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Vigilance Efficiency and Behaviour of Bohor Reedbuck Redunca redunca (Pallas 1767) in a Savanna Environment of Pendjari Biosphere Reserve (Northern Benin)

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This study was performed to gain more knowledge about the Bohor reedbuck time budget and vigilance in a savanna habitat. Detailed data on the time-budget were collected through focal animal observation technique to determine whether the time budget activities of Bohor reedbuck was affected by vegetation cover and to test if herd size, position in herd, the age and sex affect vigilance efficiency. We fit generalized linear mixed-effect models to assess how herd size, sex, animal position in the herd and age affected individual time allocation to vigilance behaviour. The most parsimonious model averaging clearly showed how vigilance behaviour among Bohor reedbuck was strongly, affected by age, sex, the total herd size and animal position in the herd accounting for the lowest Δ value of Akaike Information Criterion (AIC). Vigilance and feeding behaviour occupied the largest percent of Bohor reedbuck daylight time budget. Herd size effects were significant on different behaviour categories only when the Bohor reedbuck was observed in sympatric association. Our findings support the herd size effect hypothesis only in sympatric grouping system. However, further study is needed to investigate the possible sex-specific functions and targets of vigilance behaviour with respect to the herd direction in the field.
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Vigilance Efficiency and Behaviour of Bohor Reedbuck
Redunca redunca
(Pallas
1767) in a Savanna Environment of Pendjari Biosphere Reserve (Northern
Benin)
Author(s): Chabi A. M. S. Djagoun , Bruno A. Djossa , Guy A. Mensah and Brice A. Sinsin
Source: Mammal Study, 38(2):81-89. 2013.
Published By: Mammal Society of Japan
DOI: http://dx.doi.org/10.3106/041.038.0203
URL: http://www.bioone.org/doi/full/10.3106/041.038.0203
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Mammal Study 38: 81–89 (2013)
© The Mammal Society of Japan
Vigilance efficiency and behaviour of Bohor reedbuck
Redunca redunca (Pallas 1767) in a savanna environment
of Pendjari Biosphere Reserve (Northern Benin)
Chabi A. M. S. Djagoun
1,
*, Bruno A. Djossa
1
, Guy A. Mensah
1,2
and Brice A. Sinsin
1
1
Laboratory of Applied Ecology, Faculty of Agronomic Sciences, University of Abomey-calavi, 01BP526 LEA-FSA,
Cotonou, Benin
2
National Institute of Agricultural Research of Benin (INRAB), 01 BP2359, Cotonou, Benin
Abstract. This study was performed to gain more knowledge about the Bohor reedbuck time budget
and vigilance in a savanna habitat. Detailed data on the time-budget were collected through focal
animal observation technique to determine whether the time budget activities of Bohor reedbuck was
affected by vegetation cover and to test if herd size, position in herd, the age and sex affect vigilance
efficiency. We fit generalized linear mixed-effect models to assess how herd size, sex, animal
position in the herd and age affected individual time allocation to vigilance behaviour. The most
parsimonious model averaging clearly showed how vigilance behaviour among Bohor reedbuck was
strongly affected by age, sex, the total herd size and animal position in the herd accounting for the
lowest Δ value of Akaike Information Criterion (AIC). Vigilance and feeding behaviour occupied the
largest percent of Bohor reedbuck daylight time budget. Herd size effects were significant on
different behaviour categories only when the Bohor reedbuck was observed in sympatric association.
Our findings support the herd size effect hypothesis only in sympatric grouping system. However,
further study is needed to investigate the possible sex-specific functions and targets of vigilance
behaviour with respect to the herd direction in the field.
Key words: behaviour, herd effect, sympatric ungulate, time budget, vigilance.
