Seasonality of meal patterns and hormonal correlates in red deer

Two groups of six adult, castrated, male red deer were housed under natural daylength conditions and at ambient temperature at 57 degrees N and fed ad lib. (AL) or at a fixed rate of 50 g/kg(0.75) initial liveweight per day t hroughout the study (restricted, R). Mean daily intakes of AL animals were higher during periods of long daylength than during short daylength (p < 0. 001). The higher rates of food intake during periods of long daylength were a function of greater meal durations (p < 0.001 shorter inter-meal interva ls (p < 0.001) and higher (p < 0.001) mean rates of ingestion (g/min). In b oth groups mean plasma concentrations of prolactin, T-3, T-4, and insulin-l ike growth factor 1 (IGF-1) were higher (p < 0.001) during long daylength t han short daylength although changes in thyroid hormone profiles were much less marked in AL animals. Insulin and growth hormone (GH) profiles exhibit ed no consistent seasonal trend. Mean plasma concentrations of T-3 were hig her in AL than in R animals. Mean plasma IGF-1 concentrations during long d ays were consistently greater in the AL than R animals. It is concluded tha t the effects of seasonal changes in daylength on appetite and food intake are expressed through changes in both the duration of daylight periods per se and in underlying seasonal changes in physiology and associated meal pat terns and eating rates. It is concluded that the roles of T-3, IGF-1, and p rolactin in the expression of seasonal changes in appetite should be invest igated further and, particularly, their effects on other hormone profiles a nd liver and gut function. (C) 1998 Elsevier Science Inc.