Download Zebra reduce predation risk in mixed-species

Behavioral
Ecology
The official journal of the
ISBE
International Society for Behavioral Ecology
Behavioral Ecology (2016), 27(4), 1073–1077. doi:10.1093/beheco/arw015
Original Article
Zebra reduce predation risk in mixed-species
herds by eavesdropping on cues from giraffe
Melissa H. Schmitt, Keenan Stears, and Adrian M. Shrader
School of Life Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa
Received 28 September 2015; revised 19 January 2016; accepted 23 January 2016; Advance Access publication 17 February 2016.
Predation risk of individuals moving in multispecies herds may be lower due to the heightened ability of the different species to detect
predators (i.e., mixed-species effect). The giraffe is the tallest land mammal, maintains high vigilance levels, and has good eyesight. As
a result, heterospecific herd members could reduce their predation risk if they keyed off the giraffe’s antipredator behaviors. However,
because giraffe rarely use audible alarm snorts, heterospecifics would need to eavesdrop on cues given off by the giraffe that indicate
predator presence (e.g., body posture), to benefit from herding with giraffe. To test this, we compared the vigilance of zebra herding
with conspecifics, with those herding with giraffe. Our results indicate that giraffe reduce zebra vigilance in zebra–giraffe herds and
that in these herds, giraffe are the primary source of information regarding predation risk. In contrast, when zebra herd with conspecifics, they rely primarily on personal information gleaned from their environment, as opposed to obtaining information from conspecifics
about predation risk.
Key words: eavesdropping, mixed-species effect, multispecies herds.
INTRODUCTION
For animals, a key advantage of group living is a reduction in predation risk through dilution and/or collective detection (Hamilton
1971; Schmitt et al. 2014). This reduced predation risks lowers
vigilance levels and allows herbivores to devote more time to other
activities. However, there are fitness advantages to maintaining personal vigilance scans (Dehn 1990). For example, individuals that
detect an approaching predator are more likely to escape compared with those that rely on collective detection (i.e., being made
aware of predator presence by another group member detecting
the predator; Elgar 1989; FitzGibbon 1989).
For some species, risk can be reduced further by herding with a
diluting partner (i.e., another species that shares a common predator; zebra and wildebeest are “diluting partners” sensu Schmitt
et al. 2014). A diluting partner may reduce predation risk from
a shared predator by having different detecting abilities. Such
mixed-species effects on risk likely reflect a heightened ability of
the mixed group to detect approaching predators compared with
single species herds. This mixed-species effect has been shown to
reduce personal vigilance in a number of species (Scheel 1993;
van der Meer et al. 2012; Schmitt et al. 2014). For example, kudu
(Tragelaphus strepsiceros) were able to drink for longer at waterholes
Address correspondence to M.H. Schmitt. E-mail: [email protected].
K.S. Coauthor is now at the Department of Ecology, Evolution, and
Marine Biology, University of California Santa Barbara, Santa Barbara,
CA 9316-9620, USA
© The Author 2016. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved. For
permissions, please e-mail: [email protected]
when herding in mixed-species groups compared with conspecific
groups (Périquet et al. 2010). Cattle (Bos taurus), when in the presence of mule deer (Odocoileus hemionus) stimuli showed a “comfort
response” whereby they reduce their vigilance levels. This reduction in vigilance allowed the cattle to spend more time foraging
(Kluever et al. 2009).
Although the benefits of mixed-species herding have been
shown (Scheel 1993; Périquet et al. 2010; van der Meer et al. 2012;
Schmitt et al. 2014), the extent to which this benefit varies across
different coherding species has scarcely been explored. For example,
body size can influence potential predation risk, with smaller species having greater predation risk and exhibiting higher vigilance
levels than larger species (Sinclair et al. 2003). Therefore, herding
with smaller species should be more beneficial than herding with
larger species, as long as the species share the same predators (i.e.,
diluting partners; Schmitt et al. 2014). Alternatively, large-bodied
species may be less negatively influenced by visual constraints, such
as vegetation, when detecting predators. Therefore, small herbivores
could potentially enhance their fitness by herding with larger species
whose detecting abilities are less diminished by habitat structure.
The association of giraffe (Giraffa camelopardalis) with other herbivores has led to the assumption that giraffe may benefit other
coherding species (e.g., Dagg and Foster 1976; Leuthold 1977).
