Ethology
Decision-Making in Przewalski Horses (Equus ferus przewalskii)
is Driven by the Ecological Contexts of Collective Movements
Marie Bourjade*, , Bernard Thierryà, Myriam Maumy§ & Odile Petità
*
à
§
Laboratoire d’Ethologie Animale et Humaine, Centre National de la Recherche Scientifique, Université de Rennes 1, Rennes, France
Association Takh pour le cheval de Przewalski, Station Biologique de la Tour du Valat, Arles, France
Département Ecologie, Physiologie & Ethologie, IPHC, Centre National de la Recherche Scientifique, Université de Strasbourg, Strasbourg, France
Institut de Recherche Mathématique Avancée, Centre National de la Recherche Scientifique, Université de Strasbourg, Strasbourg, France
Correspondence
Marie Bourjade, UMR CNRS 6552 Laboratoire
d’Ethologie Animale et Humaine – Ethos,
Université de Rennes 1, Campus de Beaulieu,
Avenue du Général Leclerc, F-35042 Rennes
Cedex, France.
E-mail: [email protected]
Received: June 18, 2008
Initial acceptance: July 31, 2008
Final acceptance: November 22, 2008
(J. Wright)
doi: 10.1111/j.1439-0310.2009.01614.x
Abstract
We addressed decision-making processes in the collective movements of
two groups of Przewalski horses (Equus ferus przewalskii) living in a semi
free-ranging population. We investigated whether different patterns of
group movement are related to certain ecological contexts (habitat use
and group activity) and analysed the possible decision-making processes
involved. We found two distinct patterns; ‘single-bout’ and ‘multiplebout’ movements occurred in both study groups. The movements were
defined by the occurrence of collective stops between bouts and differed
by their duration, distance covered and ecological context. For both
movements, we found that a preliminary period involving several horses
occurred before departure. In single-bout movements, all group members rapidly joined the first moving horse, independently of the preliminary period. In multiple-bout movements, however, the joining
process was longer; in particular when the number of decision-makers
and their pre-departure behaviour before departure increased. Multiplebout movements were more often used by horses to switch habitats and
activities. This observation demonstrates that the horses need more time
to resolve motivational conflicts before these departures. We conclude
that decision-making in Przewalski horses is based on a shared consensus process driven by ecological determinants.
Introduction
Collective patterns such as group movements are
conspicuous in animals. The joint action of individuals generates order and produces decisions at
the collective level (Thierry et al. 1996; Parrish &
Edelstein-Keshet 1999; Couzin & Krause 2003).
Group movements take the shape of foraging
swarms of ants or bees (Deneubourg & Goss 1989;
Seeley et al. 1990), schools of fish (Romey 1996;
Couzin et al. 2002) or flocks of birds (De Schutter
1997). In species where individual recognition
occurs, group movements are likely to be influenced by social relationships, for example baboons
Ethology 115 (2009) 321–330 ª 2009 Blackwell Verlag GmbH
(hamadryas baboons, Papio hamadryas: Kummer
1968; mountain baboons, Papio ursinus: Byrne et al.
1990), African buffalos (Syncerus caffer: Prins 1996)
and equids (feral horses, Equus ferus caballus: Feist &
McCullough 1976; Berger 1977; plain zebras, Equus
burchellii: Fischhoff et al. 2007). By studying group
movements, theoretical issues about the collective
processes of decision-making and its functional consequences may be addressed (Norton 1986; Conradt
& Roper 2005; Couzin et al. 2005).
One main goal is to understand how individual
decisions, i.e. shifts in individual behaviour, are integrated by groups and lead to collective phenomena
(Norton 1986; Thierry et al. 1996; Couzin & Krause
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Decision-Making in Przewalski Horses
2003), which are performed by animals when
exploiting their environment (Conradt & Roper
2003; Hewitson et al. 2007). The synchronized and
spatially coordinated patterns of movements exhibited by gregarious animals may result from an initial
change in the behaviour of one or several individuals, followed by joining responses of other group
members. In some species, obvious signals often precede departure (Black 1988; Boinski & Campbell
1995; Menzel & Beck 2000).