Herbivore vigilance and other behaviour such as forag-
ing, resting and moving are the result of a combination of
influences from both biotic and abiotic factors (Redfern
et al. 2003). It has been suggested that vigilance and
foraging behaviour are mutually exclusive and produce
trade-off (Randler 2005). High-level vigilance is often at
the expense of other activities crucial for their mainte-
nance (Inger et al. 2006). Therefore, animals should
also balance the tradeoff between vigilance and other
activities, especially foraging, resting, moving, drinking,
excreting, jumping, grooming and playing (Inger et al.
2006; Benhaiem et al. 2008). As a result of this eco-
logical shifting in time budget, animals were expected
to show behavioural adaptations that reflect their imme-
diate environments driving by vegetation height, vegeta-
tion cover, human disturbance and predation risk (Poysa
1994) as well as some intrinsic aspects such as age, sex,
position in the herd and herd size (Dalerum et al. 2008;
Shi et al. 2011). However, individuals grouping together
may benefit from a greater number of alert eyes and prey
individuals, allowing a decline in individual vigilance
and increase in other behaviour categories time budget:
the classical “herd-size” effect (Quenette 1990).
Many studies have identified factors affecting individ-
ual vigilance levels (Fernández-Juricic et al. 2007). It
has been proposed that the free ranging antelopes such as
grazers are more vigilant in open habitats than in densely
vegetated areas due to better visibility and more escape
space (Hopcraft et al. 2005) and deserve less time for
other behaviour categories such as moving, resting and
foraging. However, a recent review showed that the ex-
pected pattern of vigilance changes in relation to herd size
in sympatric association are still not well understood in
herd-forming species, and that field studies are required
which help to reconcile isolated theories (Beauchamp
2009). This also suggests questions concerning the vigi-
*To whom correspondence should be addressed. E-mail: dchabi@gmail.com
Mammal Study 38 (2013)82
lance efficiency in sympatric association interaction of
social, intrinsic factors and other environmental effects
on the anti-predator tactics of prey animals.
Within the subfamily Reduncinae, herd size varies
largely amongst species. For example, the herd size of
the Bohor reedbuck (Redunca redunca) doesn’t exceed
six individuals (Afework et al. 2009) compared to the
waterbuck (Kobus ellipsiprymnus) and kob (Kobus kob),
which can reach 35 and 65, respectively (Kingdon 1982).
The three species are savanna grazers and they forage not
far away from water sources (Kingdon 1982). They are
mostly found in Pendjari Biosphere Reserve living in
sympatric association (Kassa et al. 2007; Sinsin et al.
2008). This sympatric behaviour may be beneficial for
the species having the smallest herd size such as Bohor
reedbuck in increasing herd size to reduce the predation
risk. Because, in animals living in herds, individual time
allocation for vigilance decreases while increasing for
other behabiour activities with herd size, as more eyes and
ears are watching predators (the many-eyes hypothesis:
Pulliam 1973); in turn, individuals in herd dilute individ-
ual risk of attack (the risk-dilution hypothesis: Foster and
Treherne 1981). As a result of the ecological difference
in herd size of the three sympatric bovid species, the
Bohor reedbuck was expected to show different foraging
behaviour categories when living in sympatric association.
Grouping has been extensively studied as a form of
anti-predator behaviour in many kinds of animals, includ-
ing ungulates (Fischer and Linsenmair 2006; Dalerum
et al. 2008; Shi et al. 2011; Pays et al. 2012). Also, the
assumption that predation risk drives changes in herd
size in different habitats has been challenged in recent
studies (Creel and Winnie 2005). However, the relation-
ship between herd size, habitat type, grass height and
sympatric behaviour has not been thoroughly examined
for the Bohor reedbuck.
