A reappraisal of the reproductive ecology of Arvicanthis in Africa

Afr. J . Ecol. 1991, Volume29, pages 17-27

A reappraisal of the reproductive ecology of Arvicanthis in Africa

MARTIN F I S H E R Department oJBiology, Sultan Qaboos University. PO Box 32486 Al Khod, Muscat, Sultanate of Oman

Summary

Arvicanthis (Rodentia, Muridae) is widespread in tropical Africa, and its ecology has been studied in both East and West Africa under various climatic regimes. In order to take an overall look at the reproductive ecology of Arvicanthis in Africa, data were extracted from the literature on the climate and reproductive ecology and weight of Arvicanthis at eight localities. Relationships between aspects of reproductive ecology and climate were explored statistically.

The length of the breeding season of Arvicanthis shows a strong, non-linear relationship with the number of ‘humid’ months in a year (as determined from a standard climograrn), but weight of adult males and females, number of implanted embryos and proportion of reproductively active females do not appear to be correlated with climate. The available evidence does not support the hypothesis that populations of Arvicanthis in hotter-drier environments are more r-selected, and that populations in cooler-wetter localities are more K-selected. The necessity for consistent interpretations of r-K theory and standardization of measures of ‘stress’ and ‘predictability’ is discussed.

Key words: Arvicanthis, reproduction, Africa, r/K-selection

Rbume

Arvicanthis (Rodent&, Muridae) est rtpandu en Afrique tropicale et on a etudie son kcologie en Afrique de 1’Est et de 1’Ouest sous diffkrents regimes climatiques. Pour obtenir une vue gCnCrale de 1’Ccologie de la reproduction d’hvicanthis, en Afrique, nous avons repris des donnkes dans la litterature sur le climat, la reproduction et le poids, en huit endroits. On a ktudit les relations statistiques entre la reproduction et le climat.

La duree de la saison de reproduction d’Arvicanthis montre une relation Ctroite non linCaire avec le nombre des mois humides par an, (On l’a determine avec un clirnogramme standard), rnais le poids des adultes miles et femelles, le nombre d’embryons implantb et la proportion de femelles qui participent A la reproduction ne semblent pas lies au climat. Les chiffres ne confirment pas I’hypothese colon laquelle les populations d’drvicanthis sont plus r-selectionnbes, ni que les populations d’emplacements plus froids et plus humides sont plus K-stlectionnCes. On discute la nkcessitb d’interprktations fiables de la thkorie r-K et de la standard- isation de mesures de ‘stress’ et de ‘probabilit6‘.

18 M . Fisher

Introduction Arvicanthis (Rodentia, Muridae) is widespread in tropical Africa. Population levels are generally low, but there are occasional large increases, during which the animal can be a serious pest of agriculture (Delany, 1986). The biology of the genus has been studied in Senegal (Poulet & Poupon, 1978), Nigeria (Rabiu & Fisher, 1989), Sudan (Ghobrial & Hodieb, 1982; Happold, 1966), Ethiopia (Muller, 1977) and in various parts of East Africa (Delany, 1986; Delany & Kansiimeruhanga, 1970; Delany & Monro, 1985a,b, 1986; Delany & Roberts, 1978; Neal, 1981; Senzota, 1982; Taylor & Green, 1976). Various classifications are available for the genus Arvicanthis, some recognizing only one species (Rosevear, 1969) and others recognizing up to five species (Delany & Monro, 1986, and references therein). Most of the populations studied have been referred to as Arvicanthis niloticus (Desmarest).