Some authors suggest that a division of labour
occurs between decision-makers; ‘primers’ are
responsible for the priming role, ‘releasers’ for the
departure time and ‘leaders’ for the travel direction
(Norton 1986; Byrne et al. 1990; Erhart & Overdorff
1999). Pre-departure behaviour has been reported
before movement in several species. Baboons indicate travel choices by their location, body direction
and short moves (Kummer 1968; Sigg & Stolba
1981; Norton 1986), whereas African buffalos gaze
in the direction of subsequent movements (Prins
1996). In honey bees (Apis mellifera), Visscher & Seeley (2007) recently showed that piping signals emitted by scouts before taking-off play a priming role in
a two-step primer-releaser system.
Such complexity in inter-individual communication may illustrate an attempt to reach a collective
decision to move (Leca et al. 2003; Sueur & Petit
2008). Indeed, in permanent social groups, where
divergent individual needs and motives may lead to
group splits, group members need to resolve conflicts
of interest in order to preserve social cohesion
(Gérard & Loisel 1995; Couzin & Krause 2003; Conradt & Roper 2005; Biro et al. 2006). Up until now,
the organization of collective decisions has mostly
been explained through a self-organized approach.
The reason is because studies have focused on large
groups where only local communication may occur
between individuals (Camazine et al. 2001; Conradt
& Roper 2005). Therefore, there is a need to explore
further the decision-making processes within small
groups with a stable group composition and where
global communication may occur (Conradt & Roper
2005).
In this context, equids are worthy of scientific
study. Feral and Przewalski (Equus ferus przewalskii)
horse subspecies form family groups that constitute
stable and cohesive units, usually composed of one
adult stallion, a few adult mares and their offspring below the age of puberty (feral horses: Feist
& McCullough 1976; Berger 1977; Przewalski
horses: Boyd & Houpt 1994; King 2002). Anecdotal
reports mention that horses move in single-file
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M. Bourjade et al.
over great distances, with a preferential leader at
the head (Welsh 1975; Feist & McCullough 1976)
and that the first mover may come back to its
group when not followed by others (Berger 1977).
Most of the time senior mares have been reported
to initiate group movements (Welsh 1975; Berger
1977), except in one population where stallions
initiated most movements by herding group members (Feist & McCullough 1976). Whereas these
studies investigated whether a particular individual
walked in a front position, none focused on the
whole decision-making process occurring at group
departure.
This study addresses the dynamics and contexts of
moving patterns of Przewalski horse families living
in natural social conditions. We predicted that reaching the decision to move should take much longer
prior to movements that lead to shifts in the use of
habitats and associated activities than prior to other
types of movement. We investigated (1) which
behaviour differentiates two distinct movement patterns of horse families, (2) which behaviour occurs
before departure, (3) which joining responses are
aroused in group mates and (4) whether the duration of the decision-making is related to the number
of decision-makers and to the ecological relevance of
the movement.
Methods
Study Population
The Przewalski horse is now an extinct species in
the wild, but several free-ranging populations exist
around the world. We studied Przewalski horses kept
in a 380 hectares (ha) enclosure at Le Villaret in
Southern France (Causse Méjean, base camp office:
4415¢9¢¢N, 326¢29¢¢E). This population has produced one of the re-introduced populations in the
historic area of the species (i.e. Mongolia) and both
were managed by the Association Takh. The population of Le Villaret expanded from initial 11 individuals brought from European zoos in 1993 and 1994,
to 55 individuals in 2003. The population was closed
and free of predators; all horses, apart from the 11
founders, were born at the study site. Until 2003,
the horses were not subject to any management and
human intervention was kept to a minimum. Since
then, 12 horses were removed from Le Villaret in
Sept. 2004 and 10 in Aug. 2005 to be re-introduced
in Mongolia. Nevertheless, at both times of the
study, the population was composed of five independent families and two all-male groups.
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M. Bourjade et al.
All horses from the population were individually
recognizable by their colour variations, body or
mane shapes and stripe patterns on the legs and
were accustomed to human observations. We
observed collective movements of two families composed of different individuals. During Mar. and Apr.
2004, we followed the G12 family composed of 12
individuals [one adult stallion (8 yr), five adult
mares (4, 4, 5, 15 & 16 yr), two sub-adult females
(2 yr) and four foals (1 yr) – two males and two
females] for 88 h. During Mar. and Apr. 2006, we
followed the G6 family composed of six individuals
[one adult stallion (12 yr), three adult mares (7, 8 &
8 yr) and two colts (one-year-old males)] for 120 h.
Collective movements were observed out of the
breeding season in order to limit the effect of stallions’ intrasexual competition on family movements
(through herding behaviour).
