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1973 The Many Eyes Hypothesis
Chapter 39
1973 The Many Eyes Hypothesis THE CONCEPT Being in a group allows sharing of information about predator location and threat level. By having many eyes on sentry duty, the chances of spotting an approaching threat are increased.
THE EXPLANATION Why do animals come together in groups? Herds, schools and flocks are everyday occurrences in animal behavior. Yet their existence creates puzzles in evolutionary biology. On the surface, a group of animals is a much more concentrated and higher-value food target for a predator than individual animals would be if widely scattered in the habitat. We have already seen, in Chapter 36: 1971 Selfish Herds, that animals in groups can gain value from their association with the group. One of the explanations, the many eyes hypothesis, states that each animal needs to invest less in vigilance as group size goes up (Pulliam, 1973). As a corollary, animals should monitor how vigilant their neighbors in the group are, and adjust their behavior correspondingly. There are downsides to group membership. When in groups animals are open to observation by others of their species, so information about food discoveries, nesting sites and the like becomes public information (see Chapter 79: Public and Private Information). Many parasites and diseases are much more easily passed from animal to animal when they are close together. Living in groups seems a senseless approach if it facilitates disease, so there must be counterbalancing evolutionary forces that keep groups together. In fact, so many species of animal group together that humans have come up with an entire dictionary of species-specific names for them: a murder of crows (my favorite), a caravan of camels, a parliament of owls, and so on. There must be strong evolutionary pressures that favor living in groups. Evolutionary explanations for the ultimate causes of grouping abound (see Chapter 28: 1964 Inclusive Fitness and the Evolution of Altruism, and Chapter 35: 1971 Reciprocal Altruism, for example). These explanations Conceptual Breakthroughs in Ethology and Animal Behavior. DOI: http://dx.doi.org/10.1016/B978-0-12-809265-1.00039-3 © 2017 Elsevier Inc. All rights reserved.
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Conceptual Breakthroughs in Ethology and Animal Behavior
focus attention on fitness benefits to the individual members of the group and give us ways to account for how the benefits of group membership might outweigh the costs. Another approach is to focus on the mechanisms that facilitate information flow within groups. The many eyes hypothesis allows us to consider optimal group size. For a small group, adding another pair of eyes can substantially improve the group’s ability to spot approaching predators. Adding eyes also can reduce the vigilance burden for each group member. By incorporating the value of each member of the group in adding vigilance effort, Pulliam’s (1973) work allows modeling to predict optimal group size. This approach integrates well with Hamilton’s selfish herd concept (see Chapter 36: Selfish Herds) which shows how position within the group affects the costs and benefits of grouping. Pulliam’s (1973) short note came at a critical moment, when investigators may have been leaning too much toward just-so explanations of the advantages of group living. In clear terms, with a simple mathematical model in support, Pulliam showed that the value of shared vigilance for predators, even if each animal contributes equally, rises as the group size increases and then levels off. Larger groups can arise, though, if animals join on the chance that they will achieve more sheltered central positions in the group (see chapter: Selfish Herds).
IMPACT: 2 Following Hamilton’s (1971) paper on animal groupings (see chapter: Selfish Herds) and Pulliam’s (1973) contribution about flocking, a spurt of literature on the problem of grouping and vigilance presented tests of the reasons why animals come together in groups. Lima’s (1995) work is a particularly insightful test, looking at the effect of reduced individual vigilance in some group members, due to hunger, on the responses of other group members.
SEE ALSO Chapter 28, 1964 Inclusive Fitness and the Evolution of Altruism, and Chapter 35, 1971 Reciprocal Altruism; Chapter 36, Selfish Herds; Chapter 45, 1975 Group Selection.
REFERENCES AND SUGGESTED READING Hamilton, W.D., 1971. Geometry for the selfish herd. J. Theor. Biol. 31, 295 311. Lima, S.L., 1995. Back to the basics of antipredatory vigilance-the group-size effect. Anim. Behav. 49, 11 20. Pulliam, H.R., 1973. On the advantages of flocking. J. Theor. Biol. 38, 419 422. Treves, A., 2000. Theory and method in studies of vigilance and aggregation. Anim. Behav. 60, 711 722.
