Ecological conditions can influence decisions relating to antipredator behaviour through impacts ... more 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.► Ecology affects sentinel decisions. ► Dense habitats and high wind shortens bout latency. ► Densely vegetated habitats increase the time a sentinel stays on guard. ► Sentinels choose perches according to wind condition to enhance signal transmission.
Performing correct anti-predator behaviour is crucial for prey to survive. But, are such abilitie... more Performing correct anti-predator behaviour is crucial for prey to survive. But, are such abilities lost in species or populations living in predator-free environments? How individuals respond to the loss of predators has been shown to depend on factors such as the degree to which anti-predator behaviour relies on experience, the type of cues evoking the behaviour, the cost of expressing the behaviour and the number of generations under which relaxed selection has taken place. Here we investigated whether captive-born populations of meerkats (Suricata suricatta) used the same repertoire of alarm calls previously documented in wild populations and whether captive animals, as wild ones, could recognize potential predators through olfactory cues. We found that all alarm calls that have been documented in the wild also occurred in captivity and were given in broadly similar contexts. Furthermore, without prior experience of odours from predators, captive meerkats seemed to distinguish between faeces of potential predators (carnivores) and non-predators (herbivores). Despite slight structural differences, the alarm calls given in response to the faeces largely resembled those recorded in similar contexts in the wild. These results from captive populations suggest that direct, physical interaction with predators is not necessary for meerkats to perform correct anti-predator behaviour in terms of alarm-call usage and olfactory predator recognition. Such behaviour may have been retained in captivity because relatively little experience seems necessary for correct performance in the wild and/or because of the recency of relaxed selection on these populations.
A great deal of variation is known to underlie the vocalizations of animals. Calls can for exampl... more A great deal of variation is known to underlie the vocalizations of animals. Calls can for example vary between individuals or between social and behavioural contexts. Calls also have the potential to vary between groups. Many group-living animals are known to produce stereotyped group-specific calls and such group signatures are thought to play a role in territory defence or indeed mate choice. Group signatures are generally found in long-distance call variants that work to maintain contact between group members, sometimes referred to as ‘contact calls’. Cooperatively breeding, territorial meerkats, Suricata suricatta, also use contact calls, potentially to maintain social organization during foraging. However, these contact calls are generally quieter than long-distance calls in other species, and better described as ‘close calls’. We investigated whether these similar call types also possess group-specific signatures and whether any such variation is used by receivers. We recorded close calls from 71 individuals belonging to 10 meerkat groups. We found that such close calls indeed possessed group signatures, but that this underlying variation did not appear to be used by receivers, possibly because meerkats use other sensory systems to identify nongroup members. We stress the importance of conducting playback experiments when investigating group-specific vocal signatures and use our results as a basis for predicting which animals may rely on group information encoded within close calls.
In all social species, information relevant to survival and reproduction can be obtained in two m... more In all social species, information relevant to survival and reproduction can be obtained in two main ways: through personal interaction with the environment (i.e. 'personal' information) and from the performance of others (i.e. 'public' information). While public information is less costly to obtain than personal information, it may be inappropriate or inaccurate. When deciding how much to rely on public information, individuals should therefore assess its potential quality, but this possibility requires empirical testing in animals. Here, we use the sentinel system of cooperatively breeding pied babblers (Turdoides bicolor) to investigate how behavioural decisions of foragers are influenced by potential variation in the quality of anti-predator information from a vigilant groupmate. When sentinels moved to a higher position, from where their probability of detecting predators is likely to be greater, foragers reduced their vigilance, spread out more widely and were more likely to venture into the open. Consequently, they spent more time foraging and increased their foraging efficiency, resulting in a profound increase in biomass intake rate. The opposite behavioural changes, and consequent foraging outcomes, were found when sentinels moved lower. A playback experiment demonstrated that foragers can use vocal cues alone to assess sentinel height. This is the first study to link explicitly a measure of the potential quality of public information with a fitness measure from those relying on the information, and our results emphasize that a full understanding of the evolution of communication in complex societies requires consideration of the reliability of information.
