Video and cine films of mammals running at the trot-gallop transition were analysed to measure breathing frequencies. Breathing frequency at the trot-gallop transition (fb, in Hz) was shown to decrease with increasing body mass (M, in kg) and was described by the equation fb = 5.08 M-0.14. The stiffness of the thorax and diaphragm of mice, rats, rabbits and wallabies was calculated and this, together with the mass of the viscera, was used to calculate the natural frequency of the system (nft, in Hz). The relationship between nft and body mass can be described by the equation nft = 5.02M-0.18. The significance of these results is discussed in relation to models of mechanical linkage between respiratory and locomotory movements.

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