In general Arvicanthis breeds during the rainy season, except at high altitudes in Ethiopia, where it breeds after the rains (Muller, 1977). Breeding begins shortly after the onset of the rains in east African populations, but just before the rains in Nigeria (Rabiu & Fisher, 1989). Neal (1981) compared the reproductive biology and life-history strategy of Arvicanthis in five localities: two in west Uganda, one in central Kenya, one in Ethiopia (Muller, 1977) and one in west Kenya (Taylor & Green, 1976). Variations in the ranking of body size, litter size, reproductive rate, growth rate and age at first reproduction between localities were found to be related to a crude ranking based on the environmental ‘variability’ or ‘stress’ of each location. In environments regarded by Neal (198 1) as more variable or stressful (more arid, greater temperature variations, disruption of food supply by agriculture) individuals were smaller and had higher potential growth rates, in general agreement with the theory of r/K-selection (MacArthur & Wilson, 1967; Pianka, 1970). Similarly, Fleming (1974) studied two species of Costa Rican heteromyid rodents, and found that Liomys salvini, which inhabits deciduous, seasonal tropical forest, displays more r-selected characteristics than Heteromys desmarestianus, which inhabits wet, relatively non-seasonal tropical forests. However, populations of Arvicanthis in semi-arid areas of West Africa which would normally be regarded as ‘stressful’, have both a high average weight and large litter sizes (Poulet & Poupon, 1978; Rabiu & Fisher, 1989), in apparent contradiction to Neal’s assessment of the animals’ reproductive strategy.

In order to take an overall look at the reproductive ecology of Arvicanthis in Africa, 1 have extracted data from the literature on the climate and reproductive ecology and weight of Arvicanthis at eight African localities. Since the taxonomy of the genus Arvicanthis is still in debate (Delany & Monro, 1986), these data may represent several species within the genus Arvicanthis, populations or subspecies of A . niloticus, or a mixture of both species and subspecies. Since the breeding of Arvicanthis is modified when supplemental food is supplied (Taylor & Green, 1976), I did not include data on the breeding of Arvicanthis on permanently cultivated land in the Nile valley in Sudan (Ghobrial & Hodieb, 1982).

In this paper I explore the relationships between size and reproduction in Arvicanthis, and the influence of climate on size and reproductive ecology. The analysis gives an overall picture of certain aspects of the reproductive ecology of Arvicanthis in Africa. 1 also test the hypothesis of Neal (1981) that Arvicanthis exhibits r-selected characteristics (lower weight, larger litter sizes) in more variable or stressful localities, and K-selected characteristics (greater body weight, smaller

Reproductive ecology of Arvicanthis 19

litter sizes) in less variable, less stressful localities, and I discuss this with reference to developments of the theory of r- and K-selection (Boyce, 1984; Greenslade, 1983; Grime, 1977; Southwood, 1977, 1988; Stearns, 1977; Taylor, Aarssen & Loehle, 1990).

Methods Data extractedjrom the literature The data on Arvicanthis that could be obtained for most of the sites were the mean number of implanted embryos, the length of the female breeding season, the minimum weight at which females become reproductively active, the proportion of adult females in a year that are reproductively active and the mean weight of both adult males and females. Data were obtained for twelve months at eight localities, and for two years at one of them, giving nine one-year data sets in all (Table 1).

The number of implanted embryos was used rather than litter size because the former is more readily available in the literature. Litter size is normally lower than the number ofimplanted embryos (Neal, 1981; Rabiu & Fisher, 1989). I am making the assumption that litter size as a proportion of the number of implanted embryos is the same for all localities. The number of months in which females were pregnant or lactating was used as the ‘basic’ definition of length of breeding season, since it is the most reliable evidence of breeding. Where this was not available the assessment of length of breeding season was matched as closely as possible to this definition.

Mean annual temperature, which for most of the sites is the mean of the mean annual minimum and maximum, was not available for three localities: Kitale (Taylor & Green, 1978), Nakuru (Delany & Monro, 1986) and Semien National Park (Muller, 1977). Since Kitale and Nakuru are climatically similar to, and at approximately the same altitude as Nairobi, the mean annual temperature of Nairobi was substituted. The mean annual temperature of Addis Ababa for 23 years (White, 1983) was substituted for Semien National Park. Since Addis Ababa is at 2440 m and the site is at 3700 m the mean temperature used is probably an overestimate. Neal (1981) did not give climatic data for one of his two Ugandan sites (Crater Track). However, since Crater Track is only 10 km from the site at which climate was recorded (Mweya), and both sites are at much the same altitude, I have used the climatic data at Mweya for both localities.

For each locality the number of ‘humid’ and ‘dry’ months were calculated. Months in which the rainfall curve falls below the temperature curve on a standard climogram have been empirically established to be relatively dry, and months in which the rainfall curve rises above the temperature curve are relatively humid (White, 1983). Since the humid and dry months in a year total twelve, either, but not both, can be used as a climatic variable. I arbitrarily used the proportion of humid months in a year.