This may come from the giraffes’ increased ability to detect predators due to their height (Young and Isbell 1991), good eyesight
(Mitchell et al. 2013), and high vigilance levels (Creel et al. 2014).
However, there is no scientific evidence to support this assumption. Moreover, giraffe only rarely produce audible alarm snorts
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(Schaller 1972; Moss 1975). Thus, eavesdropping on alarm calls
cannot be the main mechanism by which coherding species reduce
their predation risk. Nevertheless, giraffe do exhibit an easily identifiable staring posture when in the presence of predators (Dagg
1971). This posture is most likely a cue, which is an incidental feature that does not have intended meaning to a receiver (Saleh et al.
2007), unlike a signal (e.g., alarm call/snort), which is believed to
elicit a hard-wired response (Beauchamp et al. 1976). As a result, it
may be possible that individuals can learn about potential threats
by observing the reaction (a cue) of other members in their group
(e.g., giraffe), even in the absence of an alarm call. For example, in
gray kangaroos (Macropus giganteus), a species that does not produce
alarm calls, conspecifics cue off the reactions of other group members to predators to gain information regarding threats (Pays et al.
2013).
However, whether one species can react to the posture of another
(i.e., a cue; Maynard Smith and Harper 2003) in the absence of an
alarm call, has scarcely been explored. Consequently, the aims of
this study were to 1) determine whether the presence of giraffe
can reduce zebra vigilance because they share the same key predator (i.e., lion; Hayward and Kerley 2005), 2) compare the impact
between giraffe and a known “diluting partner” (wildebeest) on
zebra vigilance, and 3) quantify the degree to which giraffe alter
the manner in which zebra assess risk. We expected that due to
the height advantage of giraffe, that zebra would reduce their time
spent vigilant and rely more heavily on information gathered from
coherding giraffe. Additionally, we predicted that giraffe may have
a greater impact on zebra vigilance behavior compared with wildebeest because not only are they a diluting partner, but due to
their height advantage, zebra may perceive the quality of information from giraffe to be better. If this was the case, then we expected
that zebra would focus more of their vigilance toward giraffe and
rely less on personal scanning of the environment. Alternatively,
we may find that zebra are unable to decipher nonaudible cues
(i.e., the giraffe’s behavioral posture indicating the presence of a
predator), and thus, giraffe would not influence zebra vigilance
behavior.
MATERIAL AND METHODS
All aspects of the research design were approved by our institution (Ethics Code: 13/11/Animal). We conducted fieldwork during August–September (late dry season) 2013 and December (wet
season) 2014. We collected data from Hluhluwe-iMfolozi Park and
Kruger National Park, South Africa. To minimize habitat differences in perceived predation risk, we limited observations to herds
feeding in savannas with visibly similar tree cover. However, within
a savanna, there can be differences in the tree:grass ratio that could
influence perceived predation risk (Stears and Shrader 2015) and
ultimately vigilance (Metcalfe 1984). Giraffe tend to avoid woody
habitats, particularly with young, and select open savannas where
personal vigilance and speed of escape are facilitated (Young and
Isbell 1991). Thus, we assumed in our study that giraffe selected
similar habitats and small-scale differences in tree:grass ratios
would not result in differences in vigilance levels. To control for
variation across our study sites, we included a site effect in our statistical analysis. Additionally, we collected data 2 h after first light
and 2 h before last light to maintain the same level of perceived
predation risk across days (as per Scheel 1993; Schmitt et al. 2014).
All observations occurred from a stationary vehicle using binoculars. To avoid potential behavioral changes due to vehicles, we only
Behavioral Ecology
collected data when no other vehicles were present and when the
zebra were more than 20 m from the road.
In a study of lion prey preferences, Hayward and Kerley (2005)
found that across 48 different lion populations, giraffes were the
4th most preferred prey item, whereas zebra were the 7th most
preferred prey item. Because both zebra and giraffe are categorized as favored prey items for lions (Hayward and Kerley 2005),
these herbivores gain the benefit of both dilution and detection
when herding together (i.e., diluting partners; Schmitt et al. 2014).
