Long and Short Term Changes in Gonadal Activity of Field Voles and Bank Voles

Seasonal breeding in field voles Microtus agrestis and bank voles Clethrionomys glareolus, involving long-term gonadal changes, is brought about by alterations in the pituitary content of gonadotrophins. Photoperiod is the most important environmental factor initiating these changes. In voles there appears to be an endogeneous circadian rhythm of sensitivity to light whose peak may be 12 hours after dawn. Day length measurement seems also to involve the pineal gland. Other factors probably modify photoperiod effects: light energy, plane of nutrition, ambient temperature, social influences, genotype. Bearing in mind these modifying factors, breeding seasons could vary in length according to the phase of the population cycle. Extrapolation from laboratory studies suggests that social factors could influence short-term reproductive changes, such as ovulation, corpus luteum function and pregnancy. First pregnancy in sexually mature bank voles may require humoral priming of the reproductive tract. Field voles and bank voles readily conceive post partum, but during lactation there is reduction in fertility which is lifted towards the end of the suckling period.