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On the Move: How and Why Animals move in groups. Sue Boinski and Paul A. Garber, eds. Chicago University Press, Chicago, 2000, 811 pp. (ISBN 0-226-06339-2).
This is a book mainly about primates for primatologists, although five of the 22 essays are on other animals.
In the field, we distinguish socially coherent groups of animals from mere aggregations because groups move together from place to place, while individual animals join an aggregation at a resource, and individually leave when satiated. Thus the topics of the title potentially encompass all social behaviour. Why the subjects of the chapters move seems repetitively simple: like individuals, groups move from one food source to the next, avoiding the risk of predation as far as possible, and being constrained by the movements of adjacent groups. There is general agreement that individuals in a group are somewhat protected from predators, while being somewhat constrained in their own foraging by staying in the group. Young or newly arrived group members may gain in foraging efficiency by following others which already know the terrain.
The "how" questions are more challenging. Decisions must be made about which route to take. Do the animals have a cognitive map of their home range, and if so what sort of map is it? The primates described here feed for the most part on fruit and shoots and insects. Much has been made of the complexity of a forest environment with many fruiting trees bearing at different seasons, and the bigger brain needed to handle it (R. A. Barton), so that F. C. Dyer's chapter on the movement of swarms of bees and ants, in which very small brains coordinate the movements of exceedingly large numbers of individuals may be seen as a useful corrective. For the forest primates I think the difficulty of the problem has been exaggerated by observers who fail to appreciate the time scale. Fruiting trees stay put, and will provide a resource for many generations of monkeys; in each generation a few trees will be lost and a few added, but the continuity of the group, and the longevity of the overlapping generations of its members, make possible a detailed knowledge of the resources and dangers in a home range, and how they change through the seasons, which is perhaps difficult for a briefly visiting observer to appreciate. Jansen's paper illustrates another possible limitation, in using, for the most part a model of foraging in which a group moves from one large fruiting tree to the next, all animals feeding on the one resource together. Monkeys I have observed rarely all ate in the same tree, even a large one, many foraged in adjacent trees for a range of foods at the same time. This may reflect differences in forest ecology or between monkeys, but it does suggest caution about generalising from over simple models.
On the other hand, fruit and shoot-eating primates share the problem general to all herbivores: while possible foods stay put, they vary in dietary value, not only from time to time but also, often, from plant to plant of the same species. Thus animals must take small amounts from a variety of sources to both acquire a balanced diet and avoid too many toxins. In contrast, flesh of any other animals has a similar, high food value, if you can catch it. Thus, comparing the movements of groups of primates with those of pack-hunting land carnivores like hyenas, or with dolphins which hunt pelagic fish, or even with insectivorous birds following ant caravans, is not likely to be particularly illuminating because their movements are in response to such very different needs. Milton's comparison with parrots was more promising, but too little is known about parrots to take it very far. Besides, parrots and bats fly between feeding sites, and so have an entirely different energetic balance. Another useful comparison would have been with herding ungulates; for example elephants have been studied in comparable ways. The concluding chapter by the editors gives rather a mixed message: there doesn't seem to be any evidence, from group movement patterns, of superior cognitive ability between primates or for primates relative to other group living mammals, but there must be really, because primates have big brains—and we are primates too.
The key chapter of the book is that by D. S. Wilson, expounding multi-level selection. The subject has been stalled for decades because of a semantic confusion, between "groups" that are more or less closed demes, as defined by population geneticists, and "groups" as the social entities we actually observe, whose members may or may not be closely related, and which exchange members with other groups within a much wider breeding population. For a long time the population geneticists' criteria have been applied to the social entities, and "group selectionist" has been a term of abuse. Recognition that selection occurs at every level of complexity simultaneously should stimulate some exciting new approaches. Wilson proposes that any behaviours of a group-living animal, which does not, apparently, give it some immediate advantage, should be examined as possible evidence for selection occurring at the supra-individual level of the group. He gives as an example route-proposing behaviour, described in the volume in several primate species, as well as honeybees. The remaining essays in the book can all be read in the light of Wilson's, and this approach can propel the reader through this very thick book. Does each provide evidence of group-level selection, on the anatomy, physiology, and behaviour of the individuals, as well as on the behaviour of the group as a whole? I would say yes, that there are many examples of possible mechanisms of selection on both why and how groups move, although this is not how the authors present them.
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