Statistical analyses Initially, Pearson product-moment correlation coefficients were calculated in order to explore the relationships between the biological and climatic variables. To determine whether correlations between the weight and reproductive variables of females are influenced by climate, partial correlation coefficients, controlling for correlation with the number of humid months, were then calculated. The influence of climate on weight and reproduction was further explored using regression analysis. SYSTAT (Wilkinson, 1987) was used for all statistical analyses.

h)

0 F

Tab

le 1.

Dat

a ext

ract

ed fr

om th

e lite

ratu

re o

n: (a

) the

geo

grap

hy a

nd cl

imat

e of

eigh

t Afr

ican

loca

litie

s whe

re A

rvic

anfh

is ha

s bee

n st

udie

d, an

d (b

) the

wei

ght a

nd re

prod

uctiv

e ec

olog

y of A

rvic

anfh

is a

t the

se lo

calit

ies.

Stan

dard

erro

rs ar

e gw

en in

pare

nthe

ses i

f ava

ilabl

e. T

he cl

imat

ic d

ata w

as ta

ken

from

the s

ourc

es gi

ven

in (b

) unl

ess o

ther

wis

e not

ed

(a) G

eogr

aphy

and

clim

ate

Loca

lity

Ann

ual

Mea

n an

nual

N

umbe

r of

Alti

tude

Ti

me

rain

fall

tem

pera

ture

hu

mid

mon

ths

(m)

Latit

ude

Long

itude

pe

riod

(mm

) ("

C)

in y

ear

Cra

ter T

rack

, Uga

nda"

Fe

te-O

le, S

eneg

al

Kita

le, K

enya

K

ano,

Nig

eria

K

ano,

Nig

eria

M

wey

a, U

gand

a N

akur

u, K

enya

R

ojw

ero,

Ken

ya

Sem

ien

Nat

iona

l Par

k. E

thio

pia

1050

10

0 19

00

470

470

950

I920

65

0 37

00

o"o6

's 2 15

"N

1"O

I'N

12"0

3'N

lT

03"

O"1

I'S

0" 17

's o"

1 I'N

x

13"

N

2Y54

E

x 15

"W

35"O

I'E

8"32

'E

8"32

'E

29"5

4'E

36" 1

OE

38=l

OE

x38"

E

Apr

'65-

Mar

'66

Jan-

Dec

'76

Jan-

Dec

'70

Dec

'84-

Nov

'85

Apr

'6SM

af66

A

pr'8

ILM

ar'8

2 Ju

1'74-

Jun'7

5 A

pr'7

2-M

ar'7

3

Dec

'83-

NO

~'84

-

338

1082

46

3 65

3 81

2 999

543

1200

-

29

1 8b

28

27

24

1 8b

28

16'

-

2 10 3 4 9 8 4 8

Th

e cl

imat

ic d

ata

for M

wey

a was

used fo

r thi

s site

(see

text

for d

etai

ls).

hTem

pera

ture

data

is fo

r Nai

robi

(se

e tex

t for

det

ails

). 'T

empe

ratu

re d

ata

is fo

r Add

is A

baba

(see t

ext f

or d

etai

ls).

(b) B

iolo

gica

l dat

a of

Arv

ican

fhis

Loca

lity

Leng

th o

f M

inim

um w

eigh

t fe

mal

e M

ean

of f

emal

es

bree

ding

nu

mbe

r of

M

ean

adul

t Pr

opor

tion

at fi

rst

Mea

n ad

ult

seas

on

impl

ante

d fe

mal

e of

fem

ales

re

prod

uctio

n m

ale

(mon

ths)

em

broy

s w

eigh

t (9)

br

eedi

ng

(g)

wei

ght (

g)

Lite

ratu

re

sour

ce

Cra

ter T

rack

, Uga

nda

Fete

-Ole

. Sen

egal

K

itale

, Ken

ya

Kan

o, N

iger

ia (1

984)

Kan

o, N

iger

ia (1

985)

Mw

eya,

Uga

nda

Nak

uru,

Ken

ya

Roj

wer

o, K

enya

Se

mie

n N

atio

nal P

ark,

Eth

iopi

a

10”

3.67

* -

9

Y

6.00

7‘

8.14

7‘ 6.