In many species, individuals suffer major mortality in their first year because of predation. Beh... more In many species, individuals suffer major mortality in their first year because of predation. Behaviours that facilitate successful escape are therefore under strong selection, but anti-predator skills often emerge gradually during an individual’s early development. Using long-term data and acoustic recordings of alarm calls collected during natural predator encounters, we aimed to elucidate two largely unsolved issues in anti-predator ontogeny: (1) whether incorrect predator assignment is adaptively age-appropriate, given that vulnerability often changes during development, or whether age-related differences reflect true mistakes made by immature individuals; and (2) the extent to which the development of adult-like competence in alarm-call production and usage is simply a function of maturational processes or dependent upon experience. We found that young meerkats (Suricata suricatta) were less likely to give alarm calls than adults, but alarmed more in response to non-threatening species compared to adults. However, stimuli that pose a greater threat to young than adults did not elicit more calling from young; this argues against age-related changes in vulnerability as the sole explanation for developmental changes in calling. Young in small groups, who were more likely to watch out for predators, alarmed more than less vigilant young in larger groups. Moreover, despite similarities in acoustic structure between alarm call types, calls appeared in the repertoire at different rates, and those that were associated with frequently encountered predators were produced relatively early on. These results indicate that experience is a more plausible explanation for such developmental trajectories than maturation.
alarm calling assessment-management call production call responses call usage development informa... more alarm calling assessment-management call production call responses call usage development information transfer learning maturation vocal communication Alarm calling is a widespread antipredator behaviour. Although the function and evolution of alarm call behaviour have long been studied in detail, only in the last decade has there been an upsurge in research into its development. Here, we review the literature on the development of alarm call production (the delivery of calls with a specific set of acoustic features), alarm call usage (the use of calls in particular contexts) and alarm call responses (the responses to calls produced by others). We detail the mechanistic processes that may underlie the development of each aspect, consider the selection pressures most likely to explain the relative importance of these processes, and discuss the substantial variation in developmental rates found both between and within species. Throughout, we interpret existing findings about age-related differences in alarm call behaviour from two major communicatory viewpoints: the idea that signals carry information from sender to receiver, with young taking time to acquire adult-like skills; and the possibility that signals are used to manage the behaviour of receivers, with young behaving adaptively for their age. We conclude that a broader use of various techniques (e.g. cross-fostering and temporary removals), the formation of stronger collaborative links with other disciplines (e.g. physiology and neurobiology) and the initiation of new research avenues (e.g. kleptoparasitism) will ensure that studies on the development of alarm call behaviour continue to enhance our understanding of such topics as the evolution of communication and language, kin selection and cognitive processing. Ó
Detection distance influences escape behaviour in two parids, Parus major and P. caeruleus. -J. A... more Detection distance influences escape behaviour in two parids, Parus major and P. caeruleus. -J. Avian Biol. 34: 233-236.
In contrast to historical assumptions about the affective nature of animal vocalizations, it is n... more In contrast to historical assumptions about the affective nature of animal vocalizations, it is now clear that many vertebrates are capable of producing specific alarm calls in response to different predators, calls that provide information that goes beyond the motivational state of a caller. However, although these calls function referentially, it does not mean that they are devoid of motivational content. Studies on meerkats (Suricata suricatta) directly support this conclusion. The acoustic structure of their alarm calls simultaneously encodes information that is both motivational (level of urgency) and referential (predator specific). In this study, we investigated whether alarm calls of young meerkats undergo developmental modification and whether the motivational or the referential aspect of calls changes more over time. We found that, based on their acoustic structure, calls of young showed a high correct assignment to low-and highurgency contexts but, in contrast to adults, low assignment to specific predator types. However, the discrimination among predator types was better in high-urgency than in low-urgency contexts. Our results suggest that acoustic features related to level of urgency are expressed earlier than those related to predator-specific information and may support the idea that referential calls evolve from motivational signals.
Given the strong selection on prey animals to escape predation, early development of correct avoi... more Given the strong selection on prey animals to escape predation, early development of correct avoidance strategies should be favoured. We studied the development of responses to conspecific alarm calls in a free-ranging population of meerkats in South Africa. Through behavioural observations of naturally occurring predator encounters and playback experiments, we monitored responses of young individuals from emergence (3 weeks) to 6 months of age and compared them with those of adults (>12 months). Although the total proportion of responses differing from those of adults was low during the observed period, the probability of responding like adults increased with age. Female young, who remained in closer contact to adults than did male young, were also more likely to show adultlike responses. The largest proportion of non-adultlike responses was shown before reaching independence at 3 months of age, and during this time young commonly ran immediately to a nearby individual when hearing an alarm call. After playbacks of alarm calls, young also reacted more slowly, resumed foraging sooner and spent less time vigilant than did adults. We conclude that young may need experience during early development to associate an alarm call correctly with the type of threat and appropriate response. Older group members may also serve as indirect models, perhaps helping young to form this association.
Many foraging animals face a fundamental tradeoff between predation and starvation . In a range o... more 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 .