A study by Schmitt et al. (2014) found that detection was the main
factor in reducing vigilance in small and medium herd sizes (2–30
individuals). In this study, we were only interested in the effects of
detection in reducing vigilance levels. Therefore, to make vigilance
comparisons between herds as a result of detection, we limited
data collection to similar small-sized zebra-only and zebra–giraffe
herds that we encountered (i.e., 2–14 individuals). Zebra-only herds
comprised a group of zebra that fed within 6 body lengths of their
nearest conspecific (~12 m), whereas zebra–giraffe herds contained
at least 1 giraffe within 12 m of a zebra. We collected data from
adult zebra of both sexes, but avoided herds that contained zebra
or giraffe juveniles. Giraffe within the herds comprised either adult
males and/or females.
Vigilance observations started when a focal zebra had its head
down and was grazing. We considered zebra being actively vigilant
when they lifted their head above grazing height and scanned for
predators, or focused their gaze and actively listened (as per Scheel
1993; Schmitt et al. 2014). Due to the placement of their eyes on
the side of their head, zebras can use monocular and binocular
vision (Barnett 2004). As a result, they can see to their side using
monocular vision while they forage (Harman et al. 1999). This
ability could potentially allow zebra to be passively vigilant while
they feed and thus detect approaching predators. However, because
zebra head placement while feeding is pointing toward the ground,
we are unable to differentiate between zebra being passively vigilant (looking for potential predators using monocular vision) or
searching for another foraging patch. Furthermore, it is unlikely
that the level of passive vigilance shown by zebra while foraging
will differ between zebra-only and zebra–giraffe herds. Thus, we
restricted our data collection to active forms of vigilance (Scheel
1993; Creel and Winnie 2005; Creel et al. 2014). Within a herd, we
did not record data from the same individual twice, but rather collected data from >75% of the individuals in the herd.
To explore whether the presence of giraffes influenced zebra
vigilance, we compared active (head raised) zebra vigilance when
herding in zebra-only herds, with zebra herding in zebra–giraffe
herds. We observed a zebra for 3 min and recorded: 1) time spent
actively vigilant, 2) intensity of each vigilance event (i.e., general
or focused scan), and 3) the source of information used to determine predation risk (i.e., herd member or the environment). We
defined a general scan as a zebra scanning with its head up without
fixing its attention (vision or ears) in a particular direction (zebraonly: n = 153 scans, zebra–giraffe: n = 79 scans). A focused scan
comprised a zebra staring in a fixed direction, with its ears pricked,
either looking toward another herd mate (i.e., zebra or giraffe) or
out toward the environment (zebra-only: n = 325 scans, zebra–
giraffe: n = 17 scans). We sampled both herd types in both reserves
(iMfolozi: zebra–giraffe: n = 11 herds, 64 individuals, zebra-only:
n = 34 herds, 106 individuals; Kruger: zebra–giraffe: n = 7 herds,
13 individuals, zebra-only: n = 17 herds, 63 individuals).
With regard to source of information, when herding in a zebraonly herd, a zebra could obtain information about predation risk
Schmitt et al. • Giraffe reduce predation risk of zebra
by looking at a conspecific, conducting its own scan of the environment, or both. When herding with giraffe, zebra had the same
options, but looking at herd mates comprised conspecifics and/
or giraffe. For zebra, binocular vision (oriented in the direction of
the muzzle) provides better depth perception compared with their
side-orientated monocular vision (Harman et al. 1999). As a result,
it is more likely that these herbivores would use binocular vision
to actively scan for approaching predators. Thus, we considered
zebra being actively vigilant when their head was raised and their
muzzle was directed toward a zebra, giraffe, or out to the environment. If zebra looked at 2 sources separately in a scan (e.g., herd
mate and the environment), we noted that as both. Additionally,
we were able to discern the direction of a zebra’s scan a majority of the time, even when the focal individual was in the center
of the herd. However, if a zebra directed a scan toward a zebra
and a giraffe, and we could not discern which individual was the
focus of the scan, we recorded the scan as toward an unknown target. We used a subset of our total samples to determine information source because we only began collecting these data midway
through our data collection (iMfolozi: zebra–giraffe: n = 7 herds,
21 individuals, zebra-only: n = 22 herds, 63 individuals; Kruger:
zebra–giraffe: n = 7 herds, 13 individuals, zebra-only: n = 10 herds,
24 individuals).
Finally, to explore the relative influence that giraffe have on
zebra vigilance behavior, we compared the amount of time zebra
spent vigilant when herding with giraffe and when herding with a
known diluting partner—wildebeest. We collected zebra vigilance
behavior in the same manner as described above. These data come
from Schmitt et al. (2014) (iMfolozi: zebra–giraffe: n = 11 herds, 32
individuals; zebra–wildebeest: n = 2 herds, 14 individuals; Kruger:
zebra–giraffe: n = 9 herds, 13 individuals, zebra–wildebeest: n = 4
herds, 32 individuals).