67

12d

4.62

(k0

.21)

* -

Y

8d

5.57

(k0

.24)

8‘

4.

88 (

k0.2

3)

74

92

66

(k3.

0)

102

101

86

(kl.

4)

70

49 (* 1.

4)

89

(kl.

8)

0.43

0.52

0.

35

-* 0.48

0.32

-* 0.80

0.

16

60

63

45

63

70

60

40

35

68

75

100 89

101

110 98

85

52

97

Nea

l (I9

8 1 )

Poul

et &

Pou

pon

(197

8)

Tayl

or &

Gre

en (1

978)

R

abiu

& F

ishe

r (19

89) a

nd

Rab

iu &

Fis

her (

unpu

b.)

Rab

iu &

Fis

her (

1989

) and

R

abiu

& F

ishe

r (un

pub.

) N

eal (

1981

) D

elan

y &

Mon

ro (1

985

Nea

l (19

81)

Mul

ler (

1977

) 9

~ ~

~ ~~

~

~~

~ ~

~ ~

~ ~

~ ~

2.

% *D

ata

not a

vaila

ble

in li

tera

ture

; “ba

sed

on N

eal (

1981

). p.

177

; bba

sed o

n Po

ulet

& P

oupo

n (1

978)

, p. 1

92; ‘

base

d on

num

ber

of fe

mal

es p

regn

ant/l

acta

ting;

dba

sed

on

CE

r,

num

ber o

f fem

ales

pre

gnan

t; ‘b

ased

on

Del

any

& M

onro

(l98

6), p

. 94

and

Fig.

7; ‘

base

d on

num

ber o

f mon

ths i

n w

hich

birt

hs o

ccur

red.

e

22 M . Fisher

Results The correlation coefficients between all biological and climatic variables are given in Table 2. Since a large number ofcorrelation coefficients were calculated the usual method of determining significance cannot be used, and because the data set is small there are insufficient degrees of freedom for the calculation of reliable Bonferroni-adjusted probabilities (Wilkinson, 1987). The matrix of correlation coefficients was therefore used only in a comparative sense: to indicate which climatic variables have a greater influence on weight and reproduction, and to indicate possible relationships between the biological variables.

The three climatic variables (rainfall, temperature and number of humid months) have high correlation coefficients with each other. For investigating the effect of climate on weight and reproduction only the number of humid months in the year was used, since, firstly, it is jointly determined by both rainfall and temperature and is therefore an ‘index’ of both, and, secondly, it has higher correlation coefficients with five out of the six biological variables than either rainfall or temperature.

Partial correlation coefficients between the biological variables, controlled for correlation with the number of humid months, are given in Table 3 . The correlation coefficients of both minimum weight at first reproduction and proportion of females reproductively active with mean female weight remain high, indicating that the relationships of the two variables with female weight (Fig. 1 ) are unaffected by climate. However, all but one of the other correlation coefficients are reduced, suggesting that any effect that climate has on female weight, number of embryos and length of breeding season is probably direct and not via intercorrelations between the biological variables.

The regressions of female weight and number of embryos with humid months are not significant (Fig. 2). Male weight was not regressed on number of humid months (Fig. 2) since the correlation coefficient was close to zero (Table 2). The relationship between length of breeding season and number of humid months (Fig. 2) is better fitted by a hyperbolic equation than by linear regression. The hyperbolic equation has corrected R2 of 0.712, as against an adjusted R2 of 0.629 for linear regression.

