Impact of increased predation risk on vigilance behaviour in a gregarious waterfowl,the Egyptian goose Alopochen aegyptiaca

Vigilance is amongst the most universal of anti‐predator strategies and commonly declines with increasing group size. We experimentally manipulated predation risk in a system with a known relationship between group size and vigilance levels to explore whether this relationship changes in response to elevated predation risk. We investigated the vigilance levels of Egyptian geese Alopochen aegyptiaca at eight golf courses in the western Cape, South Africa, to assess the perception of and reaction to predation risk. We manipulated predation risk by introducing trained Harris's hawks Parabuteo unicintus where avian predation was otherwise low or absent. The study confirmed the typical reduction in vigilance with group size on control sites, where the risk of predation is low. However, at experimental sites with elevated predation risk, a positive relationship between vigilance and group size was observed. We hypothesize that the mechanism for this relationship might be linked to social information transfer via copying behaviour and manipulation to induce vigilance. Thus, larger groups will have a higher probability of containing individuals with experience of elevated predation risk and their increased vigilance behaviour is copied by naïve individuals. This prediction is based on the intended outcome of introducing avian predation to make the geese feel less safe and to eventually leave the site as a management tool for controlling nuisance geese.     

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.     

Factors Affecting Nesting Site Choice in Chimpanzees at Tshibati,Kahuzi-Biega National Park: Influence of Sympatric Gorillas

We recorded 310 fresh chimpanzee night nests at 72 nest sites to determine their choice of tree and site for nesting vis-à-vis the effects of sympatric gorillas. Chimpanzees did not use trees for nesting according to their abundance, but instead tended to nest in fruit trees that they used as food sources. Nesting patterns of chimpanzees may vary with nesting group size, the type of vegetation, and fruit species eaten or not eaten by gorillas. When chimpanzees lodged as a small group in the secondary forest, they nested more frequently in trees bearing ripe fruits eaten only by themselves than in those with fruit eaten also by gorillas. When they lodged as a large group in the primary forest, they nested more frequently in trees bearing ripe fruits eaten by both apes. Nest group size is positively correlated with the availability of preferred ripe fruits in secondary forest. These findings not only reflect the larger foraging groups at the larger fruiting trees but also suggest that chimpanzees may have tended to occupy fruiting trees effectively by nesting in them and by forming large nest groups when the fruits attracted gorillas. Competition over fruits between gorillas and chimpanzees, due to their low productivity in the montane forest of Kahuzi, may have promoted the chimpanzee tactics.     

Vigilance and fitness in grey partridges Perdix perdix: the effects of group size and foraging-vigilance trade-offs on predation mortality

1. Vigilance increases fitness by improving predator detection but at the expense of increasing starvation risk. We related variation in vigilance among 122 radio-tagged overwintering grey partridges Perdix perdix (L.) across 20 independent farmland sites in England to predation risk (sparrowhawk Accipiter nisus L., kill rate), use of alternative antipredation behaviours (grouping and use of cover) and survival. 2. Vigilance was significantly higher when individuals fed in smaller groups and in taller vegetation. In the covey period (in early winter when partridges are in flocks), vigilance and use of taller vegetation was significantly higher at sites with higher sparrowhawk predation risk, but tall vegetation was used less by larger groups. Individuals were constrained in reducing individual vigilance by group size and habitat choice because maximum group size was determined by overall density in the area during the covey period and by the formation of pairs at the end of the winter (pair period), when there was also a significant twofold increase in the use of tall cover. 3. Over the whole winter individual survival was higher in larger groups and was lower in the pair period. However, when controlling for group size, mean survival decreased as vigilance increased in the covey period. This result, along with vigilance being higher at sites with increasing with raptor risk, suggests individual vigilance increases arose to reduce short-term predation risk from raptors but led to long-term fitness decreases probably because high individual vigilance increased starvation risk or indicated longer exposure to predation. The effect of raptors on survival was less when there were large groups in open habitats, where individual partridges can probably both detect predators and feed efficiently. 4. Our study suggests that increasing partridge density and modifying habitat to remove the need for high individual vigilance may decrease partridge mortality. It demonstrates the general principle that antipredation behaviours may reduce fitness long-term via their effects on the starvation-predation risk trade-off, even though they decrease predation risk short-term, and that it may be ecological constraints, such as poor habitat (that lead to an antipredation behaviour compromising foraging), that cause mortality, rather than the proximate effect of an antipredation behaviour such as vigilance.     

