013, d.f. 3, P ¼ 0.570, n ¼542).
On both plots the animals used runways in the grass
layer. These were more frequent in the denser grass layer
of plot 1 than in plot 2. Along the runways feeding places
were identified, consisting of cut grass stems of Bothriochloa
insculpa, Chloris gayana and Themeda triandra.
Hideouts, nests and runways of A. niloticus were located
by radio-tracking. In plot 1 all tagged individuals used, for
sleeping or resting, two termite mounds of Odontotermes
sp., which were 40 m apart. On several occasions all
individuals were using them simultaneously. Other hiding
places were grass padded hollows. All hiding places were
frequently visited by the animals during the night; they
were also used for the resting bouts during the day. In plot
2 only M3 and F5 used the same self dug hole throughout
the radio-tracking period.
Activity patterns
Daylight hours were almost entirely spent by all radio-
tracked individuals with activities outside the hideouts, and
only short resting bouts occurred (Fig. 1). In both plots the
highest activity was found from morning to midday, but
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
06 : 0
0
07 : 3
0
09 : 0
0
10 : 3
0
12 : 0
0
13 : 3
0
15 : 0
0
16 : 3
0
18 : 0
0
19 : 3
0
21 : 0
0
22 : 3
0
00 : 0
0
01 :
30
03 : 0
0
04 : 3
0
06 : 0
0
07 : 3
0
Act
ivit
y (%
)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
06 : 0
0
07 : 3
0
09 : 0
0
10 : 3
0
12 : 0
0
13 : 3
0
15 : 0
0
16 : 3
0
18 : 0
0
19 : 3
0
21 : 0
0
22 : 3
0
00 : 0
0
01 : 3
0
03 : 0
0
04 : 3
0
06 : 0
0
07 : 3
0
Act
ivit
y (%
)
plot 2
plot 1
Day Night
Fig 1 Activity records for A. niloticus in plot
1 and plot 2. Fixes (N) of all individuals (n)
in each period are pooled: plot 1 (n ¼ 4,
N ¼ 894), plot 2 (n ¼ 3, without F4, N ¼504)
74 Anke Hoffmann et al.
� 2006 The Authors
Journal Compilation � 2006 East African Wild Life Society, Afr. J. Ecol., 44, 72–76
activity patterns were different. In plot 2 activity increased
steeply from morning to midday, whereas in plot 1 more
fluctuations were found before the activity peak. Fixes taken
at night proved that in plot 1 A. niloticus reduced their
activity after dusk. From midnight to morning the tagged
individuals stayed permanently in their sleeping places,
except for a few brief excursions. On average the individuals
spent approximately 7 h resting without a break. The main
activity phase started 1–2 h after sunrise.
Discussion
Home range size and utilization
Information about home ranges of African rodents is scant,
and is mainly based on CMR studies. Little information on
home ranges exists for A. niloticus (e.g. Delany & Roberts,
1978). Our telemetry results show an inverse correlation
between home range size and population density, con-
firming Delany & Roberts (1978) and Delany & Monro
(1985). The same correlation has been found in other
African rodent species, e.g. Mastomys cf. natalensis (Leirs,
Verheyen & Verhagen, 1996; Hoffmann, 1999; Hoffmann
& Klingel, 2001a) and Lemniscomys striatus (Hoffmann,
1999; Hoffmann & Klingel, 2001b). Range sizes are con-
sidered to be correlated with density (Zeyda & Pelikan,
1969; Kucera, 1970; Cheeseman, 1975; Delany, 1982),
but also with body size and diet (Cheeseman, 1975; Wilson,
1976). Cheeseman (1975) suggested that smaller rodent
species have smaller ranges then larger species, but on the
basis of their weight herbivorous (‘cropper’) species have
smaller ranges than insectivorous and omnivorous (‘hun-
ter’) species. The results of our radio-tracking experiments
of the omnivorous species L. striatus, Mastomys cf. natalensis
(Hoffmann, 1999; Hoffmann & Klingel, 2001a,b) and the
herbivorous A. niloticus confirm these findings: home range
sizes of the large A. niloticus were found to be larger than
those of the small species L. striatus but similar to the
medium sized Mastomys cf. natalensis at similar densities.
Habitat parameters like state of vegetation, humidity
and food availability were obviously different in the two
plots. These factors are considered to be indirectly
responsible for the significantly different home range sizes,
which in turn are correlated with population density. It is
suggested that low population densities are associated with
increased mobility, which are thought to be influenced by
other ecological factors. That food availability can be a
major factor has also been suggested by Senzota (1990).
Our sample is not large enough for assessing a possible
correlation between spatial and temporal patterns of activity
and sex or sexual activity, although the males of plot 2 had
larger ranges than the females. Delany & Monro (1985)
found that A. niloticus males were generally more wide-
ranging than females. Larger ranges were also found for
males of other African rodents, e.g. for Arvicanthis abyssini-
cus (Muller, 1977), Hybomys univittatus (Genest-Villard,
1978), Saccostomus mearnsi (Keesing, 1998). Sexually active
individuals of both sexes were found to have larger ranges
than inactive ones as described for Mastomys cf. natalensis
(Leirs et al., 1996; Hoffmann, 1999). In this study a corre-
lation could not be recorded as all tracked adult A. niloticus in
both plots were sexually active. However, the sexually
inactive subadult females in plot 1 showed no difference in
home range size to that of the sexually active adults.
Activity patterns
In previous studies using trapping methods A. niloticus was
considered as being active day and night (Delany & Neal,
1966), whereas other authors described the species as
mainly diurnal (Delany & Kansiimeruhanga, 1970; Pack-
er, 1982; Senzota, 1990). The distinct increase of activity
in the early morning hours found by Delany & Kansii-
meruhanga (1970) could not be confirmed in this study. In
both plots A. niloticus was found to be highly active during
daylight hours with a peak at noon. In plot 1 additional
activity peaks were recorded just before dawn and one
after dusk and actual resting occurred only after midnight.
The activity pattern of our radio-tracked individuals cor-
responds with the lab results of Duplantier & Granjon
(1990) and Katona & Smale (1997). The former authors
found two activity peaks, one at midday and one 2 h after
sunset. The latter examined wheel-running rhythms in A.
niloticus under lab conditions, which exhibited predomin-
antly diurnal running rhythms with peaks of activity
around dawn and dusk. In their investigation no difference
in activity was found between females and males.
It is suggested that activity patterns depend on habitat
conditions and are influenced by reproductive activity. For
example, after destructive fires, Hoffmann (1999) recorded
a shift of activity toward the night hours for the diurnal
L. striatus, which can be explained by the reduced veget-
ation cover resulting in reduced protection against preda-
tors and solar radiation. A temporary shift of activity
toward darkness was also observed for A. niloticus imme-
diately after a fire in plot 1 in February 1997 (A. Hoff-
Spatial and temporal patterns in Arvicanthis niloticus 75
� 2006 The Authors
Journal Compilation � 2006 East African Wild Life Society, Afr. J. Ecol., 44, 72–76
mann, pers. obs.). Now that this methodology has been
developed, intensive investigation using larger samples is
feasible to address open questions.
Acknowledgements
We are grateful to Uganda National Council for Science
and Technology, Uganda National Parks and Uganda
Wildlife Authority for the approval for this study in Queen
Elizabeth National Park. We express our gratitude to Dr A.
Latif and Dr E. Edroma for their assistance. Special thanks
to field assistant Tom Friday Baluku and the rangers who
helped in the field.
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