Study Site
Le Villaret is located on a high calcareous plateau
(elevation ranged from 900 to 1250 m and mean
temperatures from )6 to +27C) offering a specific
Decision-Making in Przewalski Horses
grassland area, composed mostly of graminoids and
spread over combs, plains and crests (Saı̈di 1998).
According to this author, combs are clayey depressions containing high-quality meadows and, in parts,
formerly cultivated fields. The dominant species are
Bromus erectus, Bromus squarrosus, Trifolium repens and
Brachypodium pinnatum. The horses have always
shown a strong preference for this habitat in all seasons (Feh & Carton de Grammont unpubl. data).
Crests are more arid; contain succulents, some of the
roots the horse feed on, bare areas and small combs
on the top, which form natural shelters. Plains are
low-quality meadows dominated by Festuca duriscula
and Festuca glauca. Limited resources such as water
and some mineral elements are supplied ad libitum
on a discrete site called the ‘water site’. Combs,
plains, crests and the water site were all non-contiguous, distinct habitats, which were considered in our
analyses. Each habitat patch was named, mapped
and known to observers. The home-range used by
horses was approximately the entire enclosure area.
Families moved daily over different locations and
routinely used the same travel patterns, forming visible trails on the soil (Fig. 1).
Fig. 1: Schematic map of Le Villaret study site. Grey lines correspond to the fence and curves to the contour lines. Numbers indicate elevation in
metres. Circles, squares and stars represent the centre (centroı̈d) of habitat patches that observers geo-referenced using a geographical information system (ª MapInfo v. 7.5); circles: comb patch, white squares: plain patch, grey squares: crest patch and stars: water point patch. Black lines
represent common travel patterns on visible trails that were used by horses at least once during the study.
Ethology 115 (2009) 321–330 ª 2009 Blackwell Verlag GmbH
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Decision-Making in Przewalski Horses
M. Bourjade et al.
Observation Procedure
As two to five activity bouts per day are reported in
Przewalski horses (Berger et al. 1999), each group
was observed during four consecutive hours (n = 22
and n = 30 in 2004 and 2006 respectively) equally
distributed over the daylight period. Two observers
approached horses on foot at a distance of about
20 m, and data were collected using tape recorders.
The first observer recorded the location of the group
(dictating the name of the habitat patch) and the
activity of each individual by instantaneous sampling
every 5 min (Altmann 1974). Such sampling was
performed over the entire length of the observation
period, except when horses moved collectively (i.e.
at least 50% of the individuals were moving). The
second observer recorded the patterns of collective
movements using all occurrences sampling (Altmann
1974). Any moving horse was monitored with its
identity and the time of the movement recorded.
A ‘start attempt’ was then defined as a horse walking over a distance greater than the diameter of the
group, without stopping or foraging, i.e. with its
neck in a horizontal position. A collective movement
began with a start attempt, i.e. the departure of the
first mover, and ended when the last individual
arrived. From preliminary observations, we noticed
that a preliminary period existed during which individuals performed pre-departure behaviour before
the departure of the first mover (see Table 1). Similarly to Visscher & Seeley (2007), we called these
individuals the ‘primers’. Thus, the second observer
recorded any occurrence of pre-departure behaviour,
the time of the start attempt made by the first mover
and the departure time of each individual of the
group. During movements, any individual stopping
for a rest was monitored with its identity and the
time of the stop recorded, in particular if the front
individual or several individuals were involved. Two
cases involving the stop of all group members were
distinguished: (1) the arrival was determined when
all individuals had stopped with each individual displaying a different body orientation and (2) ‘collective stops’ were scored when all horses stopped
simultaneously and oriented their bodies together in
the same direction. Locations of departure and arrival were recorded and mapped. We controlled the
reliability between the different observers using the
kappa coefficient of Cohen (1960) that rated at
k = 0.90 (between M. Bourjade and M. Moulinot in
2004 and M. Bourjade, L. Wilkins and M.S. Delhing
in 2006).
Spatial Patterns and Behavioural Units
Locations of the groups in the different habitat
patches were geo-referenced using a geographical
information system (ª MapInfo v. 7.5, Pitney Bowes
Softwares, Troy, New York, USA). Centroids on
Fig. 1 correspond to the centres of gravity of habitat
patches used by the horses at least once in 2004.
