Number of neighbors instead of group size significantly affects individual vigilance levels in large animal aggregations

The group size effect states that animals living in groups gain anti‐predator benefits through reducing vigilance levels as group size increases. A basic assumption of group size effect is that all individuals are equally important for a focal individual, who may adjust its vigilance levels according to social information acquired from them. However, some studies have indicated that neighbors pose greater influences on an individual's vigilance decisions than other group members, especially in large aggregations. Vigilance has also been found to be directed to both predators (anti‐predation vigilance) and conspecifics (social vigilance). Central individuals might rely more on social vigilance than peripheral individuals. To test these hypotheses, we examined the effects of flock size, number of neighbors and position within a flock on vigilance and competition of greater white‐fronted goose Anser albifrons that form large foraging flocks in winter, controlling the effects of other variables (group identity, winter period and site). We found that individual vigilance levels were significantly affected by number of neighbors and position within a flock, whereas flock size showed no effect. Individuals devoted a large component of vigilance to nearby flock mates. Central individuals directed a relatively larger proportion of vigilance to monitor neighbors than peripheral ones, indicating that central individuals more relied on social information acquired from neighbors, possibly caused by the more blocked visual field of central individuals. Moreover, some social vigilance may function as conducting or preventing agonistic interactions since competition intensity was positively correlated with number of neighbors. Our study therefore demonstrate that the number of neighbors is more important than group size in determining individual vigilance in large animal groups. Further studies are still needed to unravel which neighbors pose greater influence on individual vigilance, and the factors that influence individuals to acquire information from their neighbors to adjust vigilance behaviors.     

Effect of Group Size on Individual and Collective Vigilance in Greater Rheas

We studied the effect of group size on the proportion of time that greater rheas, Rhea americana, allocated to vigilance and feeding during the non‐breeding season. We tested whether: (1) the proportion of time that one bird allocates to vigilance (individual vigilance) decreases with group size, and (2) the proportion of time that at least one bird of the group is vigilant (collective vigilance) increases with group size. We analyzed video‐recordings of birds that were foraging alone or in groups from two to 12 birds. The proportion of time allocated to individual vigilance decreased and the proportion of time spent feeding increased with group size. In both cases the main significant difference was between birds foraging alone and in groups. Collective vigilance did not vary with group size and it was lower than expected if vigilance bouts were random or sequential. Our results indicate that rheas foraging in large groups would not receive the benefit of an increase in collective vigilance, although they could still benefit from a reduction of predation risk by the dilution effect.     

Vigilance and grouping in the southern African ground squirrel (Xerus inauris)

Animals may form groups in response to the foraging–vigilance trade‐off, through enhanced predator detection (collective detection hypothesis) or reduced predation risk to the individual (dilution hypothesis), allowing individuals to decrease vigilance levels. Both hypotheses predict decreasing individual vigilance levels with increasing group size; however, the collective detection hypothesis also predicts increasing overall group vigilance with increasing group size. However, in species in which vigilance and foraging are not mutually exclusive, where vigilance may not be as costly, neither of these hypotheses may apply. Here, we examine the relationship between group size and vigilance in the social Cape ground squirrel (Xerus inauris), a species that can combine foraging and vigilance behaviours. Ten groups were observed using scan sampling, measuring both group and individual vigilance and group size. A negative relationship existed between individual vigilance and group size and a positive relationship between group vigilance and group size. Therefore, in Cape ground squirrels, vigilance seems to be costly even though it can be combined with foraging behaviours. Furthermore, group vigilance behaviour gives support to the collective detection hypothesis, whilst individual vigilance gives support to both hypotheses.     

