Sentinel Behaviour Research Articles

The sentinel behaviour of Arabian babbler floaters

The sentinel behaviour of 38 Arabian babbler adult floaters, who lived alone within a territory belonging to a foreign group, was studied and compared with their own sentinel behaviour in the past, when they were group members. All floaters acted as sentinels and uttered ‘alarm calls’. This suggests that sentinel activity is due at least, in part, to selfish motives. Floaters sentinelled less than they did as group members, with the decrease in sentinel activity sharper for ex-dominants than for ex-subordinates. One possible explanation for these differences is that sentinel activity is aimed not only at detecting predators, but also at detecting foreign conspecifics. Within a group, the latter incentive is stronger for breeding dominants than for subordinates, whereas all floaters alike may be trying to detect the owners of the territory in which they were roaming but also to avoid being detected by them. Other possible explanations are that floaters have less time and energy for sentinel activity because they are weaker or because foraging is more difficult in a foreign territory. This may be especially so for dominants who used to enjoy privileged access to food in their group. No significant difference was found in the rate of sentinels' ‘alarm calls’ between floaters and group members, suggesting that their main purpose is predator–prey communication, of which warning groupmates may be a side benefit.

Interactions of fungal pathogens with phagocytes

The surveillance and elimination of fungal pathogens rely heavily on the sentinel behaviour of phagocytic cells of the innate immune system, especially macrophages and neutrophils. The efficiency by which these cells recognize, uptake and kill fungal pathogens depends on the size, shape and composition of the fungal cells and the success or failure of various fungal mechanisms of immune evasion. In this Review, we describe how fungi, particularly Candida albicans, interact with phagocytic cells and discuss the many factors that contribute to fungal immune evasion and prevent host elimination of these pathogenic microorganisms.

Sexually selected sentinels? Evidence of a role for intrasexual competition in sentinel behavior.

Although the evolutionary mechanisms that favor investment in cooperative behaviors have long been a focus of research, comparatively few studies have considered the role that sexual selection may play. For example, evolutionary explanations for sentinel behavior (where 1 individual assumes an elevated position and scans the surroundings while other group members forage nearby) have traditionally focused on the inclusive fitness benefits arising from its effects on predation risk, while its potential role in defense against intrasexual competitors remains largely unexplored. Here, we provide experimental evidence of a role for sentinel behavior in intrasexual competition, in a cooperatively breeding songbird, the white-browed sparrow weaver (Plocepasser mahali). First, dominant males sentinel substantially more than other group members (even when controlling for variation in age and body condition), consistent with a role for sentineling in intrasexual competition for mates and/or territory. Second, experimental playback of an unfamiliar male's solo song elicited a marked increase in sentineling by the dominant male, and the vocal response to the playback also positively predicted his sentinel effort following the simulated intrusion. A second experiment also suggests that sentineling may facilitate mounting rapid anti-intruder responses, as responses to intruder-playback occurred significantly earlier when the dominant male was sentineling rather than foraging at playback onset. Together, our findings provide rare support for the hypothesis that sentinel behavior plays a role in intrasexual competition, and so highlight the potential for sexually selected direct benefits to shape its expression in this and other social vertebrates.

Sentinel dwarf mongooses, Helogale parvula, exhibit flexible decision making in relation to predation risk