Data analysis
Prior to statistical analysis, we calculated mean individual vigilance
per herd (i.e., herds as replicates) to avoid possible pseudoreplication. To test whether time devoted to vigilance was influenced by
herding with giraffe, we tested for herd independence to ensure the
data could be pooled. There was no herd effect; thus, individual
herds were independent. Therefore, we used the mean time a herd
was vigilant as our dependent factor. We used an analysis of covariance (ANCOVA) to test whether the mean time spent vigilant by
the herds varied with herd type (zebra-only and zebra–giraffe). The
covariates such as number of giraffe, total number of ungulates,
and season were nonsignificant and thus removed from the final
model. To control for a site effect, we used “site” as a covariate in
all analyses. Data were box–cox y transformed prior to analysis.
To determine the relative impact of giraffe herding with zebra
compared with when zebra herd with a known diluting partner,
wildebeest, we used an ANCOVA with time spent vigilant as the
dependent variable. We used herd type (zebra–giraffe and zebra–
wildebeest) as the main effect and number of zebra in a herd as the
covariate. Initially, we also included site, season, and total number
of ungulates as covariates. However, these were nonsignificant and
thus removed from the final model. These data are from Schmitt
et al. (2014) and were box–cox y transformed prior to analysis.
To test whether the intensity of vigilance events varied between
zebra in zebra-only or zebra–giraffe herds, we compared the proportion of general versus focused scans within each herd. We used
proportion of each scan type per herd as the dependent variable
in an analysis of variance (Anova) and the interaction of herd
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type (zebra-only and zebra–giraffe) and scan type (1 = general,
2 = focused) as the main factors. Therefore, each herd had a proportion for a general and a fixed scan. We were able to differentiate
between these proportions by including the independent variable,
scan type. By including both these proportions for each herd type,
we were able to determine if there was any change in the proportional use of each behavior as herd type changed.
Covariates site (iMfolozi, Kruger), total number of ungulate individuals in a herd, and season were nonsignificant and removed.
Data were transformed using arcsine square root.
Finally, to determine the source(s) that zebras used to gather
information about perceived predation risk, we used an Anova with
proportion of scans (regardless of the scan intensity) directed toward
an information source (i.e., herd mate, environment, or herd mate
plus environment), per herd as the dependent variable. We then
used the interaction of herd type (zebra-only and zebra–giraffe) and
information source as the main effects. Covariates site, season, and
total number of ungulates were nonsignificant and thus removed.
Data were arcsine square root transformed prior to analysis.
RESULTS
The presence of giraffe lowered the average time a zebra was vigilant by nearly two-thirds (ANCOVA: F2,68 = 15.75, P < 0.0001).
Zebra herding with conspecifics spent an average (mean ± standard
error) of 27 ± 2 s vigilant, whereas zebra herding with giraffe spent
only 10 ± 4 s vigilant per three-min observation period. The covariate number of giraffe was nonsignificant, indicating that the presence rather than number of giraffe reduced zebra vigilance.
When we compared the amount of time zebra spent vigilant
when herding with giraffe versus with wildebeest, we found that
the presence of giraffe lead to a 50% reduction of zebra vigilance
compared with the level zebra maintained when herding with wildebeest (ANCOVA: F2,23 = 13.280, P < 0.0001). This reduction
indicates that zebra perceive information gleaned from giraffe to be
more valuable than that from wildebeest.
In addition to altering vigilance time, the intensity of vigilance
scans by zebra differed between herds with and without giraffe
(F3,130 = 19.31, P < 0.0001). When herding alone, zebra used
focused scans proportionally more (0.70 ± 0.039) than general scans
(0.29 ± 0.039). However, when herding with giraffe, zebra used general scans proportionally more (0.81 ± 0.064) than focused scans
(0.19 ± 0.064). This pattern was consistent across sites.