To scan or not to scan? Occurrence of the group‐size effect in a seasonally nongregarious forager

Group‐living requires a compromise between safety and direct/indirect costs for individuals. The larger is the group, the greater is the collective vigilance, leading to a greater net food intake per forager because of the time saved individually from scanning behaviour. In turn, individual alertness usually decreases with increasing group size (“group‐size effect”). Information on the occurrence of group‐size effect is still unclear. Previous studies have shown that it may fail to occur or even reverse, for example when costs of interference between conspecifics are high. In turn, assessing whether the group‐size effect would occur in weakly or seasonally gregarious species may help to understand its drivers. We evaluated the occurrence and the extent of group‐size effect in a seasonally nongregarious herbivore, the roe deer Capreolus capreolus. We examined the roles of sex/age class and season as drivers of vigilance behaviour. In roe deer, the group‐size effect did not depend on sex/age class: time spent foraging increased with increasing group size; in turn, vigilance increased with decreasing group size, in all sex/age classes. Females with fawns were the most vigilant sex/age class, thus revealing the cost of offspring protection. Accordingly, the higher spring vigilance levels of does could be related to reproductive costs (e.g., defence of newborn fawns). Conversely, the greater summer vigilance of bucks could result from patrolling/defence of territories. Both adult males and females also showed the higher vigilance in winter, likely because of an increase in the perception of predation risk and/or, possibly, hormones linked to an increase in intolerance of conspecifics, in males. However, the group‐size effect occurred in all the seasons, for adult males and females. Our findings suggest that foraging benefits provided by the group‐size effect may have overcome costs of group‐living, even in a weakly gregarious forager.     

Inter‐ and intraspecific effects of body size on habitat use among sexually‐dimorphic macropodids

Body size affects key life‐history parameters including dietary requirements and predation risk. We examined these effects on diel habitat use in a community of three sexually‐dimorphic macropodid marsupial species: western grey kangaroo Macropus fuliginosus, red‐necked wallaby M. rufogriseus and swamp wallaby Wallabia bicolor. In particular, our study seeks evidence of these effects operating concurrently at the intra‐ and interspecific levels. We used radio‐tracking to quantify habitat use and characterised each used location by recording the cover of plant functional groups and the presence of plant species. During nocturnal foraging periods we predicted that smaller animals (between and within species) should use habitats with higher‐quality forage, which is often less abundant, than larger animals, as metabolic demand scales with body size. During diurnal resting periods we predicted that smaller animals (between and within species), being more vulnerable to predation, should use greater concealment cover than larger animals. Western grey kangaroos and swamp wallabies behaved as predicted during foraging periods, but red‐necked wallabies did not, using more open, poorer‐quality habitats than expected. Only western grey kangaroos showed a within‐species effect on habitat use: the relatively smaller females foraged in higher‐quality patches. Habitats used by animals during the resting period generally offered greater concealment cover than those used during the foraging period, but there were no clear body size effects on the density of vegetation used. In our system, body size alone could not explain all of the observed patterns, suggesting that there may also be individual differences in habitat requirements influenced by factors such as reproductive costs, predation risk and social facilitation.     

集群数量和采食距离对储草期布氏田鼠警戒频次的影响

警戒行为是动物对环境中潜在危险做出的反应。为探究影响警戒行为的因素,作者在内蒙古锡林郭勒典型草原区进行了集群数量和采食距离对储草期自由生活状态下布氏田鼠(Lasiopodomys brandtii)警戒频次影响的实验。通过标志重捕法和人工去除法设定3个集群数量梯度(11只、6只和3只),利用人工食物站设置4个采食距离梯度(5 m、10 m、20 m、30 m),共12个梯度组合,每个组合分别观察60次采食过程,共计观察720次,并记录采食过程中的警戒频次。对数据进行双因素方差分析,集群规模和采食距离对布氏田鼠警戒行为频次均有极显著影响(P0.01)。随着集群数量减小,布氏田鼠单次采食的警戒频次显著增加;采食距离越远,单次采食的警戒频次越高;集群数量和采食距离之间还存在极显著的交互作用(P0.01)。集群数量增大意味着采食距离的增加,为保证储草效率,布氏田鼠种群会在增加集群数量和减小采食距离之间权衡,最终集群数量维持在中等水平,支持了最优集群理论。     

Vigilance and Collective Detection of Predators in Degus (Octodon degus)