Distances between centroids, corresponding to common travel patterns of horses, were then calculated
taking elevation into account using a Vertical Mapper (ª MapInfo International). Distributions of travel
distances of observed movements are shown in
Fig. 2. For analysis, we considered once each travel
distance observed in 2004 (even if more than one
collective movement covered this distance) in order
to estimate the median distance the horses were
Table 1: Pre-departure behaviour that occurred during the preliminary period preceding departure in collective movements of Przewalski horses
Category
Pre-departure behaviour
Definition
Primary
behaviour
Moving away
An individual moves away from the group without foraging, and with its neck in
a horizontal position, over a distance shorter than the diameter of the group
An individual stays on the periphery of the group, initially without close
neighbours, and stretches the shape of the group by its location
An individual follows an individual moving away from the group at a shorter
distance than three horse-body lengths
An individual starts from the group and joins an individual displaying a peripheral
overhang
An individual joins a peripheral individual and repeatedly pauses in walk, the
body oriented in the direction of the peripheral individual
The stallion gathers its group members in approaching them from the back, ears
laid back and neck stretched out (Feist & McCullough 1976)
Displaying a peripheral overhang
Secondary
behaviour
Following an individual that is
moving away
Joining a peripheral individual
Pausing
Herding
behaviour
Primary behaviour is behaviour that is initially observed; secondary behaviour is carried out in response to it. Primary and secondary behaviour
may be performed by any group members. Herding behaviour is a stallion-specific behaviour.
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Decision-Making in Przewalski Horses
M. Bourjade et al.
foraged and oriented their body in the same direction, i.e. approximately the direction of the preceding moving bout. Scored collective stops did not
exceed 30 min. If the group started again in the
opposite direction, a new collective movement was
systematically recorded. So, the duration of the MB
movement was calculated from the last moving
bout, including collective stops.
Statistical Analyses
Fig. 2: Distributions of travel distances for movement patterns
observed in 2004 and 2006 according to movement types. Grey bars
represent the number of single-bout (SB) movements and black bars
the number of multiple-bout (MB) movements observed in each distance category.
likely to cover, i.e. 468 m. As travel distances were
only approximated, we classified each collective
movement as ‘short’ or ‘long’ in distance according
to whether it was shorter or longer than the median
distance. Activities recorded by instantaneous sampling for each individual were grazing, moving, resting-standing, lying recumbent, monitoring the
environment, maintenance and social behaviour.
The activity of the group was the one displayed by
the majority of individuals. The types of pre-departure behaviour recorded during the preliminary period are listed in Table 1. They include primary
behaviour, referring to initial conspicuous behaviour
made by primers, and secondary behaviour carried
out in response to this primary behaviour. We also
recorded the herding behaviour of stallions which
makes the group gathered, and therefore has spatial
consequences.
Moving Patterns
The duration of a movement corresponded to the
time elapsed between departure of the first mover
and arrival of the last individual. We distinguished
two categories of movement according to the presence or absence of collective stops. Single-bout
movements (SB movements) consisted of a unique
moving period. At arrival, all individuals changed
activities or dispersed in different directions to forage. Multiple-bout movements (MB movements)
consisted of several moving periods separated by
collective stops during which the group kept a conspicuous moving shape (stretched); all individuals
Ethology 115 (2009) 321–330 ª 2009 Blackwell Verlag GmbH
Analyses were initially based on 135 moving events
with all group members participating in the collective movement. We considered each collective
movement as an independent event. We compared
the duration of SB and MB movements using
Mann–Whitney U-tests. Activities and types of habitat scored just before departure and just after arrival
of each movement were compared with a uniform
law using chi-square goodness-of-fit-tests with simulated p-values and performed with R 2.6.1 software
(http://cran.r-project.org). Neu tests were then used
post hoc to determine which of the categories were
different. Fisher’s exact probability tests were used to
test for a link between the type of movement
(SB ⁄ MB) and: (1) the distance (short ⁄ long), (2) the
habitat of departure and arrival sites (comb, plain,
crest, water site) and (3) the occurrence of shifts in
activity and habitat between departure and arrival
(yes ⁄ no).
We investigated the decision-making process
through the duration of the joining process (this corresponds to the time the whole group took to start
moving after a start attempt) on 113 moving events.