In this study, we determined how different behaviour
time budget activities of Bohor reedbuck were affected
by habitat types and grass height. Finally, we deter-
mined whether the herd size (including other species or
not), the position in herd, the age and sex influenced the
Bohor reedbuck vigilance behaviour. We predicted that
the different behaviour categories of Bohor reedbuck
would be strongly affected by (i) their ecological envi-
ronment, specifically, habitat type and grass height; (ii)
in keeping with the herd-size effect, we expected that
the presence of sympatric species would lead to a signifi-
cant decrease in individual vigilance and, consequently,
increased other foraging behaviour categories time bud-
get because of early warning and a dilution effect; (iii)
males will be more vigilant than females because they
are also watching for intruding males and adults should
be more vigilant than young because young have not yet
learned to be sufficiently vigilant; (iv) edge animals
should be more vigilant than center animals because they
may be the first to encounter mammalian predators.
Methods
Study site
The Pendjari Biosphere Reserve (PBR) is situated in
the north-western Benin (10°30'–11°30N; 0°50'–2°00'E,
Fig. 1). It was declared a Game Reserve in 1954 and
upgraded to the National Park of Pendjari (NPP) in 1961.
Nowadays, it comprises a strictly protected core area
(named “National Park” covering 2,660 km²) and two ad-
jacent hunting zones named “Konkombri” and “Pendjari”
hunting zones, which are on the Eastern and South-
Western sides of the National Park, respectively, (cover-
ing together 1,971 km²). The PBR is limited by the
foothills of the Atacora massif in the East and by the
river Pendjari in the North and the West.
This reserve is under a tropical climate with a seven-
month dry period during which game can be hunted by
tourists. Mean annual precipitation is 1,000 mm, with
most rain falling between late May and early October.
The mean annual temperature is 27°C. During the rainy
season, numerous small ponds in the vicinity of the large
natural ones named Tiabiga, Fogou, Mondri, Diwouni,
Yangouali and Bali in the center of the National Park
are full of water. During the dry season, natural ponds
attract a variety of animal herds, especially large mam-
mals searching for water. The vegetation cover of the
PBR is made up of a mosaic of grasses, shrubs, trees and
woodland savannas, which are burnt every year to pro-
vide fresh pasture to herbivores that dominate the reserve,
to provide visibility to wildlife tourists and hunters, who
visit mostly during the dry season, and to avoid un-
controlled mid or late dry season fires that spread from
surrounding villages or that are lit by poachers. In the
flood plains, Mitragyna inermis, Panicum anabaptistum,
Schyzachirum rupestre, Andropogon canaliculatus,
Vetiveria nigritana, Andropogon spp. and Hyparrhenia
spp. are the most dominant. In drier areas, the main
grass species found belong to the Andropogon and
Hyparrhenia, whereas in wetter areas species such
as V. nigritana, P. anabaptistum, S. rupestre and A.
canaliculatus are found (Sokpon et al. 2001). To date,
Djagoun et al., Vigilance and behaviour of Bohor reedbuck 83
only the largest mammals (Bouché et al. 2004; Di
Silvestre 2008; Sinsin et al. 2008), amphibians (Nago et
al. 2006), bats (Djossa et al. 2008) and some rodents
(Mensah et al. 2007) living in the reserve have attracted
any scientific interest. Bovid species are among the
most targeted species by sport hunters throughout Afri-
can savannas, particularly in Pendjari Biosphere Reserve.
This reserve supports Panthera leo (Linnaeus, 1758),
Panthera pardus (Linnaeus, 1758), Acinonyx jubatus
(Schreber, 1775), Crocuta crocuta (Erxleben, 1777) and
Lycaon pictus (Temminck, 1820), which can prey on
Bohor reedbuck.
Data collection
Observations were made on herds of Bohor reedbuck
at Pendjari Biosphere Reserve from May to July 2011
during the wet season. Data were collected along 10 line
transects of 5 km each, setup along the Pendjari River
due to the fact that the Bohor reedbuck live not far away
from water sources in association with other species such
as waterbuck and kob (Estes 1991; Kassa et al. 2007).
