Photoperiodic control of testis activity in the vole, Microtus agrestis.

Under natural conditions the vole (Microtus agrestis) breeds in the spring and summer but not in the autumn and winter. This seasonal change was attributed by Baker & Ranson (1933) to the influence of hours of sunshine, and in the laboratory it has been shown that photoperiod affects the growth ofgonads and maintenance of their activity (Baker & Ranson, 1932; Clarke & Kennedy, 1967; Breed & Clarke, 1970). Testes of voles living in 6 hr light/day are smaller and less active than those from animals given 15 or 16 hr light daily. Essentially similar effects of photoperiod have been observed in Microtus arvalis (Martinet, 1963). In the hope of gaining insight into the underlying processes by which altered photoperiod is perceived, experiments have been carried out to discover which part of the light/dark cycle causes changes in gonadal activity.