Vigilance Behaviour in Grazing African Antelopes
TL;DR: Time spent looking varied with position within the group; this effect was strongest in closed habitats, where central animals tended to scan least and feed most, and within species, animals inclosed habitats, those with dense vegetation, tended to spend more time in looking than did animals in the open.
Abstract: African antelope may devote a large proportion of their foraging time to looking around. The factors affecting such vigilance behaviour are examined for grazing antelope, five species being studied in detail. The proportion of time spent looking decreased as species body weight increased. Within species, animals in closed habitats, those with dense vegetation, tended to spend more time in looking than did animals in the open. There was some evidence that vigilance, presumably for predators, was shared by group members, but in one species, impala, vigilance apparently increased with group size and with proximity to neighbours. Time spent looking varied with position within the group; this effect was strongest in closed habitats, where central animals tended to scan least and feed most. Vigilance increased as feeding success decreased, partly due to mutual interference between looking and feeding. The possible social, foraging and predator-detection values of vigilance are discussed. A simple model is introduced to help explain the effects of cover and to facilitate further discussion.
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TL;DR: This work has shown that predation is a major selective force in the evolution of several morphological and behavioral characteristics of animals and the importance of predation during evolutionary time has been underestimated.
Abstract: Predation has long been implicated as a major selective force in the evolution of several morphological and behavioral characteristics of animals. The importance of predation during evolutionary ti...
7,461 citations
TL;DR: Most of the studies fail to adequately demonstrate an unambiguous relationship between vigilance behaviour and group size, but many studies reveal interesting features of the relationship between Vigilance and Group size that should provide fruitful avenues for future research.
Abstract: One commonly cited benefit to animals that forage in groups is an increase in the probability of detecting a predator, and a decrease in the time spent in predator detection. A mathematical model (Pulliam 1973) predicts a negative relationship between group size and vigilance rates. Over fifty studies of birds and mammals report that the relationship at least partly explains why individuals forage in groups. This review evaluates the strength of these conclusions based on their evidence. Those variables that may confound the relationship between vigilance and group size are outlined, and their control is assessed for each study. The variables I consider to be important include the density and type of food; competition between individuals; the proximity to both a safe place and the observer; the presence of predators; the visibility within the habitat; the composition of the group; the ambient temperature and the time of day. Based on these assessments, most of the studies fail to adequately demonstrate an unambiguous relationship between vigilance behavior and group size. Nevertheless, many studies reveal interesting features of the relationship between vigilance and group size that should provide fruitful avenues for future research.
1,051 citations
TL;DR: Male elk and bison showed no response to the reintroduction of wolves, maintaining the lowest levels of vigilance throughout the study (12 and 7% of the time was spent vigilant, respectively).
Abstract: The elk or wapiti (Cervus elaphus) and bison (Bison bison) of Yellowstone National Park have lived in an environment free of wolves (Canis lupus) for the last 50 years. In the winter of 1994-1995, wolves were reintroduced into parts of Yellowstone National Park. Foraging theory predicts that elk and bison would respond to this threat by increasing their vigilance levels. We tested this prediction by comparing vigilance levels of elk and bison in areas with wolves with those of elk still in "wolf-free" zones of the Park. Male elk and bison showed no response to the reintroduction of wolves, maintaining the lowest levels of vigilance throughout the study (12 and 7% of the time was spent vigilant, respectively). Female elk and bison showed significantly higher vigilance levels in areas with wolves than in areas without wolves. The highest vigilance level (47.5 ± 4.1%; mean ± SE) was seen by the second year for female elk with calves in the areas with wolves and was maintained during the subsequent 3 years of...
915 citations
Cites background from "Vigilance Behaviour in Grazing Afri..."
...Indeed, various authors (Underwood 1982; Lagory 1986; Scheel 1993; Bednekoff and Ritter 1994; Molvar and Bowyer 1994) have shown this to be the case for a variety of ungulate species....
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TL;DR: Simulations based on the energy-intake model suggest a number of key environmental factors that should determine the impact of forage maturation and spatial variation on herbivore distribution patterns.
Abstract: Three general hypotheses have been proposed to explain why many large herbivores have highly aggregated patterns of distribution: dilution of predation risk, maintenance of forage in an immature bu...
