Factors affecting the breeding of the field mouse ( Microtus agrestis ). Part I.—Light

Abstract

The investigations described in this series of papers were partly experimental and partly observational. The first two papers will deal with the experimental work, and the third with the observational. The observational work has shown that Microtus agrestis has a definite breeding season in the British Isles, extending roughly from March till September. This species appeared to be a suitable one on which to experiment for the purpose of finding the factors which control the breeding season, a subject on which so little is known in any mammal. Our immediate object has been to find the factors, and not to find how they work. There seem to be two sorts of way in which they might work. Firstly, one could imagine that it might be physiologically impossible for an animal like Microtus to breed below a certain temperature, or in the absence of a certain foodstuff. There is, however, a second, quite different way in which such factors might act. It is possible that during the course of evolution Microtus might have evolved an adaptive response of stopping and starting reproduction when the temperature and food altered with the seasons beyond certain limits. One could imagine the natural selection of the offspring of those individuals which responded to the environment by breeding only at the most appropriate time for the rearing of the young. In our preliminary investigations we have not attempted to distinguish between these two possible modes of action of factors controlling the breeding season. Our object has been to find the factors, irrespective of the way in which they work. We have regarded the problem from the biological rather than the physiological point of view. No doubt we could stop Microtus from breeding by depriving it of vitamin E, or subjecting it to extremes of temperature ; but our experiments were planned in such a way that our animals were never subjected to unnatural conditions. They were never fed on unnatural foods, nor kept at higher or lower temperatures than occur naturally in Great Britain, nor given longer or shorter periods of light, nor more intense light. We were not anxious simply to stop and start the reproduction of Microtus at will, but to find what are the natural factors which control its breeding season in a state of nature.