Sex of the foetus determines the time of weaning

of the previous offspring of captive plains

zebra (Equus burchelli)

Jan Pluhacek a,b,*, Ludek Bartos a, Miroslava Dolezalova c,Jitka Bartosova-Vıchova a

a Ethology Group, Research Institute of Animal Production, Pratelstvı 815,

104 01 Praha-Uhrıneves, Czech Republicb Department of Zoology, Faculty of Science, Charles University, Vinicna 7,

128 44 Praha 2, Czech Republicc Dvur Kralove Zoological Gardens, Stefanikova 1029, 544 01 Dvur Kralove nad Labem,

Czech Republic

Accepted 29 May 2006

Available online 7 July 2006

Abstract

We examined if captive plains zebra (Equus burchelli) mares change the timing of weaning according to

the sex of the suckling foal and of the new impending foetus. We observed 19 captive plains zebra foals, in

three different formations, at the Dvur Kralove Zoo. The earliest time for weaning of any foal was recorded

at the age of only 243 days. The latest suckling that was ever recorded occurred 83 days before the dam’s

next delivery, when the foal was at the age of 355 days. We found that pregnant mares weaned their foals

sooner than the non-pregnant mares. Survival analysis revealed that the sex of the foetus was the most

influential factor in determining the weaning age of the current foal. Hence, if the foetus was a male, then

weaning occurred sooner than if the dam was carrying a female foetus. This finding is the first of this kind

among ungulates. The reproductive state of the mare and the sex of her foetus should be considered when

interpreting weaning times of equids.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Equus burchelli; Weaning time; Zebra; Maternal investment; Trivers–Willard model; Zoo

www.elsevier.com/locate/applanim

Applied Animal Behaviour Science 105 (2007) 192–204

* Corresponding author. Tel.: +420 267 009 765; fax: +420 267 710 797.

E-mail address: janpluhacek@seznam.cz (J. Pluhacek).

0168-1591/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.applanim.2006.05.019

1. Introduction

Subsequent pregnancy is one of the main factors that can affect the length of lactation of

mammalian species where the lactating female conceives shortly after birth (Bateson, 1994).

Under most environmental conditions, the optimum weaning time should occur earlier for

pregnant than for non-pregnant females (Bateson, 1994). This has been documented for various

ungulates, such as Saharan arrui (Ammotragus lervia; Cassinello, 1997) and American bison

(Bison bison; Green et al., 1993). In African elephants (Loxodonta africana), the duration of

maternal investment via nursing appeared to be controlled by the inter-birth interval of the

mother (Lee and Moss, 1986).

Differences in time spent suckling between young males and females have been reported for

many sexually dimorphic ungulates, such as red deer (Cervus elaphus; Clutton-Brock et al.,

1981), Saharan arrui (Cassinello, 1996), American bison (Wolff, 1988, but see Green and Berger,

1990), and bighorn sheep (Ovis canadensis; Festa-Bianchet, 1988). Only two reports have

suggested that feral horse (sexually monomorphic species) male foals spent a longer time

suckling than female foals during the first 8 weeks of their life (Duncan et al., 1984; Berger,

1986). On the other hand, however, no sex differences in suckling behaviour have been reported

for other horse populations (Crowell-Davis, 1985; Smith-Funk and Crowell-Davis, 1992; Wolff

and Hausberger, 1994; Cameron et al., 1999b) and sexually monomorphic guanacos (Lama

guanicoe; Sarno and Franklin, 1999) or for many sexually dimorphic ungulates, such as fallow

deer (Dama dama; Gauthier and Barrette, 1985), white-tailed deer (Odocoileus virginianus;

Gauthier and Barrette, 1985), reindeer (Rangifer tarandus; Lavigueur and Barrette, 1992), and

Saharan arrui (Cassinello, 2001). It should be noted, however, that suckling behaviour is not

always a useful predictor of milk intake (Cameron, 1998; Cameron et al., 1999b).

