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: anke-hoffmann@gmx.net

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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

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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.

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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

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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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