The annual breeding cycle of the high-arctic Svalbard ptarmigan (Lagopus mutus hyperboreus).

Gonadal size, spermatogenesis, egg production, height of the supraorbital comb, moult, and plasma luteinizing hormone (LH) levels were measured during an annual cycle in wild Svalbard ptarmigans (Lagopus mutus hyperboreus). These birds live permanently at 77-81 degrees N where daylight is continuous from the beginning of April until mid September while the average ambient temperature is above freezing only between early June and mid September. The gonads and comb size began to enlarge in March coinciding with an increase in plasma LH, but sperm production and egg laying did not begin until the end of May and early June, respectively. This long lag between the onset of reproductive functions and the time of breeding reflects a slow rate of gonadal growth which may be caused by nonphotoperiodic, inhibitory input from the environment. Gonadal regression occurred in July and was associated with a steep decline in plasma LH levels indicating that the birds became long-day refractory. A moult from white to pigmented plumage occurred in females at the beginning and in males towards the end of the breeding season. This observation is consistent with the notion of an inhibitory effect of testosterone on moult in the males. Both sexes moulted simultaneously into white plumage in late September/early October.