Descending Sloth

John Maynard Smith

  • The Mammalian Radiations: An Analysis of Trends in Evolution, Adaptation and Behaviour by John Eisenberg
    Athlone, 610 pp, £32.00, December 1981, ISBN 0 485 30008 7

‘Natural history’ is seen by some professional biologists as hardly deserving to be regarded as a part of science. Compared to the experimental sophistication of molecular biology, and the apparent generality of its conclusions, natural history is no more than a collection of particular facts of little theoretical or practical import. There are two reasons why one should dissent from this judgment. The first is that the task of biology is to explain the living world, and that world is irreduciby complex. If I am interested in molecular biology – and I am – it is because it helps to explain how that complexity arose. A biologist who was not interested in the diversity of living things would be like a historian who did not care what had actually happened in history, and confined his attention to the study of experimental psychology.

The second reason for taking natural history seriously is that the central theoretical idea in biology is that of evolution by natural selection, and this idea was formulated by men who were naturalistss first and evolutionists second. Charles Darwin collected beetles before he embarked in the Beagle, and it was only in thinking about the experiences of that voyage that he became an evolutionist. Alfred Wallace differed in that he was an evolutionist (although he had not conceived the mechanism of natural selection) before he undertook his journeys to South America and Malaysia, but it was his experience as a naturalist in this country which made him an evolutionist. A knowledge and love of natural history were not sufficient to bring Darwin and Wallace to the theory of natural selection, but they were necessary.

Although Darwin, in The Descent of Man and Selection in relation to Sex, and in The Expression of the Emotions in Man and Animals, made a decisive start on the study of the evolution of behaviour little further progress was made during the next fifty years, and it is only in the last fifty that the subject has come of age. That it has done so we owe in part to the ethologist, in particular Lorenz and Tinbergen, who taught us to look at what animals actually do and to ask questions about how and why they do it, and in part to men like Lack and MacArthur, who related the observed behaviour of particular species to their ecology, and to the selective forces acting on them. It is a striking fact that all these four men, and many of their immediate followers, worked primarily on birds (for the sake of my theme, I shall forget Tinbergen’s classic work on sticklebacks). Birds are conspicuous diurnal animals, and they exist in a rich variety of species in Europe and North America.

In contrast, mammals – about which we might be expected to have a more immediate curiosity – are much more difficult to study in the field, particularly if one happens to live in Europe. Most mammals are nocturnal. Often they rely on means of communication (scents; high-frequency sounds) which we find harder to perceive than the songs and plumage displays of birds. Many of them live underground, or in the forest canopy. The number of species living in Europe is relatively small. Yet despite all these difficulties, knowledge of mammalian behaviour has grown at an astonishing rate, particularly during the last ten to fifteen years. This has in part depended on field studies in the tropies: we know more about the behaviour of lions than we do of badgers. It has also been helped by technical advances – for example, the development of miniature radio transmitters which can be attached to animals as small as bats and mice without serious inconvenience. But in the main it has been achieved by the determination, ingenuity and endurance of a band of field naturalists. As a largely chairbound biologist, I am repeatedly amazed at the quality of the information gathered by colleagues.

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