Red kangaroos (<Emphasis Type="Italic">Macropus rufus</Emphasis>) receive an antipredator benefit from aggregation

For species that cannot seek cover to escape predators, aggregation becomes an important strategy to reduce predation risk. However, aggregation may not be entirely beneficial because aggregated animals may compete for access to limited resources and might even attract predators. Available evidence suggests that foraging competition influences time allocation in large-bodied macropodid marsupials, but previous studies have focused primarily on species in areas with protective cover. We studied red kangaroos, a species often found in open country without noticeable cover, to determine whether they experienced a net benefit by aggregation. Red kangaroos varied their time allocation as a function of group size and, importantly, more variation in time allocation to vigilance and foraging was explained by non-linear models than by linear models. This suggests red kangaroos directly translated the reduction of predation risk brought about by aggregation into greater time foraging and less time engaged in vigilance. We infer that red kangaroos received a net benefit by aggregation. Social species living in the open may be generally expected to rely on others to help manage predation risk. Communicated by K. Kotrschal     

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.     

Vigilance Patterns of Territorial Guanacos (Lama guanicoe): The Role of Reproductive Interests and Predation Risk

We conducted focal observations of territorial guanacos, a highly polygynous and social mammal, to compare time budgets between sexes and test the hypothesis that the differences in reproductive interests are associated with differential group size effects on male and female time allocation patterns. In addition, we used group instantaneous sampling to test the hypothesis that grouping improves detection capacity through increased collective vigilance. We fit GLM to assess how group size and group composition (i.e., presence or absence of calves) affected individual time allocation of males and females, and collective vigilance. As expected from differences in reproductive interests, males in family groups devoted more time to scan the surroundings and less to feeding activities compared to females. Both sexes benefited from grouping by reducing the time invested in vigilance and increased foraging effort, according to predation risk theory, but the factors affecting time allocation differed between males and females. Group size effects were significant when females were at less than five body‐lengths from their nearest neighbour, suggesting that grouping benefits arise when females are close to each other. Female time budgets were also affected by season, topography and vegetation structure. In contrast to our expectation, males reduced the time invested in vigilance as the number of females in the group increased, supporting the predation risk theory rather the intrasexual competition hypothesis. The presence of calves was associated with an increase in male individual vigilance; and vegetation type also affected the intensity of the group size effect over male time allocation. In closed habitats, collective vigilance increased with the number of adults but decreased with the number of calves present. Although male and female guanacos differed in their time allocation patterns, our results support the hypothesis that both sexes perceive significant antipredator benefits of group living.     

Foraging efficiency and vigilance behaviour of impala: the influence of herd size and neighbour density

Group foraging can be beneficial for ungulates by decreasing the time required for vigilance, but it can also prove costly because of competition. To determine responses to gregarious behaviour, we studied foraging activity and vigilance of impala ( Aepyceros melampus ) near Kruger National Park, South Africa. We measured time spent foraging, vigilant, moving, grooming, engaging in social interactions and determined herd size and group distribution (i.e. density). We calculated accepted food abundance (AFA), food ingestion rate, steps per minute and percent vigilance for female, bachelor male and herd male impala. There was no relationship between herd size and vigilance, but vigilance decreased with increasing density ( t _1,311 = 4.91, P  <0.0001). Additionally, AFA decreased ( t _1,61 = 5.96, P  <0.0001) and steps per minute increased ( t _1,311 = 14.38, P  <   0.0001) as more individuals fed in close proximity to each other. Impala could be altering their behaviour to accommodate a perceived change in resources because of intraspecific competition and these adjustments might be related more to the distribution of herd members than to herd size. Further studies should examine the behaviour of gregarious animals in relation to the distribution of herd members in addition to group size.     

Sexual,Seasonal and Group Size Differences in the Allocation of Time between Vigilance and Feeding in the Greater Rhea,Rhea americana

We studied the effect of sex and group size on the proportion of time a greater rhea, Rhea americana, allocates to vigilance and feeding during the breeding and the non-breeding seasons. We analysed 175 records of focal animals that were feeding alone or in groups of 2 to 26 birds. In both seasons, males spent more time in vigilance and less time in feeding than females. Both sexes spent more time in vigilance and less time in feeding during the breeding season. Sexual and seasonal differences in vigilance were the result of different mechanisms. Males had shorter feeding bouts than females but there were no sexual differences in the length of the vigilance bouts. On the contrary, seasonal differences were the result of males and females having longer vigilance bouts during the breeding season but there were no seasonal differences in the length of the feeding bouts. During the non-breeding season, individual vigilance was higher in rheas foraging alone than in groups. In this case, solitary birds had longer vigilance and shorter feeding bouts than birds foraging in groups. We discuss the possible effect of intragroup competition and food availability on the allocation of time between feeding and vigilance in this species.     