Transects were installed at each 5 km, perpendicularly to
the river in order to avoid replication in herd sampling
and behaviour observation. Also to minimize the risk of
non-independent samples, the behavioural data were
never recorded on more than two focal animals from the
same herd during the same day. Data were collected
through continuous focal animal observation (Altmann
1974). All observations took place during daylight
and only during active foraging time in the mornings
(between 07:00–09:00h). Focal observations were
conducted for 10-minute periods following Roux et al.
(2009) and recorded time spent in the five behavioural
categories such as feeding, vigilant, resting, moving and
other activities (e.g., drinking, excreting, jumping, groom-
ing and playing). Antelopes were categorized as for-
aging when they were standing with their head below
shoulder level, biting or chewing vegetation or if they
were walking with their muzzle close to the ground.
They were categorized as vigilant if they were standing
while watching with the head at or above the shoulder
level. Moving was defined as walking or running with
their head at or above the shoulder level. Resting was
recorded if the animal was lying down. Observations
were abandoned if the focal individual moved out of
view and if the herd varied in size. Data were collected
on a total of 68 Bohor reedbuck herds with 135 randomly
selected focal animals. The time, number of Bohor reed-
buck in the herd, the number of total animals in the herd
(including Bohor reedbuck), sex, age (adult or young),
location in the head (edge or center), the habitat types
and grass height were recorded for each observation.
Fig. 1. Locality of Pendjari Biosphere Reserve, Benin.
Mammal Study 38 (2013)84
Five different habitat types were distinguished: grassland
(defined as open grasslands existing of tall caespitose
grasses with no or rarely any trees (< 5%)), gallery forest
(defined as evergreen gallery forest characterized by tall
trees and no understory layer), wood savanna (defined
as savannas with scarce and separated tree canopies),
woodland (defined as wooded vegetation with overlapping
or bordering tree canopies) and outcrop vegetation
(defined as vegetation located on the mountain). For
grass height, a scale related to the animal’s physiognomy
was used: ‘hoof’ height when the grasses reach the hoof
of the focal animal, ‘knee’, ‘belly’ and ‘spine’. Female
Bohor reedbucks with lambs were excluded to avoid the
influence of parental care on vigilance behaviour (Li et
al. 2009).
Data analysis
Accumulated time and frequency of behaviours were
calculated, and each unit sampling of focal animal was
used as one datum unit in statistical tests. The calcula-
tion of the time-budget was based on the total time spent
on each of the behaviour elements divided by the total
observation period duration (1,350 minutes). We analysed
whether the each defined behaviour variable time was
different among habitat types and grass height by using
analysis of variance (ANOVA) test after checking the
assumption about the normality of the data. We used
Spearman’s rank correlation tests to analyze the correla-
tion between different behaviour categories applied to
focal animal observed with Bohor reedbuck herd size.
The factor affecting vigilance of Bohor reedbuck was
assessed using the generalized linear mixed-effect
models (GLMM) to highlight how the predictor vari-
ables such as herd size, sex, animal position in the herd
and age affected individual time allocation to vigilance
behaviour. The individual ID nested in the herd was
considered as random effect while factors such as total
herd size (including other sympatric species), reedbuck
herd size (including only the number of reedbuck indi-
viduals recorded in the total herd size) sex, animal posi-
tion in the herd (edge or center), and age (young or adult)
were used as fixed effects. In the model built, we could
distinguish the “total.herd.size” as the number of individ-
uals in the herd including other sympatric species, the
“reedbuck.herd.size” including only the number of reed-
buck individuals recorded in the total herd size and the
“animal.position” explains the position (edge or center)
of the focal animal in the herd. Model selection was con-
ducted using a backward stepwise procedure where we
used Akaike’s Information Criterion (AIC) (Akaike 1973)
to remove parameters stepwise (factors were excluded if
this improved the model fit by > 2 AIC units) and select
the most parsimonious model with the best fit to the
data (Bolker et al. 2008). All statistical analyses were
done using R for Microsoft Windows version 2.14.2 (R
Development Core Team 2012), using an alpha value
of 0.05 as threshold for significance.