Ecological conditions can influence decisions relating to antipredator behaviour through impacts ... more 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.► Ecology affects sentinel decisions. ► Dense habitats and high wind shortens bout latency. ► Densely vegetated habitats increase the time a sentinel stays on guard. ► Sentinels choose perches according to wind condition to enhance signal transmission.
Performing correct anti-predator behaviour is crucial for prey to survive. But, are such abilitie... more Performing correct anti-predator behaviour is crucial for prey to survive. But, are such abilities lost in species or populations living in predator-free environments? How individuals respond to the loss of predators has been shown to depend on factors such as the degree to which anti-predator behaviour relies on experience, the type of cues evoking the behaviour, the cost of expressing the behaviour and the number of generations under which relaxed selection has taken place. Here we investigated whether captive-born populations of meerkats (Suricata suricatta) used the same repertoire of alarm calls previously documented in wild populations and whether captive animals, as wild ones, could recognize potential predators through olfactory cues. We found that all alarm calls that have been documented in the wild also occurred in captivity and were given in broadly similar contexts. Furthermore, without prior experience of odours from predators, captive meerkats seemed to distinguish between faeces of potential predators (carnivores) and non-predators (herbivores). Despite slight structural differences, the alarm calls given in response to the faeces largely resembled those recorded in similar contexts in the wild. These results from captive populations suggest that direct, physical interaction with predators is not necessary for meerkats to perform correct anti-predator behaviour in terms of alarm-call usage and olfactory predator recognition. Such behaviour may have been retained in captivity because relatively little experience seems necessary for correct performance in the wild and/or because of the recency of relaxed selection on these populations.
A great deal of variation is known to underlie the vocalizations of animals. Calls can for exampl... more A great deal of variation is known to underlie the vocalizations of animals. Calls can for example vary between individuals or between social and behavioural contexts. Calls also have the potential to vary between groups. Many group-living animals are known to produce stereotyped group-specific calls and such group signatures are thought to play a role in territory defence or indeed mate choice. Group signatures are generally found in long-distance call variants that work to maintain contact between group members, sometimes referred to as ‘contact calls’. Cooperatively breeding, territorial meerkats, Suricata suricatta, also use contact calls, potentially to maintain social organization during foraging. However, these contact calls are generally quieter than long-distance calls in other species, and better described as ‘close calls’. We investigated whether these similar call types also possess group-specific signatures and whether any such variation is used by receivers. We recorded close calls from 71 individuals belonging to 10 meerkat groups. We found that such close calls indeed possessed group signatures, but that this underlying variation did not appear to be used by receivers, possibly because meerkats use other sensory systems to identify nongroup members. We stress the importance of conducting playback experiments when investigating group-specific vocal signatures and use our results as a basis for predicting which animals may rely on group information encoded within close calls.
In all social species, information relevant to survival and reproduction can be obtained in two m... more In all social species, information relevant to survival and reproduction can be obtained in two main ways: through personal interaction with the environment (i.e. 'personal' information) and from the performance of others (i.e. 'public' information). While public information is less costly to obtain than personal information, it may be inappropriate or inaccurate. When deciding how much to rely on public information, individuals should therefore assess its potential quality, but this possibility requires empirical testing in animals. Here, we use the sentinel system of cooperatively breeding pied babblers (Turdoides bicolor) to investigate how behavioural decisions of foragers are influenced by potential variation in the quality of anti-predator information from a vigilant groupmate. When sentinels moved to a higher position, from where their probability of detecting predators is likely to be greater, foragers reduced their vigilance, spread out more widely and were more likely to venture into the open. Consequently, they spent more time foraging and increased their foraging efficiency, resulting in a profound increase in biomass intake rate. The opposite behavioural changes, and consequent foraging outcomes, were found when sentinels moved lower. A playback experiment demonstrated that foragers can use vocal cues alone to assess sentinel height. This is the first study to link explicitly a measure of the potential quality of public information with a fitness measure from those relying on the information, and our results emphasize that a full understanding of the evolution of communication in complex societies requires consideration of the reliability of information.