The source of information used by zebra to assess risk depended
on whether giraffe were present or not (Figure 1; significant
herd type × information source interaction term; F5,132 = 46.46,
P < 0.0001). Zebras herding alone actively scanned the environment
(0.79 ± 0.04) significantly more than individuals herding with giraffe
(0.20 ± 0.06). In contrast, zebra herding with giraffe actively scanned
herd mates (0.72 ± 0.06) significantly more than zebras herding alone
(0.15 ± 0.04). Neither herd types extensively scanned both herd mates
and the environment. Zebra herding with giraffe looked directly at
herd mates 73% of their scans, and of these scans directed at their
herd mates, zebra looked directly at giraffe 45%, giraffe and zebra
23%, and zebra 5% of the time. In contrast, zebra herding alone
only used their herd mates ~20% of the time for information.
DISCUSSION
Moving in groups is a key way in which animals reduce predation
risk (Krause and Ruxton 2002). However, for this to be possible,
Behavioral Ecology
1076
Proportion of scans
directed towards information sources
0.8
Zebra-Giraffe Herds
Zebra-Only Herds
0.6
0.4
0.2
0.0
Herd Mate
Environment
Herd Mate+Environment
Information sources
Figure 1
Mean (± standard error) proportional use of information sources by zebra
when herding with conspecifics or giraffe.
group members must be able to obtain information about potential threats from other group members (Metcalfe 1984; Elgar 1989).
A number of studies have shown that information about predators
can be conveyed through alarm calls (Seyfarth et al. 1980; Fichtel
and Kappeler 2002; Manser et al. 2002), but this is a challenge
in species that either do not have auditory alarm calls (Pays et al.
2013), or rarely uses them (e.g., giraffe; Schaller 1972; Moss 1975).
Our results show that when one or more giraffe were present in
a group of foraging zebra, the time zebra devoted to vigilance
dropped by nearly two-thirds compared with when herding with
conspecifics. Furthermore, in mixed-species herds, the majority of
zebra scans were directed toward giraffe. Therefore, we hypothesize
that the mechanisms through which this reduction occurs is likely
because zebra are able to interpret the leaked cues (e.g., posture)
from the giraffe. As a result, zebra are able to reduce their perceived
predation risk even in the absence of alarm calls. Interestingly, the
amount of time that zebra devoted to vigilance when herding with
giraffe was half as much as when they herded with wildebeest, a
known diluting partner (Schmitt et al. 2014). Thus, it seems that
zebra perceive these leaked cues from giraffe to be more valuable,
and more reliable, than information from a similar-sized diluting
partner. Furthermore, in zebra–giraffe herds, zebra relied more
on general vigilance scans compared with the focused scans (i.e.,
staring in a particular direction with ears pricked) used in zebraonly herds. Moreover, there was a near complete switch in behavior from relying on direct personal assessment of risk in zebra-only
herds, to primarily using social information (i.e., eavesdropping) on
other herd members in zebra–giraffe herds.
Because giraffe do not frequently use alarm snorts, in fact, we
never heard them make any vocalizations during our study, it is
plausible that the zebra actively eavesdropped off the giraffe’s
behavior (i.e., cues) to assess predation risk. In zebra–giraffe herds,
the majority of social information (~45%) used by zebra involved
directing their scans toward giraffe. An additional 23% focused
toward both giraffe and conspecifics. However, the zebras’ inspections of conspecifics were more likely to protect against competition
(Artiss and Martin 1995) because zebra in zebra-only herds did not
focus on conspecifics to reduce predation risk.
The switch in information source used by zebra is surprising
because in mixed-species herds, zebra theoretically should use both
conspecific and heterospecific cues equally to better determine
predation risk (Goodale and Kotagama 2008), rather than relying
mainly on cues from giraffe. However, this shift can potentially be
explained by 2 reasons. First, when compared with zebra and other
similar-sized ungulates, the elevated position of giraffe’s eyes (and
thus a better view than what shorter species have), coupled with
good eyesight (Mitchell et al. 2013), most likely increases the distance over which a giraffe can detect approaching predators. This
is similar to other species that rely on elevated conspecific sentinels
for antipredator information, such as meerkats (Suricata suricatta)
(Santema and Clutton-Brock 2013).