Individuals of social and partially social species typically reduce their vigilance activity when foraging in groups. As a result, per capita risk of predation decreases and individuals allocate more time to foraging and other fitness rewarding activities. Reduction of per capita risk is hypothesized to occur because there are more individuals to detect potential predators. If so, collective (i.e. total) vigilance is expected to increase with foraging group size. Increased surveillance during group foraging may occur if group members scan independently of one another, or sequentially to avoid the overlapping of their vigilance bouts. Intriguingly, such coordinated vigilance assumes that individuals monitor not only the presence, but the vigilance behaviour of group mates. We used seasonal records on time budget and grouping patterns of individually marked degus (Octodon degus), a social rodent, to examine if (a) individual vigilance decreases and/or foraging increases with group size, (b) collective vigilance increases with group size and (c) foraging degus coordinate their vigilance. When foraging, degus decreased their individual vigilance and increased their foraging time when in larger groups. Despite this, degus in larger groups increased their collective vigilance, supporting the hypothesis that socially foraging degus decrease predation risk through an improved ability to detect and escape potential predators. Additionally, patterns of collective vigilance suggested that degus scan independently of each other and so, they do not coordinate their vigilance to prevent its temporal overlapping. This finding does not support that foraging degus monitor the vigilance activity of group mates.     

Costs and Benefits of Sociality Differ Between Female Guanacos Living in Contrasting Ecological Conditions

According to current theory, anti‐predator benefits promote group formation in open‐dwelling ungulates. An inverse relationship between vigilance effort and group size has been documented frequently and thought to reflect the consequent decrease in perceived predation risk as group size increases. In contrast, competition costs are supposed to set the upper limit to the number of individuals that can forage together. As anti‐predator behavior is no longer functional in the absence of predation and competition costs might be affected by resource distribution, the net benefit of aggregation will depend on the particular combination of predation risk and habitat structure experienced by the individual. To test this hypothesis, group‐size effects on female time allocation and within‐group aggression rate were compared between two guanaco populations exposed to contrasting levels of puma predation. Habitat structure within both sites consisted of mosaics of shrublands and grasslands, and group‐size effects were also compared between these habitat types. Females under predation risk showed a strong reduction in vigilance as the number of adults in the group increased, whereas females from the predator‐free population showed overall low levels of vigilance, regardless of group size. These results emphasize the anti‐predator significance of the group‐size effect on female vigilance, as well as guanaco plasticity to adjust time allocation to local conditions. On the other hand, within‐group aggression rate increased with the number of adults in the group. Aggression rate was almost null within groups located in grasslands but was significantly higher in shrublands, regardless of predation risk, suggesting that the more heterogeneous distribution of shrubs increases the interference competition level. These results strengthen the notion of predation pressure and habitat structure as major determinants of the balance between costs and benefits of group living, and highlight the potential of individual behavioral patterns to make qualitative predictions about group‐size variation within territorial ungulates.     

The effect of group size on vigilance in Ruddy Turnstones Arenaria interpres varies with foraging habitat

Foraging birds can manage time spent vigilant for predators by forming groups of various sizes. However, group size alone will not always reliably determine the optimal level of vigilance. For example, variation in predation risk or food quality between patches may also be influential. In a field setting, we assessed how simultaneous variation in predation risk and intake rate affects the relationship between vigilance and group size in foraging Ruddy Turnstones Arenaria interpres. We compared vigilance, measured as the number of ‘head‐ups’ per unit time, in habitat types that differed greatly in prey energy content and proximity to cover from which predators could launch surprise attacks. Habitats closer to predator cover provided foragers with much higher potential net energy intake rates than habitats further from cover. Foragers formed larger and denser flocks on habitats closer to cover. Individual vigilance of foragers in all habitats declined with increasing flock size and increased with flock density. However, vigilance by foragers on habitats closer to cover was always higher for a given flock size than vigilance by foragers on habitats further from cover, and habitat remained an important predictor of vigilance in models including a range of potential confounding variables. Our results suggest that foraging Ruddy Turnstones can simultaneously assess information on group size and the general likelihood of predator attack when determining their vigilance contribution.     

