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...

Behavioral decisions made under the risk of predation: a review and prospectus

Beginning with Emlen (1966) and MacArthur and Pianka (1966) and extending through the last ten years, several authors have sought to predict the foraging behavior of animals by means of mathematical models. These models are very similar,in that they all assume that the fitness of a foraging animal is a function of the efficiency of foraging measured in terms of some "currency" (Schoener, 1971) -usually energy- and that natural selection has resulted in animals that forage so as to maximize this fitness. As a result of these similarities, the models have become known as "optimal foraging models"; and the theory that embodies them, "optimal foraging theory." The situations to which optimal foraging theory has been applied, with the exception of a few recent studies, can be divided into the following four categories: (1) choice by an animal of which food types to eat (i.e., optimal diet); (2) choice of which patch type to feed in (i.e., optimal patch choice); (3) optimal allocation of time to different patches; and (4) optimal patterns and speed of movements. In this review we discuss each of these categories separately, dealing with both the theoretical developments and the data that permit tests of the predictions. The review is selective in the sense that we emphasize studies that either develop testable predictions or that attempt to test predictions in a precise quantitative manner. We also discuss what we see to be some of the future developments in the area of optimal foraging theory and how this theory can be related to other areas of biology. Our general conclusion is that the simple models so far formulated are supported are supported reasonably well by available data and that we are optimistic about the value both now and in the future of optimal foraging theory. We argue, however, that these simple models will requre much modification, espicially to deal with situations that either cannot easily be put into one or another of the above four categories or entail currencies more complicated that just energy.

Citation classic - optimal foraging - a selective review of theory and tests

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.

Predator vigilance and group size in mammals and birds: a critical review of the empirical evidence

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...

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Wolves, elk, and bison: reestablishing the "landscape of fear" in Yellowstone National Park, U.S.A.

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...

Forage Quality and Aggregation by Large Herbivores

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.

Vigilance Behaviour in Grazing African Antelopes

The second to second organization of the foraging behaviour of eleven species of African ungulates is described, with particular emphasis on locomotion while foraging and on the time spent feeding. It was predicted that foraging behaviour should change with the species' body size and stomach specialization and, within a species, with the seasonal or spatial changes in the quality and availability of the food supply. A feeding site was defined as the area which a feeding animal could reach without moving either of its forefeet. Feeding subjects thus encountered a new site every time they took a step with either forefoot and foraging behaviour could be described in terms of the organization of a series of behavioural events (steps) and bouts of activity (feeding). Five indices were used to summarize all records obtained. Only one of these, the proportion of time spent feeding, was significantly correlated with the species' body size. The other four indices include: the rate of movement (in steps per second), the mean feeding time per step, the proportion of sites encountered which carried at least some acceptable food, and the mean time spent feeding from those accepted sites. These last four indices were apparently more strongly affected by the species' stomach specialization than by its body size. The five species with the largest sample sizes (reedbuck, impala, tsessebe, wildebeeste, buffalo) were used to investigate the tendency found within all study species for foraging behaviour to vary seasonally. Firstly, both the mean feeding time per step and the mean feeding time per accepted site were found to be positively correlated with indicators of vegetation bulk and maturity, but negatively correlated with an index of fresh green growth, for all five species: it is suggested that this is due to changes in both the amount of food taken from a given site and in the time needed to ingest a given bulk of food. Secondly, the organization of events within foraging sessions (described in terms of the frequency distributions of step intervals, feeding durations, and the lengths of runs of acceptance or rejection of feeding sites) varied through the year: in particular, food patch size, indicated by the length of sequences of accepted sites, was apparently largest just before the end of the rains, when the vegetation should have been most uniform in quality. Thirdly, the reedbuck is a specialist grazer which, because of its body size, might be expected to feed very selectively within the grass layer: the feeding behaviours of the other four species most closely resembled that of the reedbuck when these four species were feeding off long grass in the dry season, conditions which presented them with much low quality food senescent grass hampering access to very small amounts of higher quality green matter. It is suggested that, as in domestic ungulates, feeding behaviour varies within a species with the proportion of low fibre, high protein, green growth in the vegetation, and in the contrast in quality between the various plant parts. A multivariate analysis was used to identify the foraging characteristics of individual ruminant species. Species which were specialist grass feeders (bulk/roughage feeders) encountered more sites more predictably, and spent more time feeding off those sites, than did species which were known to switch from grasses to other food sources to take advantage of changes in the relative quantity and abundance of food types in the habitat (intermediate feeders). It is suggested that, since pure grazers' foraging is limited to a fairly continuously distributed food supply, their foraging consists mainly of teasing out and biting off grass leaves and the organization of their foraging behaviour is determined by the structure and quality of the grass sward. Intermediate feeders, on the other hand, had the option of taking higher quality but less continuously distributed items, such as fruits, and their foraging may have involved seeking out and moving between such items. Pure grazers' foraging behaviour is thus seen as being dominated by food capture and handling events, while intermediate feeders may be more strongly influenced by food search or pursuit requirements. It is likely that grazers differed from intermediate feeders not only in the basic organization of their foraging behaviour, but also in the way that this organization was affected by the species' body size.

R. Underwood

Papers

Vigilance Behaviour in Grazing African Antelopes

The Feeding Behaviour of Grazing African Ungulates