1. Amoroso, E. C., and Marshall, F. H. A., in Marshall's Physiology of Reproduction, third ed., 1, part 2 (edit. by Parkes, A. S.), 707 (Longmans Green, London, 1960).
2. Sadleir, R. M. F. S., The Ecology of Reproduction in Wild and Domestic Mammals (Methuen, London, 1969).
3. Asdell, S. A., Patterns of Mammalian Reproduction, second ed. (Cornell University Press, Ithaca, 1964).
4. Bullough, W. S., Vertebrate Reproductive Cycles, second ed. (Methuen, London, 1961).
5. Clegg, M. T., and Ganong, W. F., in Reproduction in Domestic Animals, second ed. (edit. by Cole, H. H., and Cupps, P. T.), 473 (Academic, New York, 1969).