相关风险因子对高原鼠兔摄食行为的影响

研究了捕食风险环境中集和洞口距离对高原鼠兔摄食行为的影响。结果表明,集群数量的增加不仅降低了警觉行为,同时也减少了摄食行为,在高风险环境中,集群为1时的取食行为强度最大,低风险环境中,为0时最大,警觉行为主要出现在距洞口2m的范围内,其行为强度与洞口踪影职责负相关,当洞口距离大于3m时,风险处理区的高原鼠兔几乎无警觉行为出现,且该处理区的取食区域几乎压缩的洞口旁,研究结果表明,在捕食风险环境中,高原鼠兔摄食行为与集群和洞口距离之间具有复杂的关系,其行为决策反映了降低风险与摄取食物间的权衡,行为目标是在降低捕食风险的同时尽可能地取食食物。     

Effects of position and flock size on vigilance and foraging behaviour of the scaled dove Columbina squammata

Amongst the benefits of foraging in flocks are the enhancement of food finding and predation avoidance. Characteristics such as size, individual position, as well as position and distance between members are factors that may influence vigilance and foraging. In a study using scaled doves, Columbina squammata, I observed a negative correlation between group size and vigilance and a positive correlation with time spent foraging, which suggests a reduction of costs and an increase of benefits as a consequence of larger group sizes. Individual position in the flock appeared to be an important factor in this trade-off. Peripheral individuals were more vigilant and foraged less than central ones, suggesting an edge effect in flocks of this species. The clustering of conspecifics may be related with fast transmission of information. Overall, aggressive interactions were rarely observed; when registered, they occurred mostly in larger groups, suggesting an effect of interference competition. These results imply that predation may be a strong pressure on the scaled dove's flock formation and behaviour.     

The effect of predation pressure and predator adaptive foraging on the relative importance of consumptive and non‐consumptive predator net effects in a freshwater model system

An important challenge in community ecology is identifying the functional characteristics capable of predicting the nature and strength of predator effects on food webs. We developed an individual‐based model, based on a shallow lake model system, to evaluate the total, consumptive, and non‐consumptive indirect effect that predators have on basal resources when the predators differ in their foraging types (active adaptive foraging or sedentary foraging). Overall, both predator types caused similar total indirect effects on lower trophic levels. However, the nature net effects of predators diverged between predator foraging types. Active predators caused larger non‐consumptive effects, relative to the total indirect effect, irrespective of predation pressure levels. On the other hand, sedentary predators caused larger non‐consumptive effects for lower predation pressure levels, but consumptive effects became more important as predation pressure increased. Our simulations showed that the reliance on a particular mechanism driving consumer–resource interactions is altered by predator foraging behavior and highlight the importance of both prey and predator foraging behaviors to predict the causes and consequences of cascading effects observed in food webs.     

An Experimental Analysis of Spatial Position Effects on Foraging and Vigilance in Brown-Headed Cowbird flocks

Several observational studies have found that the costs and benefits of social foraging vary as a function of spatial position in the group. However, it is difficult to make mechanistic inferences because several confounding factors, such as food deprivation levels, food availability, neighbor distance, and group size can mask or amplify spatial position effects. We attempted to address experimentally the effect of spatial position on foraging and vigilance in a group, controlling for many confounding factors. We used enclosures that restricted physical but not visual interactions between brown-headed cowbirds and manipulated spatial position, flock size, and neighbor distance. Pecking rate (number of pecks per trial duration) was not related with position, but instantaneous pecking rate (number of pecks per foraging bout duration) was higher at the edge. The proportion of time spent head-up (scanning and food-handling) was also higher at the edge. For pecking rate and proportion of time spent scanning, changes in neighbor distance influenced the behavior of edge birds to a lesser extent than central birds. These results suggest that cowbirds at the edge perceived greater predation risk and that during the limited foraging time available, edge birds tried to compensate by foraging at a faster rate.     

