Seasonal changes of metabolism and appetite in Soay rams

Relationships between photoperiod and cycles of voluntary food intake (VFI) and maintenance energy requirements (MER) were determined in Soay rams, subjected to a 6-month photoperiodic regime. Food was offered ad libitum (no. = 5) or at a predicted maintenance level (no. = 5). All rams demonstrated 6-month cycles of VFI, growth and reproductive status. Metabolizable energy intake (MEI) was greatest in rams given food ad libitum (666 (s.e. 21·7) kJ/kg metabolic live weight (M^0·75) and food-restricted (528 (s.e. 12·2) kJ/kg M^0·75) rams during sexual quiescence. Conversely, MEI was minimal (ad libitum, 289 (s.e. 8·4) kJ/kg M^0.75; restricted, 428 (s.e. 8·1) kJ/kg M^0·75) during the rut. Distinct cycles of heat production (HP) accompanied changes in MEI. Changes in HP were similar (P > 0·05) for both groups (ad libitum, 520 (s.e. 9·2) to 394 (s.e. 9·2) kJ/kg M^0·75; restricted 503 (s.e. 14·0) to 407 (s.e. 17·5) kJ/kg M^0·75) and therefore energy retention varied more (P < 0·015) when rams were given food ad libitum (ad libitum, 131 (s.e. 43·1) to-106 (s.e. 38·2) kJ/kg M^0·75; restricted, 78·0 (s.e. 27·1) to -53·0 (s.e. 38·2) kJ/ kg M^0·75). Apparent digestibility of dietary energy varied inversely with MEI (P < 0<.01). MERs ranged from 524 (s.e. 35<0) kJ/kg M^0.75 to 401 (s.e. 27·3) kJ/kg M^0·75, a proportional fluctuation of ±0·13. Changes in metabolic rate preceded those in appetite, suggesting a causal relationship.