This variable is therefore the outcome of the decision
to move. As the relationships between the duration
of the joining process and the social variables sampled prior to movements were clearly non-linear, we
used non-parametric Spearman correlations to test
for the significance of these relationships. Social
variables scored in the 20 min before the start
attempt were: (1) the number of primers, (2) the
number of occurrences of primary and secondary
pre-departure behaviour and (3) the number of
occurrences of herding behaviour. Fisher’s exact
probability tests were used again to test for a link
between the presence of secondary behaviour in the
preliminary period (yes ⁄ no) and the number of
primers (superior ⁄ inferior to the median number).
All tests were performed using the software statistica
7.0 (Statsoft Inc., Maisons-Alfort, France) – except
when specified – and were two-tailed with a level of
significance setting at 0.05.
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Decision-Making in Przewalski Horses
Results
Multiple and Single-Bout Movements
Start attempts were followed by a collective movement of the whole group in 93% of cases (n = 135
collective movements out of 145 start attempts),
making failures infrequent. The subsequent results
are based on 135 movements except when specified.
MB movements (n = 47, involving 2–10 bouts separated by collective stops) and SB movements
(n = 88) were observed in both groups. Distances
covered by the horses ranged from about 130 m to
1600 m: families covered short (56.2%) and long
travel distances (43.8%) with MB movements, while
SB movements were mostly performed over short
distances (88.6%) (Fisher’s exact probability test:
p < 0.001). MB movements lasted significantly
longer than SB movements (Mann–Whitney test,
G12: p = 0.002; G6: p < 0.001) and durations were
shorter in G6 than in G12 whatever the movement
type (Mann–Whitney test, MB: p = 0.006; SB:
p < 0.001) (Fig. 3). Therefore, MB and SB movements were investigated separately throughout the
study.
Ecological Contexts
Horses spent respectively 10% and 8% of their daytime moving collectively in G12 and G6 groups.
Group movement and standing-rest were the second
main activities of these families, while grazing occupied 78% of their day-time budget in G12 and 76%
M. Bourjade et al.
in G6. Grazing was the prevailing activity preceding
(46.7% of SB and 67.7% of MB movements) and
following (53.3% of SB and 67.7% of MB) both
types of group movements (n = 104, chi-squared
test: p < 0.001 in all cases), whereas standing-rest
strictly preceded (42.7% of movements) and followed (33.3% of movements) SB movements only
(0% preceding or following MB movements).
Combs were the most frequent sites of departure
and arrival for SB movements (chi-squared test:
p < 0.001 for departure and arrival). A similar pattern was found for MB movements, but failed to
reach significance (chi-squared test: p = 0.390 for
departure, p = 0.150 for arrival) (Table 2). However,
movement types were associated with departure
(Fisher’s exact probability test: p = 0.001) and arrival
(Fisher’s exact probability test: p = 0.017) sites: families left or joined the water site and crests significantly more often with MB than with SB
movements, while they left or joined plains preferentially with SB than MB movements (Table 2).
Additionally, MB movements occurred significantly more often than SB movements in shifts of
habitats (MB movements: 68.7%, SB movements:
37.5%, Fisher’s exact probability test: p < 0.001) and
changes of group activities (MB movements: 48.4%,
SB movements: 24.6%, Fisher’s exact probability
test: p = 0.022). Thus, after MB movements, horses
tended to be in a different habitat and show a different activity than before departure.
Decision-Making Processes
The duration of the joining process positively correlated with the duration of MB movements of G12
group (Spearman correlation, G12: rs = 0.78,
p = 0.004; G6: rs = 0.36, p = 0.050) and of SB movements of both groups (Spearman correlation, G12:
rs = 0.78, p < 0.001; G6: rs = 0.64, p < 0.001).
Therefore, we focused on social factors preceding
departure that may affect the duration of the joining
Table 2: Percentages of multiple-bout (MB) and single-bout (SB)
movements (n = 47 and n = 88 respectively) used by the horses to
leave and join the four different habitats
Fig. 3: Durations of single-bout (SB) and multiple-bout (MB) movements in both study groups. White bars represent the G12 group of 12
individuals observed in 2004 [n (SB movement) = 36, n (MB movement) = 11].
Grey bars represent the G6 group of six individuals observed in 2006
[n (SB movement) = 38, n (MB movement) = 28]. **p < 0.01, ***p < 0.001
(Mann–Whitney test).
326
Departure
MB movement
SB movement
Arrival
MB movement
SB movement
Comb
Crest
Water site
Plain
35.42
49.43
16.67
5.75
31.25
16.09
16.67
28.74
31.25
44.83
25.00
8.05
29.17
13.79
14.58
33.33
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M. Bourjade et al.