A comparative analysis of vigilance in birds

Individual variation in vigilance is known to vary with factors such as group size but the ecological determinants of vigilance among species have not been examined thus far in a systematic fashion. Earlier analyses suggested that vigilance should be lower in larger species and in species living in larger groups. These analyses were based on a small number of species and failed to take into account phylogenetic relationships among species. Here, I examined ecological determinants of vigilance in a large sample of bird species using a phylogenetic framework. I focused on vigilance in foraging groups of birds in the non-breeding season. Among species, vigilance by solitary foragers was not influenced by body mass. However, among species, asymptotic vigilance, the plateau reached by vigilance in larger groups, decreased with increasing group size in vegetarian clades but not in carnivorous clades. Asymptotic vigilance also increased with increasing body mass in vegetarian clades but not in carnivorous clades. Increasing group size may allow species to reduce vigilance in response to decreased predation risk. Increasing body mass may allow species to increase vigilance because more non-foraging time is available in larger species. Diet may modulate the effect of body mass and group size through factors such as within-group vigilance or foraging techniques.     

Detecting predators and locating competitors while foraging: an experimental study of a medium-sized herbivore in an African savanna

Vigilance allows individuals to escape from predators, but it also reduces time for other activities which determine fitness, in particular resource acquisition. The principles determining how prey trade time between the detection of predators and food acquisition are not fully understood, particularly in herbivores because of many potential confounding factors (such as group size), and the ability of these animals to be vigilant while handling food. We designed a fertilization experiment to manipulate the quality of resources, and compared awareness (distinguishing apprehensive foraging and vigilance) of wild impalas (Aepyceros melampus) foraging on patches of different grass height and quality in a wilderness area with a full community of predators. While handling food, these animals can allocate time to other functions. The impalas were aware of their environment less often when on good food patches and when the grass was short. The animals spent more time in apprehensive foraging when grass was tall, and no other variable affected apprehensive behavior. The probability of exhibiting a vigilance posture decreased with group size. The interaction between grass height and patch enrichment also affected the time spent in vigilance, suggesting that resource quality was the main driver when visibility is good, and the risk of predation the main driver when the risk is high. We discuss various possible mechanisms underlying the perception of predation risk: foraging strategy, opportunities for scrounging, and inter-individual interference. Overall, this experiment shows that improving patch quality modifies the trade-off between vigilance and foraging in favor of feeding, but vigilance remains ultimately driven by the visibility of predators by foragers within their feeding patches.     

Stress During Adolescence Shapes Performance in Adulthood: Context‐Dependent Effects on Foraging and Vigilance

Exposure to chronic stress during adolescence can shape behaviour, cognition and physiology in adulthood, but the consequences of these long‐term changes remain unclear. Prior studies reporting altered performance following exposure to stress in adolescence have generally interpreted lasting changes as impairments. However, we have recently shown that exposure to chronic unpredictable stress during adolescence (from post‐natal days 30–70) can enhance performance in a context‐dependent manner during a foraging task. Increases in foraging performance, (previously measured by the number of rewards obtained), are often associated with trade‐offs in other behaviours, such as vigilance. Here, we examined the effect of stress exposure in adolescence on adult foraging in male Sprague Dawley rats to determine (1) whether the increase in foraging performance exhibited by animals exposed to stress in adolescence is balanced by a decrease in vigilance, and (2) whether stress in adolescence alters time allocation between foraging and vigilance behaviours in low‐ and high‐threat conditions. We found no evidence of a trade‐off between foraging and vigilance; under low‐threat conditions, rats exposed to stress in adolescence spent more time being vigilant compared with unstressed rats, suggesting that exposure to stress in adolescence enhances anticipation of threat in adulthood. Under high‐threat conditions, adolescent‐stressed and unstressed rats did not differ in foraging and vigilance behaviours. Given that we have previously found that rats exposed to stress in adolescence nearly double food intake under high‐threat, and we now show that high‐performing rats do not spend more time foraging, it appears that stress exposure in adolescence may enhance foraging efficiency (food consumed/time) under high‐threat conditions rather than time allocation between foraging and competing behaviours. We also examined the relationship, at the level of the individual, between foraging performance and foraging and vigilance behaviours. We found that changes in individual foraging performance between low‐ and high‐threat conditions were independent of behavioural changes (i.e. both highly and poorly performing rats were equally active and contacted a similar number of patches). This suggests that the ability to obtain many rewards under high‐threat conditions may be related to efficiency, rather than the frequency of foraging and effort‐related behaviours.     