Discussion Several authors have made qualitative observations on the relationship between climate and the breeding season of Arvicanthis, which has generally been observed to breed during the rainy season, and for a longer period in wetter, cooler localities (Delany, 1986; Neal, 198 1 ; Rabiu & Fisher, 1989; Taylor & Green, 1978). Figure 3 quantifies this relationship and provides a predictive equation for the length of the breeding season of Arvicanthis in Africa. The breeding season of Arvicanthis does not always coincide with the humid months: in Ethiopia Arvicanthis breeds immediately after the rainy season (Muller, 1977), and in northern Nigeria breeding begins shortly before the onset of the rains (Rabiu & Fisher, 1989). Neverthe- less, there appears to be a strong relationship between the number of humid months and length of the breeding season, presumably via the effect of the number of humid months on plant growth. Similarly, the length of the breeding season increases when supplemental food is provided for wild Arvicanthis (Taylor &

Tab

le 2

. Pea

rson

pro

duct

-mom

ent c

orre

latio

n co

effic

ient

s bet

wee

n le

ngth

of b

reed

ing

seas

on, m

ean

num

ber o

f im

plan

ted

embr

yos,

mea

n ad

ult f

emal

e wei

ght,

prop

ortio

n of

fe

mal

es in

repr

oduc

tive

cond

ition

, min

imum

wei

ght a

t whi

ch fe

mal

es b

ecom

e re

prod

uctiv

ely

activ

e an

d m

ean

adul

t mal

e w

eigh

t of A

rvic

anfh

is, a

nd a

nnua

l rai

nfal

l. m

ean

annu

al te

mpe

ratu

re a

nd n

umbe

r ofh

umid

mon

ths

in a

yea

r at e

ight

Afr

ican

loca

litie

s and

for t

wo

year

s at o

ne o

fthe

loca

litie

s. Ir

rele

vant

cor

rela

tions

hav

e be

en o

mitt

ed. n

= 9

for a

ll co

mpa

riso

ns e

xcep

t for

thos

e in

volv

ing

mea

n nu

mbe

r of i

mpl

ante

d em

bryo

s and

pro

port

ion

of fe

mal

es in

repr

oduc

tive

cond

ition

, for

whi

ch n

= 7

Leng

th o

f fe

mal

e M

ean

Min

imum

wei

ght

of fe

mal

es

Mea

n N

umbe

rof

bree

ding

nu

mbe

r of

Mea

n ad

ult

Prop

ortio

n at

firs

t M

ean

adul

t A

nnua

l an

nual

hu

mid

se

ason

im

plan

ted

fem

ale

of fe

mal

es

repr

oduc

tion

mal

e ra

infa

ll te

mpe

ratu

re

mon

ths

("C

) in

ayea

r (m

onth

s)

embr

yos

wei

ght (

g)

bree

ding

* (g

) w

eigh

t (g)

(m

m)

Leng

th o

f fem

ale b

reed

ing

seas

on

(mon

ths)

M

ean

num

ber o

f im

plan

ted

embr

yos

Mea

n ad

ult f

emal

e wei

ght (

g)

Prop

ortio

n of

fem

ales

bre

edin

g*

Min

imum

wei

ght o

f fem

ales

at fi

rst

repr

oduc

tion

(g)

Mea

n ad

ult m

ale

wei

ght (

g)

Ann

ual r

ainf

all (

mm

) M

ean

annu

al te

mpe

ratu

re ("

C)

Num

ber o

f hum

id m

onth

s in

a y

ear

1 .Ooo

- 0.

709

-0.3

73

-0.1

81

- 0.

222

0.53

9 - 0.

440

0.82

2

1 .O

oo

F

0402

1 .O

oo

0.08

8 -0

.770

I 4

00

0.91

0 -0

.827

0.

074

z 2. e rp

rp

hi

1 .O

oo

2

-0.7

18

-0.3

74

-0.3

44

-0.2

04

-0.1

12

0.86

6 -0

.821

1.

Ooo

.q

1 .om

1 .O

oo

-0.1

23

-0.0

04

0.17

8 0,

020

-0.9

72

-

-

-

0,88

9

5 - 0.

508

- 0.

237

- 04

82

0.46

2 0.

239

0.48

7 I ~

Ooo

> s.

*Arc

sine

tran

sfor

med

. 0

-3 h,

w

24 M . Fisher

Table 3. Partial correlations between length of breeding season. mean number of implanted embryos, mean weight of adult females. minimum weight at which females become reproductively active and proportion of females in reproductivecondition of Arvicunfhis, given their correlation with the number of humid months in a year. n = 9 for all comparisons except those involving mean number of implanted embryos and proportion of females in reproductive condition, for which n = 7

Length of Mean Minimum weight breeding number of Mean adult Proportion of of females at

season implanted female females first (months) embryos weight (g) breeding* reproduction (g)

-

Length of breeding season

Mean number of implanted -0.378 1 .ooo

Mean adult female weight (g) -0.070 0.337 I .om

breeding* 0.118 -0.243 -0'916 I ,000

at first reproduction (9) -0'097 0.062 0.9 I9 -0.897 I ~OOO

I .ooo (months)

embryos

Proportion of females

Minimum weight of females

*Arcsine transformed.