It has previously been found that the sex ratio of the offspring of captive equids did not differ

significantly from 1:1 (Mlıkovsky, 1988). To date, in four equid species, a few different ways of

sex-biased maternal care has been documented. Firstly, relationships between high maternal rank

and male biased offspring production, and probability of conception and inter-birth intervals

were observed in captive plains zebra (Equus burchelli; Schilder and Boer, 1987; Pluhacek et al.,

2006). Secondly, in contrast to the previous findings, dominant mountain zebra (E. zebra)

mothers produced more daughters than sons (Lloyd and Rasa, 1989). Thirdly, female-biased sex

ratios following a poor year and after the dam had produced a male in the preceding year (Monard

et al., 1997) and male-biased sex ratios for offspring in mares in better condition was observed in

feral horses (E. caballus; Cameron et al., 1999a). Finally, a male-biased sex ratios in offspring

was observed in nonprimiparous and young Asiatic wild asses (E. hemionus; Saltz and

Rubenstein, 1995; Saltz, 2001).

The plains zebra is a polygynous species in which there is relatively high competition among

males compared to females. The gestation period of plains zebra ranges from 361 to 385 days

(Wackernagel, 1965; Klingel, 1969a; Lobanov, 1983). The first fertile oestrus typically occurs

within 7–9 days post-partum (Wackernagel, 1965; Klingel, 1969a; Smuts, 1976b). Conception

rates ranging between 50 and 79% per season have been reported (Klingel, 1969a; Smuts,

1976b). At least half of the zebra mares conceive when they are still lactating. Thus, mares

frequently have to invest in two offspring at one time. It was suggested by Keiper (1979) that

there is an increased probability for feral horse mares to abort when nursing a foal in comparison

with pregnant mares that have no dependent offspring.

Under natural conditions, a plains zebra herd contains just one stallion (Klingel, 1969a,

1969b) who is always dominant over the entire herd (Klingel, 1967; Schilder and Boer, 1987;

J. Pluhacek et al. / Applied Animal Behaviour Science 105 (2007) 192–204 193

Andersen, 1992). His reproductive potential is limited only to the time he spends with the herd

(for an exception see Rubenstein, 1986), while females can reproduce throughout most of their

lifetime (Klingel, 1969a). Plains zebra foals frequently suffer from predation (in wild; Smuts,

1976a) and infanticide (in captivity; Pluhacek and Bartos, 2000, 2005). Stallion keep the herd as a

group by active herding (Andersen, 1992), which can disturb suckling behaviour. Mares that live

in herds without stallions can change their behaviour in other unexpected ways. For example, in

horses, the adult mares allogroomed more in groups without stallion than in groups with a stallion

(Sigurjonsdottir et al., 2003). Therefore, we also consider the presence of the stallion within the

herd as a possible factor influencing weaning time.

Various reports in equids have focused on the age at weaning of the foal (King, 1965; Zwolinski

and Siudzinski, 1966; Tyler, 1972; Rashek, 1976; Smuts, 1976b; Martin-Rosset et al., 1978;

Penzhorn, 1979; Duncan et al., 1984; Berger, 1986; Oom and Reis, 1986; Becker and Ginsberg,

1990). Only very few of these researchers, however, analysed any other factor that could influence

the weaning age. In horses, Asiatic wild asses and mountain zebra, pregnant mares nurse their foals

for a shorter period than the non-pregnant dams (Rashek, 1976; Duncan et al., 1984; Penzhorn,

1984; Berger, 1986; Cameron et al., 2000). However, older (over 9 years of age) horse mothers did

not significantly differ in foal weaning age in relation to their reproductive condition (Cameron

et al., 2000). These mares generally weaned their foals before 1 year of age, regardless of whether or

not they were pregnant, whereas younger mothers continued to suckle their yearling offspring if

they did not have a subsequent pregnancy in the next breeding season (Cameron et al., 2000). It has

been reported that the duration of lactation is longer for primiparous than for multiparous mares

(Duncan et al., 1984). Furthermore, lactation lasted for equal lengths of time for both male and

female foals (Duncan et al., 1984; Berger, 1986; Cameron and Linklater, 2000) and weaning dates

did not vary in relation to the condition of the mare (Berger, 1986; Cameron and Linklater, 2000).