1973 The Many Eyes Hypothesis THE CONCEPT Being in a group allows sharing of information about predator location and threat level. By having many eyes on sentry duty, the chances of spotting an approaching threat are increased.
THE EXPLANATION Why do animals come together in groups? Herds, schools and flocks are everyday occurrences in animal behavior. Yet their existence creates puzzles in evolutionary biology. On the surface, a group of animals is a much more concentrated and higher-value food target for a predator than individual animals would be if widely scattered in the habitat. We have already seen, in Chapter 36: 1971 Selfish Herds, that animals in groups can gain value from their association with the group. One of the explanations, the many eyes hypothesis, states that each animal needs to invest less in vigilance as group size goes up (Pulliam, 1973). As a corollary, animals should monitor how vigilant their neighbors in the group are, and adjust their behavior correspondingly. There are downsides to group membership. When in groups animals are open to observation by others of their species, so information about food discoveries, nesting sites and the like becomes public information (see Chapter 79: Public and Private Information). Many parasites and diseases are much more easily passed from animal to animal when they are close together. Living in groups seems a senseless approach if it facilitates disease, so there must be counterbalancing evolutionary forces that keep groups together. In fact, so many species of animal group together that humans have come up with an entire dictionary of species-specific names for them: a murder of crows (my favorite), a caravan of camels, a parliament of owls, and so on. There must be strong evolutionary pressures that favor living in groups. Evolutionary explanations for the ultimate causes of grouping abound (see Chapter 28: 1964 Inclusive Fitness and the Evolution of Altruism, and Chapter 35: 1971 Reciprocal Altruism, for example). These explanations Conceptual Breakthroughs in Ethology and Animal Behavior. DOI: http://dx.doi.org/10.1016/B978-0-12-809265-1.00039-3 © 2017 Elsevier Inc. All rights reserved.
121
122
Conceptual Breakthroughs in Ethology and Animal Behavior
focus attention on fitness benefits to the individual members of the group and give us ways to account for how the benefits of group membership might outweigh the costs. Another approach is to focus on the mechanisms that facilitate information flow within groups. The many eyes hypothesis allows us to consider optimal group size. For a small group, adding another pair of eyes can substantially improve the group’s ability to spot approaching predators. Adding eyes also can reduce the vigilance burden for each group member. By incorporating the value of each member of the group in adding vigilance effort, Pulliam’s (1973) work allows modeling to predict optimal group size. This approach integrates well with Hamilton’s selfish herd concept (see Chapter 36: Selfish Herds) which shows how position within the group affects the costs and benefits of grouping. Pulliam’s (1973) short note came at a critical moment, when investigators may have been leaning too much toward just-so explanations of the advantages of group living. In clear terms, with a simple mathematical model in support, Pulliam showed that the value of shared vigilance for predators, even if each animal contributes equally, rises as the group size increases and then levels off. Larger groups can arise, though, if animals join on the chance that they will achieve more sheltered central positions in the group (see chapter: Selfish Herds).
IMPACT: 2 Following Hamilton’s (1971) paper on animal groupings (see chapter: Selfish Herds) and Pulliam’s (1973) contribution about flocking, a spurt of literature on the problem of grouping and vigilance presented tests of the reasons why animals come together in groups. Lima’s (1995) work is a particularly insightful test, looking at the effect of reduced individual vigilance in some group members, due to hunger, on the responses of other group members.
SEE ALSO Chapter 28, 1964 Inclusive Fitness and the Evolution of Altruism, and Chapter 35, 1971 Reciprocal Altruism; Chapter 36, Selfish Herds; Chapter 45, 1975 Group Selection.
REFERENCES AND SUGGESTED READING Hamilton, W.D., 1971. Geometry for the selfish herd. J. Theor. Biol. 31, 295 311. Lima, S.L., 1995. Back to the basics of antipredatory vigilance-the group-size effect. Anim. Behav. 49, 11 20. Pulliam, H.R., 1973. On the advantages of flocking. J. Theor. Biol. 38, 419 422. Treves, A., 2000. Theory and method in studies of vigilance and aggregation. Anim. Behav. 60, 711 722.