To maximize survival, animals should adjust their behaviour flexibly in response to indicators of predation risk. Predation risk is affected by a range of ecological, social and individual variables, which can fluctuate over different timescales. In general, current risk levels are known to influence the behaviour of sentinels, individuals that adopt a raised position to scan for danger while groupmates are engaged in other activities. However, there has been little consideration of whether decisions made at different stages of a sentinel bout are affected in the same way by perceived predation risk and whether the same level of behavioural plasticity is exhibited when making these different decisions. Here we used detailed behavioural observations and a playback experiment to investigate the behavioural choices of dwarf mongoose sentinels at three different stages of a bout (before, start, during). Individuals were more likely to begin a bout, and did so sooner, following alarm calls, which are immediate, direct indicators of elevated risk. Sentinels selected an initial height from which to guard depending on factors that tended to vary in the medium term (hours), choosing higher positions in denser habitat and less windy conditions. In contrast, decisions about bout duration were made in relation to short-term (seconds/minutes) changes in information, with sentinels guarding for longer when an alarm call was given during a bout, and terminating bouts sooner when groupmates moved out of sight. Our results demonstrate that sentinel decisions are influenced by both direct and indirect indicators of likely predation risk and that sentinel behaviour is adjusted flexibly with regard to information presented on various timescales, highlighting the complexity of decision-making processes. • Sentinel decisions are influenced by direct and indirect indicators of risk. • Individuals become a sentinel and do so faster following an alarm call. • Sentinels adjust guarding height according to factors varying in the medium term. • Sentinels adjust bout duration according to factors varying in the short term.

Looking after your partner: sentinel behaviour in a socially monogamous bird

Natural selection favours those individuals with effective anti-predator defences. The presence of sentinels is known to be an effective form of defence amongst stable groups of individuals within cooperative and polygynous breeding systems. However, the presence of sentinels in the more prevalent socially monogamous breeding systems remains overlooked as an important benefit of such partnerships. Here, we describe a study in which we examined the presence and effectiveness of sentinels in a wild population of the socially monogamous zebra finch (Taeniopygia guttata). We found that when experimentally approached by a human observer during incubation, birds flushed from their nests at significantly greater distances when their reproductive partner was acting as a sentinel than when the partner was absent. The distance at which birds flushed was not influenced by the approach direction of the human observer, the gender of the incubating bird, the presence of conspecifics, the habitat type or the size of the breeding colony. Our results indicate that sentinels are an effective anti-predator defence amongst socially monogamous birds, and may represent a neglected benefit of the formation of stable social partnerships in birds. We suggest that whilst recent work has focused on the sexual conflicts that occur between males and females in socially monogamous pairs, we should not lose sight of the benefits that individuals may gain from their partner.

Meerkat helpers increase sentinel behaviour and bipedal vigilance in the presence of pups

Helpers in cooperatively breeding species may assist in a number of cooperative activities, such as nest building, incubating, babysitting and feeding of offspring. In addition, individuals may engage in sentinel behaviour and other forms of vigilance, which benefits other group members not only by reducing the risk of predation, but also by allowing them to reduce their own vigilance. It remains unclear, however, whether these vigilance behaviours are selfish, and the benefits to other group members simply an unselected by-product, or whether they are forms of cooperation, and the benefits to group members a selective force favouring the behaviour. In meerkats, Suricata suricatta, both female and male helpers were more likely to perform sentinel behaviour after dependent pups had started joining the group on foraging trips. In addition, female helpers performed more bipedal vigilance and found less food when pups were in the group. These results suggest that sentinel behaviour and bipedal vigilance represent forms of cooperation in meerkats.

Is sentinel behaviour safe? An experimental investigation

Sentinel behaviour, where individuals take turns to watch for danger and give alarm calls to approaching predators, has been observed in a number of animal societies. However, the evolutionary causes of this behaviour remain unclear. There are two main, competing hypotheses regarding the evolution of sentinel behaviour. The first hypothesis is that it is a cooperative behaviour, where group members benefit from the detection of danger but share the workload of acting as a sentinel. The second is that it is a safe, selfish behaviour. Under the second hypothesis, once an individual is satiated, being a sentinel is safer because sentinels can detect threats more readily and can therefore escape from predators faster. We examined whether sentinels are safer than foragers in a wild, free-living cooperative bird (the pied babbler, Turdoides bicolor) with a well-described sentinel system. We found that sentinel behaviour was costly because (1) sentinels were targeted by predators more often, (2) they were further from cover than foragers, and (3) they took longer to reach the safety of cover following a predator alarm. These results suggest that individuals do not become sentinels because it is safer. This is the first study to demonstrate that sentinels are at greater risk of predator attack than foraging group members and suggests sentinel activity may have evolved as a form of cooperative behaviour.