We are aware of only one study dealing with interspecific differences in weaning time among

equids that has been published (Becker and Ginsberg, 1990) and, this study did not reflect any

other factors such as pregnancy or parity of the mare, which were previously documented as

important (Rashek, 1976; Duncan et al., 1984; Berger, 1986).

Investigation of the factors affecting pregnancy and weaning provides an opportunity to test

the Trivers–Willard model (TWM; Trivers and Willard, 1973). The TWM predicts that mothers

in polygynous species, that are in good condition, would be advantaged by investing more in

sons, whereas mothers in poor condition would be advantaged by investing more in daughters.

There are two ways to accomplish this biased investment, either by allocating resources

preferentially to one sex or by adaptive adjustment of the sex ratio at conception (Cassinello,

1996). Ungulates are a useful group of mammals for evaluating different theories of offspring sex

ratio variation, because they (1) give birth to quite few young, (2) parental investment for an

individual varies, and (3) parents may provision in sons and daughters differentially (Kojola,

1999). In ungulates, the second way has been tested more frequently in the past (Rutberg, 1986;

Hewison and Gaillard, 1999; Kruuk et al., 1999; Lindstrom et al., 2002). However, ungulates

typically have a long duration of maternal investment (gestation and lactation; Saltz, 2001) and

the TWM should only be applied when three assumptions are met: (1) the condition of the young

at the end of maternal investment will tend to be correlated with the condition of the mother

during her investment; (2) differences in young at the end of the period of maternal investment

will tend to endure into adulthood; (3) adult males will be differentially helped in reproductive

success compared with adult females by slight advantageous in condition. The TWM was

successfully used in studies of feral horse populations (Cameron et al., 1999a; Cameron and

Linklater, 2000) and we believe that the plains zebra also fit all three of these assumptions.

J. Pluhacek et al. / Applied Animal Behaviour Science 105 (2007) 192–204194

We tested the following four hypotheses: (1) pregnant plains zebra mares would wean sooner

than non-pregnant ones, (2) the age of foal at weaning would increase with increasing time to

delivery of the next foal. We assumed that the captive zebra mares in this study enjoyed under

good conditions (due to management, feeding, lack of predation, etc.) and they should invest

more into sons than into daughters due to high future competition among sons. Based on the

TWM and on our assumption concerning the condition we further predicted that, (3) if the mare

was pregnant and the foetus was a male, she should wean her recent offspring sooner than when

the foetus was a female, and (4) male foals should be weaned later than female foals due to

increased investment in male offspring due to their greater competitive potential.

As potential predictors, we also tested other factors, which might affect natural weaning in

captive plains zebra, such as mortality of a new offspring, the presence of a stallion, the age of the

parents, and the experience of mother (i.e. the number of previous foals successfully reared per

mother), etc.

We are aware of three reports to date on the weaning time of plains zebra (King, 1965; Smuts,

1976b; Becker and Ginsberg, 1990) and all these reports focused on from free ranging

populations. Nevertheless, these reports substantially differed from each other. While King

(1965) reported weaning to occur at 7 months (25 weeks) of age in East Africa, Smuts (1976b)

observed that weaning occurred between 9 and 16 (‘‘normally 11’’) months in South Africa. In

the East Africa, Becker and Ginsberg (1990) reported that weaning occurred at a mean age of

15.5 (�2.2) months (n = 7). Kingdon (1997) more recently summarized that ‘‘foals suck milk for

up to 6 months’’. However, no factor influencing weaning time was considered in any of these

reports of free ranging zebra populations. The knowledge from natural maternal weaning

behaviour gives important information about the investment of a mare in her offspring. Hence,

this biological information is deemed necessary in testing for some sociobiological hypotheses

(Ebensperger, 1998; Pluhacek and Bartos, 2000, 2005).

2. Animals, materials and methods

2.1. Animals

We observed 19 foals of 11 individual plains zebra mares, split into three different herds, at the Dvur

Kralove Zoo, Czech Republic. In three cases, the mares were not pregnant and all their foals (two males and

one female) were weaned artificially (by human intervention).

