The influence of photoperiod and melatonin on testis size, body weight, and pelage colour in the Djungarian hamster (Phodopus sungorus)

Summary

The responses of testes, body weight, and pelage colour to short and long photoperiods in winter were determined in adult male hamsters with and without the implantation of melatonin. Animals in winter condition, with involuted testes and winter pelage, were kept at 20 °C under conditions of either long (16 h per day) or short (8 h per day) photoperiods beginning on 2 January. In each condition one group was implanted three times at weekly intervals with melatonin in beeswax, a control group was implanted with beeswax only, and another control group was left untreated. A further control group remained in natural day light. After 37 days testes of the control groups in long photoperiods had reached summer condition, while the group treated with melatonin was delayed in testicular development, and closely resembled both the three shortday groups and the group kept in natural daylight (Fig. 2–4). In short photoperiods there was no difference between the group treated with melatonin and the two control groups. All groups showed some testis development as compared to animals killed at the beginning of the experiment. Hamsters kept under natural daylight showed a marked annual cycle of body weight which closely paralleled gonadal activity (Fig. 5). In the experimental groups there was a corresponding increase in body weight paralleling testicular development (Fig. 6). The two control groups in long photoperiods had a significantly higher increase in body weight than all other groups, while there were no significant differences between the groups treated with melatonin, the two short-day groups and the group under natural daylight. Testis size at the end of the experiment was highly correlated with increase in body weight (Fig. 7, and Table 1).

Molt into summer pelage had started in all groups at the end of the experiment. Colour change was most advanced in the two control groups under long photoperiods, while the long-day group treated with melatonin resembled the short-day groups (Figs. 9 and 10).

It is concluded that the change in physiological state from winter to summer is based on an endogenous mechanism, which is accelerated by long photoperiods, and that melatonin inhibits or greatly diminishes this acceleration while it does not inhibit spontaneous development towards summer condition.