Effects of predation risk on foraging behaviour and group size: adaptations in a social cooperative species

There is a divergence between models examining the evolution of group living in species in which groups are largely based on families and those based on fluid aggregations of nonrelatives. In the former, the onus has been on ecological and demographic factors that select for offspring philopatry; in the latter, the importance of factors such as foraging success and predation risk are more typically emphasized. We examined the association between predation risk and both group size and foraging behaviour in the chestnut-crowned babbler, Pomatostomus ruficeps, a family-living cooperatively breeding bird that does not appear to face classic ecological or demographic constraints on dispersal and breeding. Groups were more likely to encounter, and be attacked by, avian predators when dependent young were present. Large groups were also more likely to encounter a predator, but less likely to be attacked by it, consistent with a benefit of group living through early predator detection or confusion effects. In addition, the average risk of predation for a given individual was reduced in large groups compared to small ones, owing to the dilution effect. That predation might partly select for group living in this species is boosted by findings showing reductions in ground foraging and increases in sentinel behaviour when predation risk was higher. We conclude that predation might represent an important force selecting for sociality in chestnut-crowned babblers, and highlight the need for future studies to consider more explicitly inherent benefits to group living in the evolution of vertebrate cooperative breeding systems.

Ecological conditions influence sentinel decisions

Ecological conditions can influence decisions relating to antipredator behaviour through impacts on the likelihood of detecting predators and the ability to hear vocalizations. Previous studies of antipredator behaviour have tended to focus on foragers, whose vigilance behaviour may be confounded by the type of food they are eating, and on receivers in vocal communication networks. We examined the impact of habitat and wind conditions on the behaviour of sentinels, individuals that suspend their own foraging to adopt a raised position to scan for danger while groupmates continue feeding, and that produce a variety of calls used by foragers to adjust their antipredator behaviour. Sentinels of the pied babbler, Turdoides bicolor, a cooperatively breeding bird, started guarding sooner and guarded for longer in long grass compared to more open habitats, and also initiated sentinel bouts sooner in high wind, probably because of the increased predation risk in such circumstances. Sentinels also selected positions that were both lower and closer to the foraging group when it was windy, potentially improving transmission of vocal signals that are valuable to foragers. Our results demonstrate that sentinel behaviour can be influenced by extrinsic factors, as well as the intrinsic factors previously shown, and suggest that ecological variation may affect decisions bearing both selfish and cooperative benefits. 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Calling by Concluding Sentinels: Coordinating Cooperation or Revealing Risk?

Efficient cooperation requires effective coordination of individual contributions to the cooperative behaviour. Most social birds and mammals involved in cooperation produce a range of vocalisations, which may be important in regulating both individual contributions and the combined group effort. Here we investigate the role of a specific call in regulating cooperative sentinel behaviour in pied babblers (Turdoides bicolor). ‘Fast-rate chuck’ calls are often given by sentinels as they finish guard bouts and may potentially coordinate the rotation of individuals as sentinels, minimising time without a sentinel, or may signal the presence or absence of predators, regulating the onset of the subsequent sentinel bout. We ask (i) when fast-rate chuck calls are given and (ii) what effect they have on the interval between sentinel bouts. Contrary to expectation, we find little evidence that these calls are involved in regulating the pied babbler sentinel system: observations revealed that their utterance is influenced only marginally by wind conditions and not at all by habitat, while observations and experimental playback showed that the giving of these calls has no effect on inter-bout interval. We conclude that pied babblers do not seem to call at the end of a sentinel bout to maximise the efficiency of this cooperative act, but may use vocalisations at this stage to influence more individually driven behaviours.

Bargaining babblers: vocal negotiation of cooperative behaviour in a social bird

Wherever individuals perform cooperative behaviours, each should be selected to adjust their own current contributions in relation to the likely future contributions of their collaborators. Here, we use the sentinel system of pied babblers (Turdoides bicolor) to show that individuals anticipate contributions by group mates, adjusting their own contribution in response to information about internal state broadcast by others. Specifically, we show that (i) short-term changes in state influence contributions to a cooperative behaviour, (ii) individuals communicate short-term changes in state, and (iii) individuals use information about the state of group mates to adjust their own investment in sentinel behaviour. Our results demonstrate that individual decisions about contributions to a cooperative effort can be influenced by information about the likely future contribution of others. We suggest that similar pre-emptive adjustments based on information obtained from collaborators will be a common feature of cooperative behaviour, and may play an important role in the development of complex communication in social species.