G6: rs = 0.39, p = 0.040; G12: rs = 0.55, p = 0.080)
probably due to the small sample size (n = 11). The
number of primers did not correlate with the duration of the joining process in SB movements (Spearman correlation, G12: rs = )0.13, p = 0.431; G6:
rs = 0.21, p = 0.191).
In contrast, the more frequently the herding
behaviour occurred during the preliminary period,
the shorter was the duration of the subsequent joining process in MB movements of group G6 but not
of G12 (Spearman correlation, G6: rs = )0.53,
p = 0.004; G12: rs = )0.13, p = 0.700). Once again,
the occurrence of herding behaviour did not correlate with the duration of the joining process in SB
movements of both groups (Spearman correlation,
G12: rs = )0.02, p = 0.079; G6: rs = )0.29,
p = 0.900).
Quality of pre-departure behaviour preceding MB movements
Fig. 4: Relationship between occurrences of pre-departure behaviour
(primary and secondary behaviour) and duration of the joining process
in multiple-bout (MB) movements; G6: group of six individuals
observed in 2006, G12: group of 12 individuals observed in 2004.
Spearman correlations, p < 0.05 in both cases.
process. Each moving pattern and each group were
investigated separately due to differences in group
composition and size. Sample sizes of these analyses
were – except when specified – 36 SB and 11 MB
movements for G12 and 38 SB and 28 MB movements for G6.
Occurrence of pre-departure behaviour and primers
Interestingly, the occurrences of pre-departure
behaviour performed by the primers during the preliminary period correlated with the duration of the
joining process in MB movements of both groups
(Spearman correlation, G12: rs = 0.69; G6: rs = 0.53,
p = 0.004 in both cases), but not in SB movements
(Spearman correlation, G12: rs = )0.09, p = 0.593;
G6: rs = 0.28, p = 0.082). The more frequently predeparture behaviour occurred, the longer was the
duration of the joining process in MB movements
only (Fig. 4). Likewise, the number of primers also
correlated positively with the duration of the joining
process in G6 but not in G12 (Spearman correlation,
Ethology 115 (2009) 321–330 ª 2009 Blackwell Verlag GmbH
In the G6 group (where the number of primers also
correlated positively with the duration of the joining
process), we explored which types of pre-departure
behaviour occurred during the preliminary periods
involving more or less than three primers (i.e. the
median number of primers). We found the secondary behaviour significantly associated with preliminary periods with many primers, and therefore
preceding the longest joining processes (Fisher’s
exact probability test: p = 0.002). Indeed, secondary
behaviour occurred in only 25% of preliminary periods with less than three primers (n = 8), while it
was scored in 100% of preliminary periods with
more than three primers (n = 9).
Were primers the leaders?
Simultaneous first movers were responsible for 31%
of collective movements (35 out of 113) excluding
the possibility to assess whether a first mover was
also a primer in these cases. However, we investigated this question on the 24 MB and 43 SB movements where a preliminary period and a single first
mover occurred. The first mover was one of the
primers in 62.5% of MB and 79.1% of SB movements, but was the lone primer in only 20.1% of
MB and 16.3% of SB movements. Each horse could
be a primer and a first mover in both families.
Discussion
Przewalski horses displayed two patterns of movement that were distinguished by the absence or
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Decision-Making in Przewalski Horses
presence of collective stops. SB and MB movements
differed by their duration, the distance covered by
animals and their context. Whereas SB movements
were relatively short in duration and distance, MB
movements covered short and long distances and
were long-lasting.
The preliminary periods, which preceded departures of both types of movements, involved several
group members that actively contributed to the
decision-making, thus qualifying the process as
‘partially shared consensus decision’ (Conradt &
Roper 2005; Sueur & Petit 2008). Likewise, start
attempts relied on single or simultaneous first
movers that could either be primers or not. This
indicates a distributed social process between group
members that belies the traditional report of a
consistent leader in horses (Feist & McCullough
1976; Berger 1977). After departure, walking was
the normal gait for daily movements between two
regularly attended sites, and groups rarely failed to
keep cohesion as described in feral horses (Tyler
1972; Berger 1977). While each individual had
been observed stopping for a rest during both
types of movements, the collective stops observed
in MB movements are also similar to those
reported by Tyler (1972) in feral ponies for which
stops and re-starts of first movers induced stops
and re-starts of all group members. Thus, group
cohesion during horses’ collective movements
could be underlain through the tendency of individuals to stop.
