Effects of photoperiod, melatonin implants and castration on molting and on plasma thyroxine, testosterone and prolactin levels in the european badger (Meles meles)

Abstract 1. 1. The seasonal molt, which lasts six months in the badger, begins in mid-July and ends at the beginning of winter. It occurs under natural long-day conditions, following the seasonal drop in plasma testosterone levels, concomitant with high levels of thyroxine and prolactin. 2. 2. To examine the role of the different factors involved (day length, prolactin, thyroxine, testosterone), different groups of badgers, divided into subgroups of castrated or intact animals, were subjected to the influence of long days (20L: 4D), short days (54L: 20D) or the effect of subcutaneous melatonin implants. 3. 3. In all cases, castration resulted in a significantly earlier onset of molting 1–3 months, depending on the group, regardless of the experimental conditions (20L:4D, 4L:20D, melatonin). 4. 4. However, molting started earliest in animals subjected to long days, irrespective of whether they were castrated or intact. 5. 5. In the melatonin-implanted badgers, molting started either early (castrated animals), or late or not at all (intact animals). 6. 6. Lastly, in castrated badgers subjected to experimental photoperiods (short days or long days) or melatonin implants, the period of molting was shortened from 6 months (intact outdoor animals) to 4 months. 7. 7. The advance in shedding was always related to an early drop in testosterone (or an absence of testosterone in the castrated animals) and to a higher or earlier increase in thyroxine levels. 8. 8. The role of prolactin is more difficult to define, but appears to play a synergic role favoring the molt; however, prolactin blockage at low levels (in intact animals subjected to short days or in castrated melatonin-implanted animals) did not prevent the seasonal molt from occurring, and it was even significantly earlier when the animals were castrated. 9. 9. Nevertheless, the molt was clearly promoted by high levels of thyroxine and prolactin, combined with low levels of testosterone.