Results
Time budget according to habitat types and grass height
Figures 2A and 2B give an overview of the time bud-
get proportion of the five categories of behaviour (vigi-
lance, resting, moving, foraging and other activities)
defined in this study across different habitat types and
grass height, respectively. Comparison amongst habitat
types for each of the five categories of behaviour show a
non significant shifting in time budget devoted for vigi-
lant behaviour (ANOVA: F
(4,130)
= 1.321, P = 0.265),
moving behaviour (ANOVA: F
(4,130)
= 0.672, P = 0.612)
and other activities (ANOVA: F
(4,130)
= 1.482, P = 0.211).
But the time budget of Bohor reedbuck being for-
aging (ANOVA: F
(4,130)
= 2.568, P = 0.041) and resting
(ANOVA: F
(4,130)
= 8.972, P < 0.001) were significantly
different among habitat types. The higher values occurred
in the wood savanna (36%) and woodland (33%) for the
time budget of Bohor reedbuck being foraging while the
largest value of being resting was recorded in gallery for-
est and in outcrop vegetation (31 and 17%, respectively).
Non significant difference was found in the proportion of
the time budget devoted by Bohor reedbuck for all the
five behaviour categories investigated within the grass
height classes.
Herd size effect on different behaviour categories
The herd size formed by both Bohor reedbuck and
other sympatric species was found to be negatively
correlated with vigilance behaviour (Spearman’s rank
correlation test: r = –0.57, n = 135, P < 0.001) while this
relationship was significantly positive in foraging behav-
iour (Spearman’s rank correlation test: r = 0.23, n = 135,
P = 0.005) indicating that the Bohor reedbuck spent
more time for foraging when herd size increased. It was
also found that the duration of resting follows the same
trend as foraging time and increase significantly with the
herd size which, include other species (Spearman’s rank
correlation test: r = 0.23, n = 135, P = 0.005), while
non-significant correlation was found between moving
Djagoun et al., Vigilance and behaviour of Bohor reedbuck 85
behaviour time duration and herd size (Spearman’s rank
correlation test: r = –0.10, n = 135, P = 0.24) as well as
other behaviour activities and herd size (Spearman’s
rank correlation test: r = 0.08, n = 135, P = 0.31) (Table
1). Surprisingly, no significant correlation was found
between the time spent for the different behaviour cate-
gories and the herd size only formed by Bohor reedbuck.
Linear mixed model with vigilant behaviour as response
variable
Among the 10 candidate generalized linear mixed
models (GLMMs), only two models were retained as the
most parsimonious accounting for the lowest value of
ΔAIC (Table 2). However, sex, age classes, total herd
size and position of the focal animal in the herd con-
tributed to explaining more such variability, as shown,
respectively, by the two top ranked selected models
Fig. 2. Time budget of Bohor reed buck (Redunca redunca) according to the habitat type (A): gallery forest (GF), grassland (GL), outcrop
vegetation (OV), wood savanna (WS) and woodland (WL) and glass height (B).