Experimental evidence that sentinel behaviour is affected by risk

Sentinels are a conspicuous feature of some cooperative societies and are often assumed to provide benefits in terms of increased predator detection. Similar to other cooperative behaviours, variation in investment in sentinel behaviour should reflect variation in the benefits of such behaviour. However, evidence for this is inconclusive: to date experiments have manipulated the cost of sentinel behaviour, and considerations of changes in the benefits of sentinel activity on investment patterns are lacking. Here, we experimentally manipulated the benefits of sentinel behaviour in the cooperatively breeding pied babbler (Turdoides bicolor) to assess whether this had any impact on sentinel activity. We simulated the presence of an unseen predator using playbacks of heterospecific alarm calls, and the presence of an actual predator using a model snake. In both cases, the increase in perceived predation risk caused an increase in sentinel activity, demonstrating that investment in sentinel activity increases when the benefits are greater.

Male Red-winged Blackbirds with experimentally dulled epaulets experience no disadvantage in sexual selection

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The presence of an avian co-forager reduces vigilance in a cooperative mammal

Many animals must trade-off anti-predator vigilance with other behaviours. Some species facilitate predator detection by joining mixed-species foraging parties and 'eavesdropping' on the predator warnings given by other taxa. Such use of heterospecific warnings presumably reduces the likelihood of predation, but it is unclear whether it also provides wider benefits, by allowing individuals to reduce their own vigilance. We examine whether the presence of an avian co-forager, the fork-tailed drongo (Dicrurus adsimilis), affects rates of vigilance (including sentinel behaviour) in wild dwarf mongooses (Helogale parvula). We simulate the presence of drongos-using playbacks of their non-alarm vocalizations-to show that dwarf mongooses significantly reduce their rate of vigilance when foraging with this species. This is, to our knowledge, the first study to demonstrate experimentally that a mammal reduces vigilance in the presence of an avian co-forager.

Cooperative Sentinel Calling? Foragers Gain Increased Biomass Intake

Many foraging animals face a fundamental tradeoff between predation and starvation. In a range of social species, this tradeoff has probably driven the evolution of sentinel behavior, where individuals adopt prominent positions to watch for predators while groupmates forage. Although there has been much debate about whether acting as a sentinel is a selfish or cooperative behavior, far less attention has focused on why sentinels often produce quiet vocalizations (hereafter known as "sentinel calls") to announce their presence. We use observational and experimental data to provide the first evidence that group members gain an increase in foraging success by responding to these vocal cues given by sentinels. Foraging pied babblers (Turdoides bicolor) spread out more, use more exposed patches, look up less often, and spend less time vigilant in response to sentinel calling. Crucially, we demonstrate that these behavioral alterations lead to an increase in biomass intake by foragers, which is likely to enhance survival. We argue that this benefit may be the reason for sentinel calling, making it a truly cooperative behavior.

Do Conversational Gutturals Help Florida Scrub‐Jays Coordinate Their Sentinel Behavior?

AbstractFlorida scrub‐jays, Aphelocoma coerulescens, perform sentinel behavior in which individuals alternate bouts of watchfulness with little overlap. We examined how calls might facilitate sentinel coordination. Small soft calls labeled conversational gutturals were heard more often from sentinels than from foraging birds. Calls occurred infrequently throughout sentinel bouts but were more common later in bouts. The pattern of calling does not match predictions for a ‘watchman’s song’ at regular intervals nor for a signal at the end of a sentinel bout. Thus, our quantitative assessment of calling by sentinels did not find support for either standard hypothesis. Although Florida scrub‐jays clearly have information on each other’s sentinel behavior, our results suggest that calls provide perhaps a fraction of this information.