Group‐Size Effect on Vigilance and Foraging in a Predator‐Free Population of Feral Goats (Capra hircus) on the Isle of Rum,NW Scotland

Many previous studies have found that as group size increases, individual vigilance levels decrease and forage intake increases (group‐size effect), but few such studies have considered the impact of within‐group interactions and other confounding factors on the direction of group‐size effects. A free‐ranging population of feral goats (Capra hircus), with little predation threat, was studied on the Isle of Rum (northwest Scotland), from Jun. to Nov. 2000, to investigate the effects of group size on individual vigilance levels and foraging efficiency after taking into account the effect of confounding factors (e.g. sex, season, time of day, habitat, predation risk) and within‐group interactions (via changes in movement rates while feeding). Our results show that, while group size exerted a negative influence on individual vigilance levels and a positive effect on movement rate, foraging efficiency never increased with group size and even decreased at certain times of day. There was no sex difference in individual vigilance in feral goats, but foraging efficiency was higher in females than in males. Goats were more vigilant in fall than in summer. The results imply that the benefits for foraging obtained from the reduced vigilance level in larger groups may be constrained or offset by increased interaction (or competition) within larger groups even in a population that faces negligible predation risk.     

Foraging behavior of three Tasmanian macropodid marsupials in response to present and historical predation threat

It is often essential to understand historical selection regimes to explain current traits. We studied antipredator behavior of three Tasmanian macropodid marsupials – Forester kangaroos Macropus giganteus , Bennett's wallabies M. rufogriseus , and Tasmanian pademelons Thylogale billardierii – to understand how antipredator behavior functions in a relatively intact predator community. We also compared behavior of the kangaroos and wallabies on a predator-free island where they were translocated from mainland Tasmania 30 yr ago. Both species allowed humans to get closer to them on the predator-free island; a finding consistent with a reduced risk of predation on the island. Neither kangaroos, nor wallabies, exhibited group size effects – they did not modify time allocated to foraging or antipredator vigilance as a function of group size at either site. Nor did overall time allocation vary in any consistent way. In contrast, mainland Australian sibling-species of Forester kangaroos and Bennett's wallabies have both been reported to have group size effects. It is possible either that the extinction of the thylacine Thylacinus cynocephalus in the last century has led to an evolutionary loss of group size effects and other antipredator behavior, or that thylacines were never that important a predator on Tasmanian subspecies. In contrast, Tasmanian pademelons studied on the Tasmanian mainland modified time allocation as a function of group size suggesting that they perceived safety in numbers. Pademelons, because of their body size, are relatively more vulnerable than larger-bodied macropodids to the rich community of marsupial carnivores in Tasmania, and used a mix of social and individual strategies to manage predation risk.     

The loss of anti-predator behaviour following isolation on islands

When isolated from predators, costly and no longer functional anti-predator behaviour should be selected against. Predator naiveté is often pronounced on islands, where species are found with few or no predators. However, isolation on islands involves other processes, such as founder effects, that might be responsible for naiveté or reduced anti-predator behaviour. We report the first comparative evidence that, in macropodid marsupials, isolation on islands may lead to a systematic loss of 'group size effects'-a behaviour whereby individuals reduce anti-predator vigilance and allocate more time to foraging as group size increases. Moreover, insular animals forage more, and are less vigilant, than mainland ones. However, we found no evidence that animals on the mainland are 'flightier' than those on islands. Remarkably, we also found no evidence that isolation from all predators per se is responsible for these effects. Together, these results demonstrate that anti-predator behaviour may indeed be lost or modified when animals are isolated on islands, but it is premature to assume that all such behaviour is affected.     

The influence of flock size and geometry on the scanning behaviour of spotted turtle doves,Streptopelia chinensis

Abstract The negative correlation between the time individuals spend scanning the environment for predators and group size is usually explained by the benefit of corporate vigilance. However, this negative correlation may be explained additionally in terms of the ‘dilution effect’ and ‘selfish herd geometry’. Our experimental investigation of the scanning behaviour of free-living spotted turtle doves foraging at different shaped feeders revealed that flock geometry influenced individual scanning rates. The time spent scanning declined with group size less rapidly among birds foraging in linear flocks than among those foraging in more two-dimensional flocks. These results were not confounded by aggressive behaviour, and indicate that the benefits of foraging in groups include the so-called selfish herd geometry.     

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.     

Vigilance in Greater Flamingos Wintering in Southern Tunisia: Age-Dependent Flock Size Effect

Decrease in individual vigilance with flock size is a widely recognized pattern in group‐living species. However such a relationship may be affected by other factors, such as age and flock composition. For instance, because young animals generally lack experience and have higher nutritional needs than adults, they can be expected not only to be less vigilant than adults but also to decrease their vigilance level by a greater extent when flock size increases than adults do. We investigated this issue using data on greater flamingos wintering in the gulf of Gabès, in southern Tunisia. Flamingos tended to congregate in small single‐age flocks for feeding, but as flock size increased, flocks became mixed. We found that when flock size increased, young flamingos significantly decreased their vigilance time, while adult did not, suggesting an age‐dependent flock size effect on vigilance. However, when flock composition (single‐age vs. mixed) was taken into account, a more complex pattern was found. Within single‐age and small flocks, no difference was found between young flamingos and adult ones regarding their vigilance level and their response to increasing flock size. However, within mixed and large flocks, adult flamingos were more vigilant than young ones, while variation in flock size did not result in a significant change in vigilance. These results suggest that young birds relied on the presence of adults, and hence more experienced individuals in detecting dangers, to reduce their vigilance and to increase their foraging time in order to satisfy their higher nutritional requirements. They could also be interpreted as a possible consequence of increasing competition with flock size which constrained more nutritionally stressed young flamingos to increase their foraging time to the detriment of vigilance.     

