Effect of light entrainment and temperature on the reproductive cycle in the male hedgehog (Erinaceus europaeus)

Abstract

The role of photoperiod in the entrainment and synchronization of the reproductive cycle of male hedgehogs, seasonal breeders and hibernating mammals, was investigated. Groups of adult hedgehogs were either maintained outdoors (controls, n = 6) or submitted to accelerated 6-month artificial light regimens under constant ambient temperatures (20 ± 2°C versus 5 ± 1°C) in light-proofed rooms. The daily duration of light was varied sinusoidally to produce an amplitude change from 8 h (winter solstice) to 16 h (summer solstice) during the 6-month light cycle. Animals were transferred from outdoors to a high ambient temperature (20 ± 2°C) and submitted to accelerated 6-month light regimens at two times of the year: from winter solstice (Group 1, n = 14) with increasing daylengths (from 8 to 16 h) and from summer solstice (Group 2,n = 8) with decreasing daylengths (from 16 to 8h). The light regimens were then reversed for Groups 1 and 2. After the first 6-month cycle, the animals in Group I were allocated to two groups and maintained under the same initial light regimen but submitted to two ambient temperatures: Group 1 (n = 7) was maintained at 20 ± 2°C and Group 3 (n = 7) was transferred to a cold environment (5 ± 1°C). In control and experimental animals, testicular volume was estimated and blood samples were obtained twice a month to measure plasma testosterone and LH concentrations by radioimmunoassay. In all groups, all the parameters of the reproductive activity studied (testicular volume, testosterone and LH concentrations) were entrained and synchronized by the 6-month light rhythm and two cycles were observed in a year. Reproductive activity was maximum during the long days (light > 12 h) and minimum during the short days (light < 12 h). In the experimental animals and in the controls, the amplitude of variations in the parameters studied were similar. The recrudescence of reproductive activity took place just before the artificial spring equinox (short and increasing daylengths), whereas regression always occurred near the autumn equinox (12 h light:12 h dark), as in the controls kept in a natural environment. The regular incidence of involution at the autumn equinox indicates that there is a period of photosensitivity to decreasing daylengths in summer. In the experimental animals, the resting season was usually 2 months. A comparison of Groups 1 and 2 that had undergone reversed light regimens also showed that the reproductive parameters were driven in opposition. In Groups 1 and 3, no significant effect of ambient temperature (high or low) on the entrainment of the reproductive cycle by the photoperiodic rhythm was observed. These results clearly indicate that photoperiod is of prime importance among the environmental factors controlling reproduction in hedgehogs and can entrain and synchronize the seasonal changes of the reproductive cycle.