In all other cases (16) the mares (10 individual) were pregnant at the time of weaning. Ten of the foals

were weaned naturally by their mothers (9 individual mothers). Six of these foals were males and four were

females. It was later found that five of the subsequent foetuses were female and five were male. Three of the

mares delivered stillborn foals; of the stillborns, there was one male and two females.

The six foals of the remaining pregnant mares (five individual) were weaned by human intervention.

These foals were presented in each of the observed herds. Four of these foals were males and two were

females. It was also later found that one of the foetuses was male and the five others were females. Two of

these mares delivered stillborn foals, which were both females.

There was at least one female foal in each herd and one female foetus in each herd. The herd sizes ranged

between two and six breeding mares, aged from 4 to 20 years. All of the observed adult females were

multiparous. The size of the enclosures for the three herds were 800, 1350 and 1750 m2, respectively. There

was almost no vegetation present in any of the enclosures and food was provided to the zebras ad libitum.

They were given fresh food (hay and carrot in winter, grass in summer) daily, usually in the morning. The

criterion used to establish the survival status of the following offspring was set at 6 months of age. In one

herd the stallion died, in two other herds he was separated from the herd for several months and after he was

returned to its ‘‘own’’ herd for management purposes. There was at least one case of absence and presence of

J. Pluhacek et al. / Applied Animal Behaviour Science 105 (2007) 192–204 195

the stallion in each herd at the time of weaning, giving us a chance to assess the possible role of the stallion in

the weaning process. All of the stallions involved were fathers of the observed foals and no case of

aggression of the stallion towards his foals was observed. The dominance hierarchy among the adult females

was determined by biting and offensive kicking as per Pluhacek et al. (2006). Based on dyadic interactions

between mares, for each mare during each period, we estimated rank by a simple criterion of the number of

mares who dominated her (further referred to as ‘‘number of dominant mares’’).

Each group was observed at least once a week (on Saturday or Sunday) from January 1999 to January

2000 and from September 2001 to March 2002. We performed the observations in four different sessions

each week. Each session lasted for 180 min, either from 0800 to 1100 h or from 1400 to 1700 h on Saturday

or Sunday. We observed only one herd during each session (i.e. on Saturday morning we observed herd no.

1, in the afternoon herd no. 2, on Sunday morning herd no. 3 and in the afternoon again herd no. 1, the next

Saturday we began with herd no. 2, etc.). Data concerning all suckling events were recorded in each

observation session.

From February 2000 to August 2001, we performed additional observations of weaning. During these

periods we observed the foals only until they started suckling or until 180 min elapsed.

With regard to the many different definitions of natural weaning in mammals (Martin, 1984; Counsilman

and Lim, 1985; Green et al., 1993) we used the term natural weaning to indicate the cessation of suckling

(Counsilman and Lim, 1985). Thus, if the foal was not observed to be suckling during the 180 min observation

period, then the foal was regarded as weaned. Moreover, we checked for suckling during the next 180 min

period and we asked the zoo keepers to record daily any suckling events when working with the animals. We

defined the day of weaning to be the last day on which a suckling event was seen for each individual foal.

All of the other data concerning the animals (i.e. date of birth of the foals and parents, number of previous

foals of the mother, etc.) were obtained from curators of the Dvur Kralove Zoo. The study was approved by

Institutional Animal Care and Use Committee of the Research Institute of Animal Production and was

conducted in accordance with Czech Central Committee for Protection of Animals number 13803/2003-

1020.