Decision-making processes differed according to
movement patterns. The presence of a preliminary
period did not affect the joining process of SB movements. In these short movements, all group members joined the first moving horse sooner than in
MB movements. As previously reported for whitefaced capuchins (Meunier et al. 2006), a mechanism
of social facilitation at departure might underlie such
agreement, i.e. the departure of one or more first
movers increases the probability of the others moving. Such an obvious consensus at departure would
result from a decision taken prior to the movement.
Indeed, the primary spatial behaviour of preliminary
periods such as ‘peripheral overhang’ resemble the
postural behaviour of yellow baboons, Papio cynocephalus (Norton 1986) and African buffalos (Prins
1996) which precede their prompt departures. As
described in scout bees, the decision that it is time to
leave could be transmitted through a two-time process in which the preliminary period plays a priming
role, while the start attempt acts as a releaser (see
Visscher & Seeley 2007).
328
M. Bourjade et al.
For MB movements the pre-departure behaviour
appeared to constrain the joining process. The more
numerous the primers and occurrences of pre-departure behaviour during the preliminary period, the
longer was the duration of the joining process. Preliminary periods involving many primers included
several occurrences of primary behaviour performed
in the same or different directions, together with
several occurrences of secondary behaviour such as
joining or following a peripheral individual. Many
decision-makers were associated with slow subsequent joining processes, indicating that horses were
quite reluctant to move in the chosen direction and
that a consensus needed more time to emerge. Prior
to these movements, the more numerous the herding behaviour of stallions, the shorter were the subsequent joining processes. It appears that preventing
large spatial dispersion by gathering group mates
may lead stallions to resolve motivational conflicts.
Such elaborated communication in horses illustrates
a negotiation process that may request stable group
composition. A similar process was observed among
hamadryas baboons, where males indicate the travel
directions they favour by aggregating – with females
and offspring – in different columns oriented in corresponding directions; the troop eventually departs
in the direction indicated by the larger column
(Kummer 1968).
The varying patterns of group movements found
in Przewalski horses make sense when looking at
environmental determinants. Processes of decisionmaking matched ecological contexts. Short SB movements usually occurred after resting or were used to
change foraging sites, with horses staying in the
same habitat and keeping the same activity. In comparison, MB movements often led to shifts in habitats and changes in activities. These movements
occurred more often when horses left or entered
into the water site and crests, two habitats devoted
to specific activities; drinking and licking mineral
elements at the water site, searching specific roots
and succulents or licking calcareous rocks and bare
soil on crests. In herbivores, decisions to leave a
patch in heterogeneous habitats are generally based
on trade-offs between diet selection and grouping
(Dumont et al. 2002); patches can be left prematurely (Parsons & Dumont 2003) to preserve social
cohesion (Boissy & Dumont 2002). As Przewalski
horses stay relatively cohesive, they also achieve
similar trade-offs. Primary behaviour can act as stimuli to which horses responded differently according
to contexts. It should be added that the quality,
intensity and simultaneity of pre-departure and
Ethology 115 (2009) 321–330 ª 2009 Blackwell Verlag GmbH
M. Bourjade et al.
departure behaviour may have played a role in the
decision-making of Przewalski horses. The habitat
use and group activities related to MB movements
obviously revealed conflicts of interest between
group members.
As MB movements led to a wider use of habitat
types and to more diverse activities, it is understandable that horses faced larger motivational divergences during decision periods. As expected from
theory, when conflicts of interest emerge between
group-mates, the need to negotiate terms of acceptance slows decision processes (see Conradt & Roper
2005). Such conclusions stand for two families of
Przewalski horses in semi-free ranging conditions.
Futures studies should address whether matching
the decision-making process to ecological contexts is
of a general rule in wild animals.
Acknowledgements
We acknowledge the MAVA Foundation and the Station Biologique de la Tour du Valat for financial and
logistic support. We are grateful to all ‘Villaret
observers’ for their contribution: Maı̈c Moulinot,
Lucie Wilkins and Marie-Sophie Delhing. Additional
thanks are due to Ruth Knowles for English language advice, Marc Pichaud for working on MapInfo
and Martine Hausberger and Claudia Feh for their
critical comments.
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