Vigilance in Single- and Mixed-Species Groups of Tamarins (Saguinus labiatus and Saguinus fuscicollis)

By forming larger sizes of groups, individuals benefit from a decrease in vigilance, but the collective vigilance of the group as a whole is not compromised. We examined whether this group size effect is apparent in mixed-species groups of red-bellied tamarins (S. labiatus) and saddleback tamarins (S. fuscicollis) which form stable and permanent associations in the wild. We studied general vigilance and responses to hidden threatening stimuli in five captive groups of each species, while they were housed in single- and mixed-species groups. For vigilance, the individual rate was lower in the larger mixed-species groups than in the smaller single-species groups. In addition, the amount of time when at least one individual was vigilant was higher in mixed-species groups. This suggests that the tamarins alter their vigilance behavior in the presence of the other species. In response to hidden threats, both species performed brief vigilance checks and frequencies of checking did not differ in single- and mixed-species groups. However, both species had a significant reduction in the mean duration per check, and there was a reduced total amount of time spent vigilance checking in the mixed-species groups compared to the single-species groups, demonstrating the group size effect. Overall the mixed-species groups had a higher number and mean duration of checking than the smaller single-species groups. Given that the two species share a common set of predators, and respond to each other's alarm calls, these findings provide strong evidence that individuals of both tamarin species may be able to benefit from forming mixed-species groups via improved vigilance and monitoring of threats.     

Effect of Male Emigration on the Vigilance Behavior of Coresident Males in White-Faced Capuchins (Cebus capucinus)

I examined the vigilance behavior of male Cebus capucinus residing in four groups in Santa Rosa Park, Costa Rica. One male emigrated from each of three study groups, providing ideal experimental conditions for examining the effect of coresident males on male vigilance behavior (social and non-social). Following the predator and conspecific defense theories for the adaptive value of male vigilance behavior and the occurrence of multimale groups, I predicted that male nonsocial vigilance would increase after the emigration of a coresident male. My prediction was supported in only one of the three study groups. Males in two groups decreased their nonsocial vigilance after the emigration of a coresident male, which was probably influenced by seasonal changes and the peripherality of one group's coresident male before his emigration. The social vigilance hypothesis proposes that for species with rigid dominance hierarchies, social vigilance should increase with an increase in same sex competitors. Therefore, I predicted that male social vigilance would decrease after the emigration of a coresident male. The hypothesis is generally supported in my findings, as two of the three groups displayed a decrease in social vigilance after the emigration of a coresident male. Increased social vigilance in the third group is best explained by the change in the alpha male's social behavior after the emigration of his only coresident male.     

Vigilance and its complex synchrony in the red-necked pademelon, Thylogale thetis

Several adaptive functions, including gaining information fromother group members and detecting predators, are generally ascribedto vigilance in groups of animals subject to predation. Moststudies of the effects of neighbors on vigilance have focusedon individual vigilance. We investigated the effects of neighborson vigilance in wild red-necked pademelons Thylogale thetisforaging at night in nonpersistent aggregations in a clearingin rain forest. Neither the total number of pademelons in theclearing nor the numbers at various distances around focal individualsaffected the individual vigilance of focal animals. However,focal animals’ individual vigilance did change with thedistance to their nearest neighbor and also with distance tocover. Pairs of individuals closer than 10 m apart tended tosynchronize their bouts of individual vigilance and foraging.The degree of synchrony within pairs increased with both distanceto cover and the total number of pademelons foraging in thearea and decreased with increasing distance to the pair's nearestneighbor but did not vary with the distance separating the membersof the pair. Thus, despite their individual vigilance beingunaffected by the number of other pademelons in the feedingaggregation, pademelons were nonetheless sensitive to the presenceof conspecifics and adjusted their behavior in relation to theirseparation from neighbors. Thus, some vigilance benefits maybe obtained from the presence of conspecifics even in speciesthat aggregate only temporarily on food patches without formingmore permanent social groups.     

On the Advantages of Mixed‐Species Groups: Impalas Adjust Their Vigilance When Associated With Larger Prey Herbivores

Prey can obtain valuable benefits from associating with other species if heterospecifics help to detect predators or locate good food patches. In mixed‐species groups, how species respond to the presence of other species remains a poorly explored question although it might give crucial insights into mechanisms underlying the interspecific coexistence. We studied temporary mixed‐species groups of large herbivores in Hwange National Park (Zimbabwe) between the common impala (Aepyceros melampus), the focal species here, and bigger species including the plains zebra (Equus quagga), the greater kudu (Tragelaphus strepsiceros) or the blue wildebeest (Connochaetes taurinus). In the Hwange savanna, the focal and smaller species are exposed to a larger range of predators than the associated species. In this context, we investigated how impalas adjusted their vigilance with group size comparing impala‐only and mixed‐species groups and whether the identity of heterospecifics affected vigilance of impalas. Our study showed that the time impalas spent in vigilance significantly decreased with group size when they formed impala‐only groups, whereas it did not significantly vary with group size in mixed‐species groups. Moreover, in mixed‐species groups, impalas did not adjust their time spent in vigilance with the proportion of conspecifics and the identity of the associated species. Thus, the mechanism underlying the difference of impalas' behavioural adjustment of vigilance with group size between single‐ and mixed‐species groups seemed to be related to the presence but not to the number and the identity of heteropecifics. Finally, we discuss the concept that larger and dominant heterospecifics were likely to increase competition for food access, thereby forcing higher vigilance of impalas, outweighing any reduction from collective vigilance.     