Table 1. Spearman correlation test investigating the correlation between different behaviour categories applied to focal animal observed and
Bohor reedbuck herd size (n is the number of focal animals)
Behaviour
Herd size with only Bohor reedbuck Herd size including other species
nr95%IC P-value nr95%IC P-value
Vigilance duration (s) 37 –0.141 [–0.30;0.03] 0.100 135 –0.57 [–0.61;–0.36] < 0.001
Resting duration (s) 37 0.142 [–0.02;0.30] 0.090 135 0.34 [0.19;0.48] < 0.001
Moving duration (s) 37 –0.067 [–0.23;0.10] 0.438 135 –0.10 [–0.26;0.06] 0.240
Foraging duration (s) 37 0.095 [–0.07;0.26] 0.273 135 0.23 [0.06;0.38] 0.005
Other activities duration (s) 37 0.014 [–0.15;0.18] 0.866 135 0.08 [–0.08;0.25] 0.310
Mammal Study 38 (2013)86
(ΔAIC < 2; Table 2). The most parsimonious model
averaging (Table 3 and top panel model in Table 2)
clearly showed how vigilance behaviour among Bohor
reedbuck was strongly affected by age, sex, the total herd
size and position in the herd accounting for the lowest
value of ΔAIC. From this model, the vigilance behav-
iour was negatively associated to the herd size (–0.036 ±
0.004) and young animal (–0.451 ± 0.167). The edge
position (0.150 ± 0.139), the young animal located in
the edge position (0.404 ± 0.161) and the interaction
between herd size and young animal (0.019 ± 0.007)
were found to increase significantly the vigilance behav-
iour time budget (Table 3).
Discussion
A statistically significant negative correlation between
herd size and individual vigilance level was observed in
this study of Bohor reedbuck. In a large herd, an individ-
ual benefits not only from the vigilance activity of the
other herd members, but also from a dilution effect (Li et
al. 2009). This finding is in accordance with many other
previous studies in birds and mammals (Beauchamp
2008). Indeed, as individuals may benefit from both a
dilution effect (i.e. the probability of any one individual
being targeted by a predator decreases with herd size;
Hamilton 1971), and a many-eyes effect (i.e. the chance
that at least one individual in a herd is vigilant at any
given moment increases with herd size; Pulliam 1973),
individuals can afford to decrease their own level of vig-
ilance in larger herds. This pattern has been reported in
many taxa (Pays et al. 2007), but how individuals com-
bine both their duration and frequency of vigilance to
reduce the risk of predation is not well understood (Sirot
and Pays 2011). It is also likely that larger herds may
allow animals to rely on apprehensive foraging, whereas
animals in smaller herds have sometimes to dedicate
more time vigilance as the herd size is too small to rely
on the many-eyes effect. Higher perceived risk of preda-
tion leads to increased individual vigilance in many spe-
cies (Pays et al. 2007). However, the extent to which
prey adjust their vigilance to the context (vegetation
cover, grass height, herd size), or to a perceived risk of
predation associated with the immediate presence of
predators, or both, remains poorly understood. There-
fore, we might except through our study that in the dense
vegetation (such as gallery forest and woodland) the pre-
dation risk is higher than in the open habitat (such as
grassland, wood savanna and out crop vegetation) as the
probability to detect the predator reduces, and hence
individuals have to devote more time for vigilance. But
for the Bohor reedbuck, we found that the duration in
vigilance is not statistically different according to the
Table 3. Estimates for the most parsimonious model of the vigilant
behaviour, for further details see Table 2 (SE: Standard Error)
Estimates SE z-value P-value
intercept 2.430 0.120 20.207 < 0.001
total.herd.size –0.036 0.004 –7.861 < 0.001
young –0.451 0.167 –2.697 0.007
male 0.098 0.122 0.807 0.041
edge 0.150 0.139 –1.077 0.028
young × edge 0.404 0.161 2.504 0.012
total.herd.size × young 0.019 0.007 2.758 0.005
male × edge 0.049 0.155 0.321 0.748
Table 2. Set of the ten most parsimonious and global generalised linear mixed model with vigilant behaviour as response variable
Model –2LogLik AIC ΔAIC
total.herd.size + age + sex + animal.position + age × animal.position + total.herd.size × age + sex × animal.position
153.5 168.3 0
total.herd.size + age + animal.position + age × animal.position + total.herd.size × age
154.3 169.5 1.2
total.herd.size + reedbuck.herd.size + sex + total.herd.size × sex
151.4 171.1 2.8
total.herd.size + age + animal.position + sex + sex × animal.position + age × animal.position + total.herd.size × age
153.4 171.4 3.1
total.herd.size + age + animal.position + total.herd.size × age
160.1 172.1 3.8
total.herd.size + age + sex + sex × age + age × total.herd.size
159.5 173.5 5.2
total.herd.size + age + animal.position + sex + sex × animal.position + age × total.herd.size
159.4 175.4 7.1
total.herd.size + age + animal.position + sex + sex × animal.position + age × animal.position
160.9 176.9 8.6
total.herd.size + age + animal.position + age × animal.position + total.herd.size × age
154.1 178.1 9.8
total.herd.size + age + animal.position + age × animal.position + total.herd.size × age + sex × age
150.6 179.6 11.3
The models were ranked by the corrected Akaike Information Criterion corrected for small samples (AIC). (ΔAIC = difference in AIC between the
best and the actual model; the most parsimonious model is on the top of the list)
Djagoun et al., Vigilance and behaviour of Bohor reedbuck 87
habitat types and grass height classes. This finding
suggests that change in the vegetation pattern did not
affect the vigilance behaviour of the Bohor reedbuck.