In many species, individuals suffer major mortality in their first year because of predation. Beh... more In many species, individuals suffer major mortality in their first year because of predation. Behaviours that facilitate successful escape are therefore under strong selection, but anti-predator skills often emerge gradually during an individual’s early development. Using long-term data and acoustic recordings of alarm calls collected during natural predator encounters, we aimed to elucidate two largely unsolved issues in anti-predator ontogeny: (1) whether incorrect predator assignment is adaptively age-appropriate, given that vulnerability often changes during development, or whether age-related differences reflect true mistakes made by immature individuals; and (2) the extent to which the development of adult-like competence in alarm-call production and usage is simply a function of maturational processes or dependent upon experience. We found that young meerkats (Suricata suricatta) were less likely to give alarm calls than adults, but alarmed more in response to non-threatening species compared to adults. However, stimuli that pose a greater threat to young than adults did not elicit more calling from young; this argues against age-related changes in vulnerability as the sole explanation for developmental changes in calling. Young in small groups, who were more likely to watch out for predators, alarmed more than less vigilant young in larger groups. Moreover, despite similarities in acoustic structure between alarm call types, calls appeared in the repertoire at different rates, and those that were associated with frequently encountered predators were produced relatively early on. These results indicate that experience is a more plausible explanation for such developmental trajectories than maturation.
alarm calling assessment-management call production call responses call usage development informa... more alarm calling assessment-management call production call responses call usage development information transfer learning maturation vocal communication Alarm calling is a widespread antipredator behaviour. Although the function and evolution of alarm call behaviour have long been studied in detail, only in the last decade has there been an upsurge in research into its development. Here, we review the literature on the development of alarm call production (the delivery of calls with a specific set of acoustic features), alarm call usage (the use of calls in particular contexts) and alarm call responses (the responses to calls produced by others). We detail the mechanistic processes that may underlie the development of each aspect, consider the selection pressures most likely to explain the relative importance of these processes, and discuss the substantial variation in developmental rates found both between and within species. Throughout, we interpret existing findings about age-related differences in alarm call behaviour from two major communicatory viewpoints: the idea that signals carry information from sender to receiver, with young taking time to acquire adult-like skills; and the possibility that signals are used to manage the behaviour of receivers, with young behaving adaptively for their age. We conclude that a broader use of various techniques (e.g. cross-fostering and temporary removals), the formation of stronger collaborative links with other disciplines (e.g. physiology and neurobiology) and the initiation of new research avenues (e.g. kleptoparasitism) will ensure that studies on the development of alarm call behaviour continue to enhance our understanding of such topics as the evolution of communication and language, kin selection and cognitive processing. Ó
Detection distance influences escape behaviour in two parids, Parus major and P. caeruleus. -J. A... more Detection distance influences escape behaviour in two parids, Parus major and P. caeruleus. -J. Avian Biol. 34: 233-236.
In contrast to historical assumptions about the affective nature of animal vocalizations, it is n... more In contrast to historical assumptions about the affective nature of animal vocalizations, it is now clear that many vertebrates are capable of producing specific alarm calls in response to different predators, calls that provide information that goes beyond the motivational state of a caller. However, although these calls function referentially, it does not mean that they are devoid of motivational content. Studies on meerkats (Suricata suricatta) directly support this conclusion. The acoustic structure of their alarm calls simultaneously encodes information that is both motivational (level of urgency) and referential (predator specific). In this study, we investigated whether alarm calls of young meerkats undergo developmental modification and whether the motivational or the referential aspect of calls changes more over time. We found that, based on their acoustic structure, calls of young showed a high correct assignment to low-and highurgency contexts but, in contrast to adults, low assignment to specific predator types. However, the discrimination among predator types was better in high-urgency than in low-urgency contexts. Our results suggest that acoustic features related to level of urgency are expressed earlier than those related to predator-specific information and may support the idea that referential calls evolve from motivational signals.
Given the strong selection on prey animals to escape predation, early development of correct avoi... more Given the strong selection on prey animals to escape predation, early development of correct avoidance strategies should be favoured. We studied the development of responses to conspecific alarm calls in a free-ranging population of meerkats in South Africa. Through behavioural observations of naturally occurring predator encounters and playback experiments, we monitored responses of young individuals from emergence (3 weeks) to 6 months of age and compared them with those of adults (>12 months). Although the total proportion of responses differing from those of adults was low during the observed period, the probability of responding like adults increased with age. Female young, who remained in closer contact to adults than did male young, were also more likely to show adultlike responses. The largest proportion of non-adultlike responses was shown before reaching independence at 3 months of age, and during this time young commonly ran immediately to a nearby individual when hearing an alarm call. After playbacks of alarm calls, young also reacted more slowly, resumed foraging sooner and spent less time vigilant than did adults. We conclude that young may need experience during early development to associate an alarm call correctly with the type of threat and appropriate response. Older group members may also serve as indirect models, perhaps helping young to form this association.
Many foraging animals face a fundamental tradeoff between predation and starvation . In a range o... more 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 .
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Papers by Linda Hollén