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.     

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.     

How do vigilance and feeding by common cranes Grus grus depend on age,habitat, and flock size?

Animals often spend less time vigilant and more time feeding when foraging in larger groups. This group-size effect does not, however, consider if larger groups differ systematically from smaller ones: Large groups could form in different habitats than small groups or be composed of a different mix of ages or classes than small groups. We examined how habitat differences and flock size and composition explain feeding and vigilance rates in common cranes Grus grus , wintering in holm oak Quercus ilex dehesas of Spain. Flock size and composition were related to habitat type in cranes: flocks formed in areas sown with cereal crops were larger than flocks formed in set aside areas. Vigilance rate depended on habitat but decreased with increasing flock size in a similar way across all habitats. Juveniles were less vigilant than adults and showed little change in vigilance with flock size. Vigilance increased and feeding time decreased over months from November through February. Our results show that vigilance is affected by habitat but that the group size effect on vigilance is not the product of differences between habitats in group size or composition.     

Influences of sex,group size,and spatial position on vigilance behavior of Przewalski’s gazelles

Group-living animals may need to spend less time being vigilant, consequently, having more time for other important activities such as foraging (i.e., group size effect). Przewalski’s gazelle (Procapra przewalskii) is a group-living social animal, and a study was conducted in Qinghai Province of China during June–August 2006 by using a continuous focal sampling method to investigate the influences of group size, sex, within-group spatial position, and nearest-neighbor distance on individual vigilance level (defined as scanning frequency per minute). Male gazelles were more vigilant than females. The gazelle’s vigilance level decreased with group size (group size effect), but only for females. The individuals at the central positions within a group were less vigilant than those at the peripheral positions, but the nearest-neighbor distance did not have any significant influence on the individual vigilance level. Our results support the hypotheses of group size effect and edge effects, but the sexual difference in vigilance level and in the response to group size effect on vigilance suggests that there may be sexual difference in the function and targets of vigilance behavior of Przewalski’s gazelles, which warrants more investigation, with incorporation of within-group spatial position, to better understand the mechanism underlying the group size effect and edge effect.     

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.     

Foraging and vigilance time allocation in a guanaco (<Emphasis Type="Italic">Lama guanicoe</Emphasis>) population reintroduced in Quebrada del Condorito National Park (Córdoba,Argentina)

In reintroduction projects, ethology studies play a significant role in evaluating the behaviour of the individuals in habitats where they are reintroduced. We studied foraging and vigilance time allocation of a guanaco (Lama guanicoe) population reintroduced in Quebrada del Condorito National Park (QCNP), in the central mountains of Córdoba, Argentina. On average, individuals showed a higher proportion of time invested in vigilance and a lower proportion of time invested in foraging than a previously studied guanaco population belonging to the same ecological region as the source population, suggesting that at the time of this study, the reintroduced population was not fully adapted to the new habitat or suffered from an increased predation pressure. On the other hand, as expected for the species, throughout the study period, males allocated more time to vigilance than females, both sexes increasing vigilance during the reproductive period, and females increasing foraging time allocation during the post-reproductive period. Taking into account that further reinforcement to the existing population is planned, the present results may contribute to the elaboration of management strategies aimed at the successful establishment of guanaco in QCNP.     

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.     

Does vigilance always covary negatively with group size? Effects of foraging strategy

The possible effects of foraging strategy on the relationship between vigilance and group size were studied in three species of waders with different foraging strategies. I predict that (1) pause–travel species should show no relationship between scanning and group size, because these species scan for prey as well as for predators; (2) continuous-tactile foraging species should show a positive relationship between flock size and vigilance level, because in their large groups vigilance towards other birds could be used to avoid interference and aggression; and (3) continuous-visual searching species should show the general pattern of decreasing vigilance when group size increases as predicted by both the 'many eyes' and the 'predatory risk' hypotheses. Results support the predictions for the influence of foraging strategy on the relationship between vigilance level and group size. The mutual exclusion of foraging and scanning can determine the importance of the 'many eyes' hypothesis. Such exclusion seems to be determined by foraging strategy. The influence of foraging on scanning must be taken into account in future studies on the group-size effect. Electronic Publication     

Vigilance and group size in impala (Aepyceros melampus Lichtenstein): a study in Nairobi National Park, Kenya

One of the advantages of living in groups is that individuals may need to be less vigilant, allowing them more time for other important activities, such as foraging. This relationship between group size and per cent time spent being vigilant was investigated by observing impala in Nairobi National Park, Kenya. Three types of individual were observed: females, territorial males and bachelor males. Only females showed the predicted negative relationship between per cent vigilance and herd size. Both types of male showed no significant change of vigilance with increasing group size. There was no difference in levels of vigilance in open or closed habitats and no difference in vigilance between herds ‘alone’ and herds with other species that might have provided ‘extra eyes’.