1. 1.

    A simple optical method was used to observe speed, stride length and stepping frequency in undisturbed walking or running wild animals. Most measurements were made on the gnu (adult and young calf) and Thomson's gazelle.

  2. 2.

    The hypothesis that stepping frequency is independent of speed is not tenable for any gait.

  3. 3.

    The data are discussed in terms of N, the average number of feet in contact with the ground. It is suggested that the value required of N by the animal is the main consideration determining its choice of gait.

  4. 4.

    The height of the vertical jumps in galloping does not need to increase indefinitely with speed, and the power needed for the jumps is not likely to limit the maximum galloping speed.

  5. 5.

    Stepping frequency observations in the walk, trot and canter were analysed for 14 mammal species plus the gnu calf. Stepping frequency was found to vary with about the - 0.5 power of the linear dimensions in all three gaits.

  6. 6.

    It is suggested that only three gaits should be recognized, the walk, trot and canter, and that they should be distinguished on the basis of (a) the symmetry or otherwise of the stepping pattern, (b) the range of N over which the gait can be used, and (c) the nature of the energy transformations which occur at each step.

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