Endocrine changes associated with fat deposition and mobilization in svalbard ptarmigan (Lagopus mutus hyperboreus)

Plasma concentrations of Triiodothyronine (T3), thyroxine (T4) and growth hormone (GH) have been measured in blood samples taken from Svalbard ptarmigans (Lagopus mutus hyperboreus), shot throughout 1 whole year at Svalbard (79 degrees N). Plasma T3 levels varied in a monophasic pattern with low levels in winter and a peak in August, whereas plasma T4 levels remained constant throughout the year. High plasma T3 levels coincide with molt and a large food intake while low plasma levels of T3 coincide with molt arrest and a low food intake. Plasma GH levels were highest in winter and lowest in May and September. The low plasma GH levels in early autumn coincide with elevated liver weights and maximum rate of fat deposition. High GH levels in midwinter coincide with low liver weights and the mobilization of fat stores. A possible relationship between molt, food intake, fat deposition/mobilization, and plasma levels of T3 and GH is discussed.