0.6

0.2 - 04 1 1 I I I I

40 50 60 70 80 90 100 110

Weight of adult females (g)

Fig. 1. The relationship of (a) the minimum weight of reproductively active females and (b) proportion of females in reproductive condition in a year with mean adult female weight of Arvicunrhis Regression results: (a)adjusted R*=0.804, F=33.903, P=O.OOI (b)Adjusted R2=0.547, F=6.037, P=0.057.

Green, 1976). High protein foods such as seeds and insects are more abundant during the rainy season and at the beginning of the dry season, during which time they form a major component of the omnivorous diet of Arvicanthis (Delany & Monro, 1986; Neal, 1981; Rabiu & Fisher, 1989; Taylor & Green, 1976).


90- - 0 E t 80-

g 70- E .’ 60-

-0

s 50

0

a

0

- 0

m U al .-

?! n

70

60

50

Lc 0

a - 0 0

-

- 0

f m

f

6-

5-

4-

40 I I 1 I 1 I

0

0

0 0

0

0

t 9 0

c 0

40‘ 1 I I I I 1

u) g n E m c 0

7

“0 2 4 6 8 10 12 Number of humid months in year

Fig. 2. The relationships of (a) mean adult male weight, (b) mean adult female weight, (c) length of annual breeding season and (d) mean number of implanted embryos of Arvicanlhis with the number of humid months in the year. Regression results for M (b) R’=0.007, F = 1,059, P=0,338 (c) for fit of hyperbolic curve (length of breeding season = 12.736* number of humid months/[2~812+number of humid months]) corrected R’=0.712. F=24593l. PiO.001 (d)adjusted Rz=0.418, F=5.315, P=0.069.

If populations of Arvicanthis conform to the general predictions of r/K theory, then correlations would be expected between environmental ‘stress’ (measured here as the number of humid months in a year) and adult weight and the number of embryos. The absence of such correlations contradicts the hypothesis (Neal, 198 1) that populations of Arvicanthis are more r-selected in hotter, drier localities and more K-selected in cooler-wetter places. Additionally, there is no indication that the climatically equable Kitale in western Kenya, which was classified as ‘stressful’ because of disruption of the animals’ food supply by agriculture (Neal, 1981), is any more ‘stressful’ than other climatically similar localities. That the reproductive ecology of Arvicanfhis does not fit in with r/K theory is not in itself startling: Stearns (1977) reviewed thirty-five case studies and found that only eighteen fitted the r/K scheme. The populations of Arvicanthis reviewed here are probably all r-selected sensu Pianka (1970).

26 M . Fisher

Models have been proposed in which various types of ‘stress’ and ‘predictability’, and their effects on life history strategies, are summarized in a small number of axes (Greenslade, 1977; Grime, 1977; Southwood, 1977, 1988; Taylor et a/., 1990). Whatever the merits of the various approaches (Boyce, 1984; Southwood, 1988; Taylor et al., 1990) consistency of interpretation of r- and K-selection (Boyce, 1984), and standardization of quantitative measures for the assessment of ‘stress’ and ‘predictability’ are required. I have used the number of humid months in the year as a simple measure of the environmental ‘stress’ on different populations of Arvicanthis, but other methods could be tried. The ideas of Colwell (1974; see also Weis & Schwartz, 1988) could be utilized to estimate the ‘predictability’ of periodic phenomena such as climate.

Acknowledgments I would like to thank the Department of Biology, Sultan Qaboos University for research facilities, Y. Yesilicay for statistical advice and D. Gardner, S. A. Ghazanfar and R. Victor for comments on the manuscript.

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(Manuscript accepted 6 November 1990)

Documents

A reappraisal of the reproductive ecology of Arvicanthis in Africa