2.2. Statistics

We used Kruskal–Wallis test for determining whether pregnancy influenced weaning time or not in terms

of the duration of lactation. Then we analysed weaning time in pregnant mares, only. The numeric variables

that were analysed are shown in Table 1. The categorical predictors that were tested were the sex of the

weaned foal, the sex of the foetus, the presence or absence of a stallion in the herd at time of weaning and the

survival status of the next offspring. To utilize the information of the incomplete data and to shed light on the

possible interactions among the predictors, we applied a survival analysis using the Cox proportional

hazards model (Cox, 1972) using PROC PHREG (Patteta, 2001). An intrinsic characteristic of survival data

is the possibility for censoring of observations, that is, the actual time until the event is not observed. Such

J. Pluhacek et al. / Applied Animal Behaviour Science 105 (2007) 192–204196

Table 1

The numerical variables used in the model (n = 16 foals weaned by a pregnant mother)

Variable Unit Mean � S.D. Range

Foal’s age Days 313.3 � 62.9 176–461

Mother’s age Years 13.7 � 5.1 5.2–18.3

Time to next parturition Days 162 � 53 83–285

Father’s age Years 16.9 � 2.5 12.2–19.3

Number of dominant mares Animals 1.88 � 1.45 0–4

Herd size Animals 7.8 � 1.8 3–11

Number of other suckling foals Animals 1.7 � 1.1 0–3

Number of previous foals successfully reared per mother Animals 6.0 � 3.5 0–10

Number of previous foals per mother Animals 7.7 � 4.4 1–13

censoring can arise from withdrawal from the experiment or termination of the experiment. Since the

response was duration, some of the possible events may not yet have occurred when the period for data

collection had terminated. The dependent variable was the time which elapsed from the birth of a foal to its

weaning (age at weaning). Weaning occurred either naturally, being induced by the mare (an event weaning

time), or it was induced earlier by humans (a censored weaning time). For the age at weaning, a full model

containing all of the explanatory variables and the first-order interaction terms was initially fitted. Each term

was then sequentially dropped from the full model unless doing so had a significant effect on the fit of the

model. All of the data were analysed using the SAS System, version 8.2.

In a study with horses, Cameron et al. (1999a) found a tendency for female offspring to be conceived

after more cycles than for male offspring. To check this in captive plains zebra we collected data from the

herds studied (in years 1990–2004) and applied multivariate general linear mixed model (GLMM) with

interval duration between introduction of a male into the herd (including non-pregnant mares) and date of

delivery as the dependent variable and sex of the foal as independent variable. To account for the repeated

measures on the same individuals across different years, the analysis was performed using mixed model

analysis with individual mare as a random factor, using PROC MIXED (SAS, version 9.1).

For obvious technical reasons our data set was limited. Therefore, we analysed a statistical test’s power

(the probability that the test procedure will result in statistical significance) using The SAS/STAT power and

sample size (PSS) application comparing two survival curves. The power was related to the sample size, the

size of the type 1 (alpha) error, the actual size of the effect and the size of experimental error.

3. Results

In total, 859 h (during 166 days) of observations of the animals were collected and 3301 suckling

bouts were recorded. In addition, 314 suckling bouts were recorded during the additional

observations (191 days; see Section 2.1) aimed at recording the end of the suckling period.

The earliest weaning that was recorded occurred when a (male) foal was only 243 days old.

His mother gave birth to the next offspring 160 days after. The earliest weaning of the female foal

was recorded when the foal was 284 days old. The latest suckling that was ever recorded occurred

J. Pluhacek et al. / Applied Animal Behaviour Science 105 (2007) 192–204 197

Table 2

The average � S.E. age of the foal when weaned and time to the next parturition of the mare based on data from 10 foals

naturally weaned by their mothers (not censored cases) according to the sex of foetus, their own sex, mortality of the next

offspring and stallion’s presence at the time of weaning

n Age of foal when

weaned (days)

Time to the next parturition

of the mare (days)

Sex of the foetus

M 6 294 � 13 134 � 13

F 4 377 � 35 140 � 22

Foal’s sex

M 5 268 � 11 151 � 13

F 5 368 � 30 125 � 18

Mortality of the

next offspring

No 7 304 � 14 137 � 12

Yes 3 383 � 51 142 � 30

Stallion’s presence

Yes 8 325 � 25 133 � 13

No 2 328 � 18 158 � 17

83 days before the next delivery, when the (female) foal was at the age of 355 days. The average

time of weaning of 10 foals who were weaned naturally is showed in Table 2.