Vigilance in ursine black and white colobus monkeys (Colobus vellerosus): an examination of the effects of conspecific threat and predation

Vigilance is thought to have evolved as an antipredator defense but, in primates, conspecific threat often better explains this behavior. We examined vigilance in one group of Colobus vellerosus inhabiting the Boabeng‐Fiema Monkey Sanctuary in Ghana. We aimed to (1) describe factors affecting vigilance in this population, and (2) examine the importance of predation avoidance and conspecific threat in explaining vigilance patterns. Because of a male takeover preceding the study, our focal group (B2) consisted of six adult males and three adult females. We collected 490 10‐min focal samples (82 hr) from all adults in the group (N=9) from July to November, 2001. To avoid predators, individuals should be more vigilant (i) with fewer neighbors, and (ii) in areas of the canopy with higher predation risks. Conspecific threats can be divided into extra‐ and intra‐group threats. Extra‐group threats should lead to higher vigilance levels (iii) during intergroup encounters, and (iv) in areas where the home range overlaps with other groups of colobus. Intra‐group threats should lead to greater vigilance (v) in the presence of neighbors and (vi) while feeding or occupying food patches (if resources are limiting). Our results best support the hypothesis that vigilance functions primarily to detect extra‐group, conspecific threats. Individuals were most vigilant during intergroup encounters and in areas of range overlap, and monthly vigilance rates were associated with monthly encounter rates. Individuals tended to scan less in proximity to familiar vs. unfamiliar group mates, suggesting that relationship quality may affect colobus vigilance. Finally, predation pressures or anthropogenic disturbances might have influenced vigilance, as individuals were more vigilant low in the canopy. However, this last result could also be due to the lower visibility because of dense foliage or to the fact that the monkeys have access to fewer escape routes when ranging lower in the canopy. Am. J. Primatol. 71:919–927, 2009. © 2009 Wiley‐Liss, Inc.     

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’.     

Self-induced Increase of Gut Motility and the Control of Parasitic Infections in Wild Chimpanzees

When physiological adaptation is insufficient, hosts have developed behavioral responses to avoid or limit contact with parasites. One such behavior, leaf-swallowing, occurs widely among the African great apes. This behavior involves the slow and deliberate swallowing without chewing of whole bristly leaves. Folded one at a time between tongue and palate, the leaves pass through the gastro-intestinal (GI) tract visibly unchanged. Independent studies in two populations of chimpanzees (Pan troglodytes schweinfurthii) showed significant correlations between the swallowing of whole leaves and the expulsion of the nodule worm Oesophagostomum stephanostomum and a species of tapeworm (Bertiella studeri). We integrate behavioral, parasitological and physiological observations pertaining to leaf-swallowing to elucidate the behavioral mechanism responsible for the expulsion and control of nodule worm infections by the ape host. Physical irritation produced by bristly leaves swallowed on an empty stomach, increases motility and secretion resulting in diarrhea which rapidly moves leaves through the GI tract. In the proximal hindgut, the site of third-stage larvae (L3) cyst formation and adult worm attachment, motility, secretion and the scouring effect of rough leaves is enhanced by haustral contractions and peristalsis-antiperistalsis. Frequently, at the peak of reinfection, a proportion of nonencysted L3 is also predictably vulnerable. These factors should result in the disruption of the life cycle of Oesophagostomum spp. Repeated flushing during peak periods of reinfection is probably responsible for long-run reduction of worm burdens at certain times of the year. Accordingly, leaf-swallowing can be viewed as a deliberate adaptive behavioral strategy with physiological consequences for the host. The expulsion of worms based on the activation of basic physiological responses in the host is a novel hitherto undescribed form of parasitic control.     

The group size effect and synchronization of vigilance in the Tibetan wild ass

Vigilance behavior is considered as an effective strategy for prey species to detect predators.An individual benefits from living in a group by reducing the time spent being vigilant without affecting the probability of detecting a predator.However,the mechanism producing a decrease in vigilance with increasing group size is unclear.Many models of vigilance assume that group members scan independently of one another.Yet in recent studies,the other 2 patterns of vigilance,coordination and synchronization,were reported in some species.In 2 summers(2018 and 2019),we studied the group-size effect on vigilance and foraging of Tibetan wild ass in Chang Tang Nature Reserve of Tibet.We also tested whether individuals scan the environment independently,tend to coordinate their scans,or tend to synchronize their vigilance.The results showed that individuals decreased the time spent on vigilance with increasing group size,while increased the time spent foraging.Group members scanned the environment at the same time more frequently and there was a positive correlation between group members'behaviors,indicating that Tibetan wild asses tend to synchronize their vigilance.     