While similar results have been reported in prey species
(Blumstein et al. 2003), other studies have reported that
vigilance of prey may either increase (Pays et al. 2009)
or decrease (Beauchamp 2010) with distance to cover.
Such variability seems to be related to many factors
including whether the prey perceived cover as obstruc-
tive or protective (Li et al. 2009).
The Bohor reedbuck species have to deal with a large
range of potential predators in savanna ecosystems.
Hence, it is not surprising that habitat type and grass
height classes are not a useful proxy for predation risk in
the context of the Pendjari Biosphere Reserve. Surpris-
ingly, the time spent foraging and resting by Bohor reed-
buck was influenced by the habitat types with the most
time spent foraging occurring in the wood savanna and
the least in the gallery forest. The gallery forest mostly
used for resting could be because grass provides very
little cover in gallery forest, making it advantageous to
lie down when being vigilant, as standing animals are
very conspicuous in open habitats. Also, due to the
openness of grassland, Bohor reedbuck can still scan the
surroundings when they are feeding, which is harder to
do in woodland or gallery forest.
One of the fundamental assumptions underlying most
vigilance models is that animals are unable to be vigilant
while feeding (Pulliam et al. 1982), but the advantage
of being in a herd is that it allows individuals to feed
while benefiting from the vigilance of others in the herd
(Pulliam et al. 1982). The results of the present study
also support the prediction of “edge-effect” because indi-
vidual Bohor reedbuck at the periphery of a herd was
more vigilant than those in the centre, which corroborates
with the findings of previous investigators (Blanchard
et al. 2008). Although the lower vigilance level of
central individuals may be because those individuals
concentrate around a high-quality patch of gradually
depleted food and increase their intake rate (Hirsch
2007). This is less likely to happen in our study because
the food resources were sparsely and evenly distributed
across the study area (Kassa et al. 2007). Di Blanco
and Hirsch (2006) have demonstrated that the spatial
position with respect to herd direction (e.g., front edge,
end edge, etc.) is an important factor in determining
individual vigilance level, but most of such studies
(including our present one) have not measured the spa-
tial positions with respect to the direction of the herd
(Black et al. 1992), which suggest the need for further
investigation.