The age of a foal at the natural weaning by pregnant mares was lower (327.4 � 64.0 days,

n = 10) than that at the artificial weaning of non-pregnant ones (444.0 � 26.9 days, n = 3;

d.f. = 1; x2 = 4.48; P < 0.05).

Multifactorial survival analysis was employed and the 10 cases of event weaning time and six

cases of censored weaning time were entered into the model. The final model, containing two

predictors (sex of the foetus and the time to delivery), rejected the null hypothesis that all of the

regression coefficients of the model were equal to 0 (likelihood ratio, d.f. = 2; x2 = 12.43;

P < 0.001). The sex of the foetus appeared to be the most important factor for weaning time

(x2 = 5.48; P = 0.019, hazard ratio = 0.064). The time to delivery almost reached a level of

significance (x2 = 3.31; P = 0.069, hazard ratio = 0.98) and hence we kept it in the model. The

Cox proportional ‘‘hazard ratio’’ is the ‘‘hazard’’ for one group of predictor variable (male

foetus) divided by the ‘‘hazard’’ for another group of predictor variable (female foetus; Patteta,

2001). The estimate is 0.064, implying that the ‘‘hazard function’’ for male foetus is smaller than

the ‘‘hazard function’’ for female foetus. In other words, mares bearing a male foetus would wean

her foal earlier than a mare bearing a female foetus. Fig. 1 shows the pattern of the survivor

function estimate of the age of the foal at weaning (corrected for the time to the next delivery)

according to sex of the mother’s foetus. At the age of about 1 year, the foals of the mares bearing a

male foetus were likely to be weaned over 100 days earlier than those of the mares bearing a

female foetus. Except the time to delivery and the sex of the foetus, none of the factors shown in

Tables 1 and 2 affected weaning time significantly.

Then we calculated the power analysis for two survival curves. For a log-rank test comparing

two survival curves with a two-sided significance level of 0.05, a sample size of 18 per group is

required to obtain a power of at least 0.8 for the two piecewise linear curves, ‘‘Curve 1’’ and

‘‘Curve 2’’. The actual power was 0.820.

J. Pluhacek et al. / Applied Animal Behaviour Science 105 (2007) 192–204198

Fig. 1. Survivor function estimate plotted against the weaning age of the current foal according to the sex of the foetus.

For checking possible difference between the sex of the foals in interval between introduction

of a male into the herd and date of delivery, we collected data on 29 foals delivered by 15 mares.

GLMM revealed no difference in this interval between the sex of the foal (mean � S.E., for males

411.14 � 10.36 days, for females 393.27 � 13.45 days, F = 1.23; d.f. = 1, 28.9; n.s.).

4. Discussion

Pregnant captive plains zebra mares weaned their foals sooner than the non-pregnant ones.

Two individual mares were observed in both situations as pregnant and non-pregnant and one of

them having male foals only and the other mare produced female foals. When pregnant, they

weaned their foals more than 50 days earlier than when they were non-pregnant and all weaning

cases of non-pregnant mares were artificial. Thus, the variation in weaning time between

pregnant and non-pregnant mares is in fact much grater. Moreover, keepers in the zoo reported

observation of several other non-pregnant mares suckling much older foals than those of the

pregnant dams included in our analyses (Bella, personal communication). The significance of

pregnancy in the weaning process of older feral horse mares was doubted in one recent report on

weaning in feral horses (Cameron et al., 2000). The zebra mares included in our analyses were

aged from 5 to 18 years and their age did not influence the timing of weaning in our study. Two of

three of the non-pregnant mares were 14 and 15 years old (classified as old in Cameron et al.,

2000), and all of these mares nursed foals for more than one and a half year.

The time to delivery ranged from 83 to 185 days and is comparable with that in feral horses

(Duncan et al., 1984; Berger, 1986) and wild mountain zebra (Joubert, 1972). We did not record

any indications of prematurity or dysmaturity among the neonates. The power analysis revealed

that the optimal data set required to obtain a power of at least 0.8 for the two piecewise, linear

curves should be twice as much as the one available. This means that time to delivery with

significance of only P = 0.069 is in fact potentially of high importance in weaning in plains zebra,

as reported for horses, asses and mountain zebra (Tyler, 1972; Rashek, 1976; Duncan et al., 1984;

Penzhorn, 1984) and suggested for plains zebra (Smuts, 1976b). This factor has been neglected in

earlier reports on plains zebra (King, 1965; Becker and Ginsberg, 1990).