Second, the large body size and the characteristic body language
displayed by giraffe when they detect the presence of lion (i.e.,
freeze and stare posture Dagg 1971) facilitate visual eavesdropping
on these cues by zebra. To do this, it is likely that zebra focus on
the entire body posture of the giraffe and the fact that the giraffe
has/have ceased feeding and are focusing their gaze, and thus their
head, in 1 direction, indicating the presence of predators. The
information gained by zebra from giraffe is perhaps more accurate
than information from conspecifics or other coherding species, such
as wildebeest, because the height of the giraffe makes it less likely
to be obstructed by vegetation. For example, a study on 2 species
of shorebirds (Arenaria interpres and Calidris maritime) found that with
decreasing visibility there was an increase in individual vigilance
(Metcalfe 1984). This was because the shorebirds could not rely as
much on antipredator information gathered from other members
in the flock because they were obstructed from view, in addition
to a greater risk of predation from hidden predators with decreasing visibility (Metcalfe 1984). Thus, the information from giraffe is
likely more valuable to zebra than information from conspecifics or
similar-sized heterospecifics sharing the same predation risk. This
is likely because zebra can gain more information about predators
(e.g., direction of approach) from giraffe than they can from potentially obstructed conspecifics within the herd.
If our interpretation is correct, this then raises the question, if
zebra gain so much by moving with giraffe, then why do not they
always herd with them? A potential reason for this is that giraffe
may experience lower levels of predation by lions compared with
zebra. In general, bigger-bodied animals are less sensitive to predation (Sinclair et al. 2003) and therefore should have lower vigilance
levels than smaller-bodied herbivores. For example, Sinclair et al.
(2003) found that in the Serengeti, lions hardly preyed on giraffe.
However, a meta-analysis of the prey preference of lions that covered 48 different lion populations found that giraffe were determined to be the 4th most preferred prey item, whereas zebra were
only the 7th most preferred prey item (Hayward and Kerley 2005).
Thus, due to the risk of predation from lions, giraffe should maintain high vigilance levels. Although this level may be less than that
of zebras, our results suggest that the quality of the information
(i.e., unobstructed view) still benefits zebra. Otherwise, why would
they rely so heavily on the giraffe? However, if this is the case, then
it does not explain why zebra and giraffe do not always move in
mixed herds.
A more likely reason for the lack of herding of zebra with giraffe
may have to do with information that is gained beyond predator
presence. In our study, it was common for zebra to herd with giraffe,
but they more frequently herded with wildebeest, impala, or a combination of these 2 herbivores. Specifically, because zebra are grazers, they do not gain foraging information by moving with browsing
giraffe. In contrast, when herding with other grazers/mixed feeders
(e.g., wildebeest and impala), zebra can not only reduce their predation risk (Schmitt et al. 2014), but also obtain social information
Schmitt et al. • Giraffe reduce predation risk of zebra
about the availability and location of food (Valone and Templeton
2002; Shrader et al. 2007; Pays et al. 2014). Thus, the combination
of these two factors may be more beneficial than the reduced predation risk provided by giraffe, hence explaining the more frequent
herding by zebra with grazers and mixed feeders. However, when
zebra herded with giraffe they did follow them and continue to feed
in close proximity when the giraffe moved off (Schmitt MH, Stears
K, personal observation).
During our study, we did see giraffe herding with other browsing
herbivore species including kudu and impala (Schmitt MH, Stears
K, personal observation). However, kudu and impala browse at a
much lower height than giraffe (Du Toit 1990). Therefore, similar
to zebra, these browsing herbivores only gain a reduction in predation risk and no benefit from social information about the availability and location of food. This could explain why other browsing
species do not continuously herd with giraffe.
Ultimately, our data suggest that when in zebra-only herds, zebra
primarily assess predation risk by directly scrutinizing their outward
environment (i.e., personal information). However, when moving
in zebra–giraffe herds, they shift and primarily rely on giraffe as
a way of detecting predators. The combination of these data suggests that zebra are not reliant on auditory alarm snorts/calls, and
can possibly interpret leaked cues from giraffe to reduce their vigilance behavior. Thus, it would seem that for zebra in zebra–giraffe
herds that the reliability of the information about predation risk is
far greater than the benefits that zebra obtain by scanning the environment themselves. Additionally, the survival and fitness benefits
obtained by eavesdropping on giraffe are greater than when herding with conspecifics or similar-sized diluting partners.
FUNDING
This project was funded through a grant to A.M.S. (IPRR) from
the National Research Foundation.
We thank R.J. Schmitt and S.J. Holbrook who provided statistical advice
and valuable comments on earlier drafts as well as S. Swarbrick for constructive discussions. We would also thank T. Caro and an anonymous
reviewer for their constructive comments and suggestions.
Handling editor: Johanna Mappes
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