Vigilance duration was detected to increase with male
in Bohor reedbuck this could be related to the fact that
males often watch for mates and competitors as well as
predators. However, as has been shown for waterbuck
(Kobus ellipsiprymnus) and kob (Kobus kob) (Burger
and Gochfeld 1994), males are often more vigilant than
females, which support our findings. In the vigilance
model, young Bohor reedbuck was less vigilant, con-
firming our initial prediction, and corroborating others
results with ungulates (Boukhriss et al. 2007). But, the
lowered vigilance can be partially offset by parental
vigilance and by increased adult vigilance (Burger and
Gochfeld 1994). The interaction of young animal and
edge side was also found to increase the Bohor reedbuck
vigilance duration, suggesting that the strength of the
edge-side effect varies with animal age, which supports
the findings by Boukhriss et al. (2007), who stated that
the position in the herd independently from animal age
had a significant effect on vigilance. The vigilance time
budget was found to increase in the interaction of young
animal and herd size. This is contradictory to the find-
ings in literature as young animals generally lack experi-
ence and have higher nutritional needs than adults, they
can be expected not only to be less vigilant than adults,
but also to decrease their vigilance level by a greater
extent when troupe size increases than adults do
(Teichroeb and Sicotte 2012). This result could also be
interpreted as a possible consequence of increasing
competition for territory with herd size in sympatric
living system, which constrained more stressed young
Bohor reedbuck to increase their vigilance time.
In short, our study adds to the large amount of
evidence demonstrating the herd-size effect on ungulate
vigilance, and also adds the importance of sympatric
grouping system in Bohor reedbuck behaviour and
vigilance efficiency. In addition, habitat type and grass
height did not affect the vigilance behaviour of the Bohor
reedbuck. Our study has also shown that within-herd
spatial position is an important factor to be taken into
account in the study of vigilance behaviour. However,
further study is needed to investigate the possible sex-
specific functions and targets of vigilance behaviour and
the effect of within-herd spatial position with respect to
the herd direction in the field.
Acknowledgments: This study was funded by German
LOEWE Research program and SCCS scholarship that
Mammal Study 38 (2013)88
funded a one month internship for the lead author to visit
Professor Tim Coulson’s lab at Imperial College, London.
We thank two anonymous reviewers for helpful com-
ments to improve this paper. We are also grateful to the
local communities, who participated in this research.
Our acknowledgments also go to our field guide, Mr.
Elysé and Tchabi, who have helped us during the field
works.
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Flocking birds frequently look up or “scan” while they are feeding on the ground. High scanning rates increase the probability that the birds in a flock will see an approaching predator in time to avoid predation; however, high scanning rates also decrease the feeding rates of the scanning individuals. Since the scanning rate that maximizes the survival probability of one individual depends on how frequently other birds in the same flock are scanning, the optimal scanning behavior must be modeled as a game. We develop a realistic model of scanning behavior and use it to find two game theoretical solutions—the “co-operative” or Pareto optimum and the “selfish” or Nash optimum. The observed scanning rates do not differ significantly from the co-operative optimal scanning rate. We argue that in a game where players meet again and again such apparent co-operation may be an evolutionarily stable strategy.
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The time devoted to eating, vigilance (or scanning), walking and aggression was studied in one-minute focal animal samples as a function of sex, age, and motherhood for several African animals including African elephant, Burchell's zebra, Cape buffalo, Defassa waterbuck, impala, and Uganda kob. Between 24 and 45 percent of the variation in vigilance behavior was accounted for by location (center or edge of herd), sex, age and whether a female had young. Time of day entered the linear multiple regression models for two species, and surprisingly group size did not enter any of the linear multiple regression models in 1993. In general, females with young were more vigilant than their young or females without young. There were few gender differences in vigilance among animals without young, except that male zebra, wildebeest and waterbuck were more vigilant than females. Animals on the edge of herds devoted more time to vigilance than intermediate or central animals (regardless of sex or age class). Although there was no relationship between group size and vigilance in 1993 when all herds were considered, there was a decrease in vigilance with increasing herd size for impala and kob for herds less than 50. Moreover, herd size and vigilance were negatively correlated in our 1984-1985 study, due partly to differences in group sizes. In 1984-1985 animals were not migrating, and were in relatively small, discrete groups, whereas in 1993 some species were migrating in large herds that stretched for several km. We suggest that in 1993 herd size was above a threshold where increases in group size can lead to further decreases in vigilance. Under these circumstances, location in the herd becomes paramount: Outside animals can be directly exposed to attack. Consequently animals on the edge devote more time to vigilance than central animals.