In the present study, the sex of the foetus appeared to be the most significant factor influencing

weaning time (P = 0.019). Our available data set was only 50% of the optimal number revealed

by the power analysis. This could mean that the sex of the foetus would be a very strong factor

affecting the weaning time of the suckling foal. That is, if the next offspring was to be male, then

the mare was more likely to wean her currently suckling offspring earlier than if the next

offspring was to be a female. This in part may be likely due to preferential investment in male

offspring compared to female offspring during the time when the offspring (impending foetus) is

completely dependent on maternal investment. As far as we know, this is the first record of this

kind in ungulates (compare with review by Hewison and Gaillard, 1999). In our study of captive

plains zebra, we did not find any tendency for male offspring to be conceived later (longer after

foaling) than for female offspring as suggested for horses (Cameron et al., 1999a). Thus, captive

plains zebra mares appear to be changing the length of time between weaning and, delivery of the

next foal, suggesting they might be trading off a value of current and future offspring.

Our study did not reveal any influence of the sex of the weaned foal on the timing of the

weaning. Similarly, no differences were reported in the weaning ages of male and female foals in

studies on feral horses (Duncan et al., 1984; Cameron and Linklater, 2000). Since the suckling

foals were also consuming solid food with increasing age, the pregnant mare could invest

preferentially into the foetus, and this action would bear little risk for the survival of the suckling

J. Pluhacek et al. / Applied Animal Behaviour Science 105 (2007) 192–204 199

foal. The mare could then wean her suckling foal when the demand of the suckling foal is too

high.

In ungulates, the TWM has been tested predominantly on sexually dimorphic species

(Hewison and Gaillard, 1999; Sheldon and West, 2004). In these species the male offspring are

already bigger and grow faster than the female offspring irrespective of the maternal investment

(Cameron and Linklater, 2000; Cameron, 2004). Consequently it is much more difficult to

measure the differential investment by mothers in daughters when they are in poor condition

(Cameron and Linklater, 2000). Thus, the more sexually dimorphic a species is, the more difficult

it is to test the TWM. Equid species, including the plains zebra may be a more appropriate species

than sexually dimorphic cervids or bovids to test the TWM (Cameron et al., 1999a; Cameron and

Linklater, 2000). We assumed that all of the mothers in captivity were in good condition (due to

management, feeding, lack of predation, etc.) and would follow the TWM model for good

condition mothers (Trivers and Willard, 1973). On the other hand, the behavioural dominance

may give a better index of maternal ability than morphological or physiological measuring of

condition due to the following arguments (Sheldon and West, 2004). Firstly, resources available

for reproduction are likely to be determined more by continued future access than by prior access.

Future access to resources would seem more likely to be related to behavioural dominance than to

condition itself. Secondly, the data on behavioural dominance are measures that require repeated

sampling events and of higher quality than data relating to purely physical condition (Sheldon

and West, 2004). In our case, this might mean that dominant mares would prefer sons whereas

submissive mares would invest mainly in daughters. However, we did not find any influence

among dominant mares (behavioural dominance) on weaning time. Indeed, our results appear to

suggest that all of the mares preferred investment into male offspring. Thus, our findings do not

seem to be consistent with the TWM.

The weaning process did not influence the mortality rate of the newborn foals (Table 2). This

result suggests that mortality of the newborn zebra foals at the zoo is probably not caused by

nutritional stress, since plentiful resources were available.

Evidence of an effect on the weaning process by the stallion had not been detected in our

study. Under natural conditions, just one stallion is present in each herd of plains zebra (Klingel,

1964, 1967, 1969b; Smuts, 1976a). Therefore, the stallion uninterruptedly present within his

harem can be certain about the paternity of the foals (but see Rubenstein, 1986). This is quite

different from the situation of horses, where more than one stallion can live within a group at the

same time (Linklater et al., 1999; Linklater, 2000). It must be noted, however, that the number of

stallions in a group did not affect the timing of weaning in horses either (Cameron et al., 2003).