437 citations
TL;DR: A simple model shows that maximal energetic efficiency is associated with immediate consumption, whereas (under the field conditions studied) carrying items to the safety of trees provides for minimal exposure to predation.
372 citations
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124 citations
TL;DR: The gregarious feeding behaviour of Wood-pigeons Columba palumbus was studied in the January-March period in a Cambridgeshire study area when the birds were feeding on clover fields and it was found that birds feeding in isolation obtained even less food per unit time than subordinate individuals living in the flock.
Abstract: The gregarious feeding behaviour of Wood-pigeons Columba palumbus was studied in the January-March period in a Cambridgeshire study area when the birds were feeding on clover fields; many individuals were marked with wing tags enabling them to be identified under field conditions. Normal and underweight (arbitrarily under 450 g) adults survived equally well provided they remained in the flocks living within the study area, where they held dominant positions in the social hierarchy. Juveniles weighing over 450 g had a poorer survival and underweight juveniles the lowest survival of all within the area. Juveniles were more likely than adults to move to places outside the study area (5 to 53 miles) where they had the same survival rate as any adults of similar weight which moved more than five miles. Displaced adults presumably lost the advantages of their position in the established hierarchy and of local experience and now competed on more equal terms with the juveniles. The mean weight of pigeons was highest in those flocks feeding where clover leaf density was highest. Smaller flocks, containing a higher proportion of under-weight birds and fewer juveniles, were found on fields where the leaf density was low (less than 100 leaves/ft2 Dominant birds co-ordinated their searching (paces/min.) and pecking actions to obtain the best components of the feed. Subordinate birds were probably inexperienced individuals which preferred to frequent the front of the flock in order to determine what experienced birds were eating and some other experimental evidence for this view is discussed. It is shown that the number of pecks/min. taken by subordinate birds depended almost entirely on the pecking rate of dominant birds, although subordinates took fewer peckslmin. In addition, subordinates had to avoid supplanting attacks and, because they progressed by stops and starts, were prevented from co-ordinating their feeding and searching movements and in consequence were relatively unselective in what they ate. These differences became greater if numbers were high relative to clover density so that a mechanism is demonstrated which adjusted flock size to the amount of food available; adjustment of numbers occurred when the total quantity of food available appeared to be non-limiting judged by superficial impressions. Subordinate birds had lower weights and reduced survival prospects compared with dominant birds. Moreover, the adrenal cortex of subordinates became hypertrophied with the cortical cells exhibiting an increase in nuclear volume and RNA activity. Subordinate birds unable to feed successfully in one flock attempted to establish themselves in other feeding groups but if low food densities prevented this they were eventually forced to feed alone. It was found that birds feeding in isolation obtained even less food per unit time than subordinate individuals living in the flock. Nevertheless, solitary birds can feed successfully if the food supply is adequate. Solitary birds devoted much time to looking around as if afraid of being surprised by a predator. But it is more probable that when feeding on certain items pigeaons benefit from the combined from the combined feeding experience of the group and this possibility is discussed. Because a solitarily feeding Wood-pigeon cannot exploit clover efficiently, social feeding behaviour improves the survival chances of the individual and in turn enables population size to be increased; social behaviour does not limit numbers before environmental resources become limiting and any apparent self-regulatory processes are shown to be artifacts.
123 citations
TL;DR: Experiments were conducted to investigate whether different swarm regions were differentially preyed upon, finding that stragglers were more often attacked than any portion of a swarm itself, natural selection favours all individual swarm members, although differentially.
Abstract: Using three-spined sticklebacks (Gasterosteus aculeatus) as predators and water fleas (Daphnia magna) as prey, experiments were conducted to investigate whether different swarm regions were differentially preyed upon. The specific location of a region within the swarm, its density, and the motivational state of the predator all influenced the risk of predation. As stragglers were more often attacked than any portion of a swarm itself, natural selection favours all individual swarm members, although differentially.
122 citations
TL;DR: A model based on renewal processes is derived for the predator's net rate of energy intake, used to explore the optimal mode of search, search height, pause duration (“giving-up time”) and move length, and the following predictions emerge.
105 citations
TL;DR: It is concluded that small flocks are likely to be advantageous to the predator and quite possibly disadvantageous to the prey, although the “selfish” advantage of flocking is likely to remain high for prey animals in medium or large sized flocks.
85 citations