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.     

The effects of group size and reproductive status on vigilance in captive Callithrix jacchus

Previous work on mammals and birds has often demonstrated a negative relationship between group size and individual vigilance. However, this relationship has received only weak support in nonhuman primates. This result may be due to the failure to distinguish different forms of vigilance such as antipredatory vigilance and social monitoring. Here, we tested the effects of group size, reproductive status (breeding vs. nonbreeding), and sex on antipredatory vigilance and social monitoring in captive common marmosets (Callithrix jacchus). Behavioral observations using one-zero sampling were conducted on adult members of three captive groups of small, medium, and large size. Data were analyzed using a series of general linear models (GLMs) analyses of covariance (ANCOVAs). We found an overall negative group size effect on antipredatory vigilance and that breeders, especially breeding males, were significantly more vigilant than nonbreeders. Conversely, we found that social monitoring increased with group size. Unlike the results for antipredatory vigilance, neither breeders and nonbreeders nor males and females differed in their amounts of social monitoring. However, the effect of group size appeared to differ for nonbreeding males compared to all other adults. Our results generally support the idea that individuals in larger groups are safer with breeding males likely playing a prominent role in protection from predation. The increase in social monitoring may be related to increased reproductive competition with the presence of adult offspring, but future studies need to clarify the target of social monitoring in both breeders and nonbreeders. Overall, the study underlines the importance of distinguishing different forms of vigilance and other factors as they may confound the effects of group size on antipredatory vigilance.     

Sleeping Parties and Nest Distribution of Chimpanzees in the Savanna Woodland, Ugalla, Tanzania

We conducted ecological studies of chimpanzees (Pan troglodytes) in the Ugalla area, western Tanzania. Ugalla is one of the driest habitats of chimpanzees and the Ugalla River is the eastern boundary of chimpanzee distribution. Most of Ugalla is occupied by savanna woodlands dominated by deciduous trees of Brachystegia and Julbernardia. Chimpanzees tended not to make nests in riverine forests in plains, but in small patchy forests dominated by Monopetalanthus richardsiae and valley forests dominated by Julbernardia unijugata on slopes in mountainous areas. We estimated population density of chimpanzees to be 7–9 × 10⁻² individuals/km² based on nest censuses, suggesting that 2–3 × 10² individuals inhabited the 3352 km² area of Ugalla. The size of the largest nest cluster (n=23) suggests that 1 unit group (community) comprised 30–35 individuals. In the daytime, chimpanzees formed small feeding parties (mean 2.0 individuals), but larger ones in the evening (mean 4.8 individuals and 5.2 individuals based on fresh nest clusters). The pattern might reduce the predation risk from large nocturnal carnivores such as lions and leopards. The sleeping sites may function as both a safe sleeping site and a meeting point for chimpanzees with a huge home range that may have difficulty in finding other members of their unit group.     

On how risk and group size interact to influence vigilance

Vigilance allows animals to monitor their surroundings for signs of danger associated with predators or rivals. As vigilance is costly, models predict that it should increase when the risk posed by predators or rivals increases. In addition, vigilance is expected to decrease in larger groups that provide more safety against predators. Risk and group size are thus two key determinants of vigilance. Together, they could have additive or interactive effects. If risk and group size interacted, the magnitude of the group‐size effect on vigilance would vary depending on the level of risk experienced, implying that the benefits of sociality in terms of vigilance vary with risk. Depending on the model, vigilance is predicted to decrease more rapidly with group size at low risk or at high risk. Little work has focused directly on the interaction between risk and group size, making it difficult to understand under which conditions particular interactive effects arise and whether interactive effects are common in natural systems. I review the vast literature on vigilance in birds and mammals to assess whether interactive effects between risk and group size are common, and if present, which pattern occurs more frequently. In studies involving predation risk, the greatest proportion reported no statistically significant interactive effects. In other cases, vigilance decreased with group size more rapidly at low or high risk in a similar proportion of studies. In studies involving risk posed by rivals (social risk), most documented a more rapid decrease in vigilance with group size at low than at high risk, as predicted if the need to monitor rivals increases in larger groups. Low statistical power to detect interactive effects might have been an issue in several studies. The absence of interactive effects, on the other hand, might suggest constraints or limits on the ability of animals to adjust vigilance to current risk or group sizes. Interactive effects on vigilance have implications for the evolution of sociality and for our understanding of the phenotypic plasticity of predator‐ and competitor‐induced defences and deserve more attention in future studies.