We did not find an influence on the weaning process by any of the following other factors that

were included in the statistical model: age of the parents, herd size, number of other suckling

foals in the herd or experience of the mother (measured as the number of parturitions as well as

the number of successful rearings per mother).

The earliest weaning age recorded in our study (243 days of age) was higher than in King’s

report (King, 1965), but lower than in two other reports (Smuts, 1976b; Becker and Ginsberg,

1990). Those reports were based on observations of wild populations, and, they neglected other

possible factors such as pregnancy, sex of foals, foetuses, etc. Thus, the weaning time of captive

plains zebra ranges within that reported for the mares in wild population. Our weaning age

finding was higher than what has been reported for feral or domestic horses, where the youngest

weaned foal was 196 (Cameron et al., 2003), 140 (Duncan et al., 1984) or 143 (Zwolinski and

Siudzinski, 1966) days old. The earliest weaning age reported for Asiatic wild ass mares (236

days; Rashek, 1976) is also lower than that in our study. It has been previously reported that, the

J. Pluhacek et al. / Applied Animal Behaviour Science 105 (2007) 192–204200

age of the 10 months has been recorded as the time of weaning in mountain zebra (Joubert, 1972;

Penzhorn, 1984). Therefore, the plains and mountain zebra mares may wean their young at a later

age than other equids. Still, as our study shows, in comparing weaning times, other factors such as

the mare’s pregnancy, the sex of her foetus and probably also the sex of her suckling foal, should

be considered. It is very likely that interspecific differences at least among horses, plains and

mountain zebras are based on differences in their gestation periods (Wackernagel, 1965; Joubert,

1974; Jones, 1976; Grubb, 1981; Lobanov, 1983; Penzhorn, 1988; Churcher, 1993). In horses,

plains and mountain zebras, the time between weaning and the next foal delivery seems to be the

same. Under optimal conditions, mares are mated shortly post-partum and the inter-birth

intervals differ due to different gestation period lengths. This might be the main reason why

horses and Asiatic wild asses wean their offspring sooner than zebra do.

Even though many interspecies differences in weaning age among equid species have been

previously discussed, factors such as sex of the foetus and time to next delivery, at least, affecting

weaning have been neglected (Becker and Ginsberg, 1990). Based on results of this study, we do

not agree with the possibility of interspecific differences in terms of weaning time among equids

as proposed by Becker and Ginsberg (1990). In captive conditions, we are able more easily to

identify some factors that affect the weaning process, which we are typically unable to measure in

the wild (e.g. age of parents, experience of the mother, etc.). In a recent meta-analysis, no

evidence was found that wild and captive ungulate populations differ in respect to effect of

maternal condition on sex ratio in relation to maternal condition (Sheldon and West, 2004).

However, we cannot fully reject the possibility that the captive condition affected our results.

5. Conclusion

In conclusion, our results suggest that there are at least two main factors: (a) pregnancy of the

mare and (b) the sex of the foetus if she is pregnant, which appears to influence the timing of

weaning in captive plains zebra. The reproductive state of the mare and the sex of her foetus

should be considered when interpreting weaning times of equids.

Acknowledgments

The authors gratefully acknowledge the assistance of the staff of Zoological Garden at Dvur

Kralove, in particular to Ludek Culık, Ales Kopecky, Dagmar Jankova, Barbara Rakova, Martina

Trohorova, Kristina Tomasova, Miroslav Sprachal, Pavel Moucha, Dana Holeckova, Ludek Bella

and Jan Parık. We thank Wayne L. Linklater, Pavel Stopka and two anonymous referees for their

helpful comments on the earlier draft of the manuscript. We are grateful to Trevor de Vries and

Jana Kalnova for improving English. The project was partly supported by grants from the Grant

Agency of the Czech Republic (523/03/H076) and from the Ministry of Agriculture of the Czech

Republic (MZE 0002701402).

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