Visual obstruction and vigilance: a natural experiment

Visual obstructions can cause an increase in antipredator vigilance in prey animals by making predator detection more difficult. However, visual obstructions can also skew the perception of group size and inter‐individual distances and impair the detection of alarm signals by conspecifics. These changes within the group alone can cause an increase in vigilance. To disentangle the contribution of these various factors to changes in vigilance, I documented vigilance in a gregarious species, the semipalmated sandpiper Calidris pusilla, foraging in a habitat where a naturally‐occurring visual barrier partially prevented predator detection without altering the transfer of information about predation risk within the group. I used a matched sampling design to collect vigilance data for birds using adjacent areas with and without the visual barrier. In the visually‐obstructed area, sandpipers maintained a higher level of vigilance, occurred farther away from cover and in smaller flocks, and preferentially scanned the area of danger with one eye in particular. All these changes suggest that visual obstruction increased perceived predation risk. I conclude that it is the inability to get a good view of any approaching predator, rather than changes in intra‐group communication that caused the increase in vigilance in the visually‐obstructed area.     

Prey size affects the costs and benefits of group predation in nymphs of the predatory stink bug Andrallus spinidens (Heteroptera: Pentatomidae)

Group predation promotes foraging efficiency because it increases the size of prey that can be killed and improves hunting success compared to solitary predation. However, group predation may increase competition among group members during feeding. Earlier studies have focused on the advantages of group predation, but little is known about the costs and benefits of group predation for individual members of the group. Here, we show that the costs and benefits of group predation for individuals of the predatory stink bug Andrallus spinidens vary with prey size in laboratory experiments. We found that when A. spinidens fed on small prey, group predation did not significantly increase foraging efficiency but did increase competition for food among group members. In contrast, when prey was large, group predation promoted foraging efficiency, and competition over food was not detected. Our results suggest that group predation by A. spinidens nymphs is advantageous for individual members because it enables each member to hunt larger prey that could not be hunted alone. However, when group size was large or prey size was small, group predation increased competition among group members.     

Vigilance in Przewalski's gazelle: effects of sex, predation risk and group size

Vigilance in social animals is often aimed at detecting predators. Many social and environmental factors influence vigilance, including sex, predation risk and group size. During the summer of 2007, we studied Przewalski's gazelle Procapra przewalskii , an endemic ungulate to the Qinghai-Tibet Plateau, to test whether and how these three factors affect vigilance. We distinguished groups consisting of males, mothers with lambs and females without lambs making observations on groups in the presence or absence of nearby predators. We assessed the group-size effect on vigilance and how this varied with levels of predation risk and sex. Males and mothers scanned longer and with a higher frequency than females without lambs. Individuals were more vigilant under direct predation threat. Although vigilance generally decreased with group size, the extent of the decrease was independent of predation risk and was not significant in males. The results suggest that mothers are more vigilant suggesting greater vulnerability and that males may have increased their vigilance to compete for higher social ranks. The positive correlation between vigilance and predation risk and the negative correlation between vigilance and group size are consistent with earlier findings, but we failed to find an interaction between group size and predation risk on vigilance perhaps because vigilance levels are low even in small groups, thus making similar vigilant upward adjustments in both small and large groups.     

Group size effects on foraging and vigilance in migratory Tibetan antelope

Large group sizes have been hypothesized to decrease predation risk and increase food competition. We investigated group size effects on vigilance and foraging behaviour during the migratory period in female Tibetan antelope Pantholops hodgsoni, in the Kekexili Nature Reserve of Qinghai Province, China. During June to August, adult female antelope and yearling females gather in large migratory groups and cross the Qinghai-Tibet highway to calving grounds within the Nature Reserve and return to Qumalai county after calving. Large groups of antelope aggregate in the migratory corridor where they compete for limited food resources and attract the attention of mammalian and avian predators and scavengers. We restricted our sampling to groups of less than 30 antelopes and thus limit our inference accordingly. Focal-animal sampling was used to record the behaviour of the free-ranging antelope except for those with lambs. Tibetan antelope spent more time foraging in larger groups but frequency of foraging bouts was not affected by group size. Conversely, the time spent vigilant and frequency of vigilance bouts decreased with increased group size. We suggest that these results are best explained by competition for food and risk of predation.