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/kg0.75 initial liveweight per day throughout 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 intervals (p < 0.001) and higher (p < 0.001) mean rates of ingestion (g/min). In both groups mean plasma concentrations of prolactin, T3, T4, and insulin-like growth factor 1 (IGF-1) were higher (p < 0.001) during long daylength than short daylength although changes in thyroid hormone profiles were much less marked in AL animals. Insulin and growth hormone (GH) profiles exhibited no consistent seasonal trend. Mean plasma concentrations of T3 were higher in AL than in R animals. Mean plasma IGF-1 concentrations during long days were consistently greater in the AL than R animals. It is concluded that 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 patterns and eating rates. It is concluded that the roles of T3, IGF-1, and prolactin in the expression of seasonal changes in appetite should be investigated further and, particularly, their effects on other hormone profiles and liver and gut function.