霊長類と他の動物の混群現象についての研究の現状

I reviewed the studies on polyspecific associations between primates and non-primate animals to appreciate the present situation about this topic. I collected 104 case studies from 50 publications. The partner animals included 19 mammal species, 33-34 bird species, 1 reptile species, and 1-2 insect species, respectively. Among them, associations with non-primate mammals (n=43) and birds (n=59) occupied most of the reported cases. Many polyspecific associations between primates and other mammal species were reported from Asia and Africa, while those between primates and bird species were reported from Central and South America. 34 primate species worldwide formed associations with non-primate animals.Polyspecific associations between primates and other animals included 1) that primates (or other animals) provide food resources to their partner animals and 2) that primates (or other animals) provide anti-predator behavior (e.g., sentinel behavior, loud call, and alarm call). The former association increases foraging benefits to the partner animals, and the latter association decreases predation risk of the partner animals. Most studies, however, pointed out that benefit from given associations are commensalisic, (or even parasitism), i. e. biased to non-primate species.Most of these studies are anecdotal, and do not show quantitative data for the frequency of the association or the benefit of partner animals from participating in the associations. Thus, this interesting phenomenon has not been discussed from an ecological perspective. I recommend several approaches for how to overcome these present challenges.

Florida Scrub‐Jays Compensate for the Sentinel Behavior of Flockmates

AbstractSentinel coordination requires that individuals react to the sentinel behavior of others. Previous work showed that Florida scrub‐jays are sentinels more often when given supplemental food. Here we measured how birds in pairs reacted when their mates were fed. Scrub‐jays were sentinels less when their mates were fed, demonstrating compensation. Indirect evidence suggests that this compensatory decrease in sentinel behavior was smaller than the increase in sentinel behavior by their mates. In addition, males in newly established groups were sentinels less often.

Florida Scrub‐Jays (Aphelocoma coerulescens) are Sentinels More When Well‐Fed (Even with no Kin Nearby)

AbstractSentinels occupy high, exposed positions while other group members forage nearby. If sentinel behavior involves a foraging–predation risk trade‐off, animals should be sentinels more when fed supplemental food. When individual Florida scrub‐jays (Aphelocoma coerulescens) were fed fragments of peanuts, during the following 30 min they shifted 30% of their time from foraging to sentinel behavior. In a follow‐up experiment, we fed either one or two members in each group. As before, the jays reduced their foraging and spent much more time as sentinels when given supplemental food. In each treatment, pairs were sentinels simultaneously considerably less often than expected by chance. The dramatic shift from foraging to sentinel behavior suggests that for Florida scrub‐jays sentinel behavior brings substantial benefits for no greater cost than that of lost opportunities to forage. Because the results held for simple mated pairs of scrub‐jays, we argue that kin selection and social prestige are not necessary to explain sentinel behavior.

Safe selfish sentinels in a cooperative bird

SummarySentinel behaviour involves a unique cooperative system of dedicated look‐outs which protect members of their social group from attack by predators. Using detailed observations from groups of Arabian babblers,Turdoides squamiceps,we tested the original theoretical suggestion that cooperative sentinel systems are simply the result of individually selfish state‐dependent patterns of behaviour.Sentinel effort and the number of sentinel bouts per individual per hour were greater for males than females, and both increased with individual dominance status within the group.Sentinel behaviour was unaffected by group social structure, in terms of patterns of relatedness and the number of potential breeders.Total group sentinel effort increased with group size, while the effort per individual decreased irrespective of sex and dominance rank. Simultaneous sentinel behaviour by two or more birds was very rare, but increased with group size.Sentinel effort followed seasonal fluctuations in food availability, but not peaks of raptor migration through the study site.Body mass was greater for males than females and was positively related to dominance rank. Overall, body mass explained much of the variation in individual sentinel effort both within and between birds. However, data on individual foraging performance could not be related to changes in body mass and thus to differences in sentinel effort.Sentinel behaviour in groups of Arabian babblers therefore confirms many of the predictions arising from state‐dependent models of cooperative, yet individually selfish, sentinel behaviour.