Food and winter hypometabolism of red deer

Red deer (Cervus elaphus) live in temperate environments where they face a large seasonal fluctuation in food supply and thermal conditions. Like many temperate animals, red deer rely extensively on their accumulated body energy stores over winter and adaptations for energy savings are therefore paramount for survival. Indeed, northern ungulates show a strong seasonal change in metabolic rate, which is in red deer up to 60% lower in winter than spring and summer. As food digestion causes a subsequent increase in metabolic rate (heat increment of feeding), higher food intake contributes partly to a higher metabolic rate in summer. However, lower food intake in winter is not simply a function of lower food availability, but is also caused by an endogenous reduction in appetite. An endogenous circannual rhythm is well known to act via hormonal pathways in cueing numerous winter adaptations, such as pre-winter fattening, growing a thick pelage, and reducing activity. It was recently shown that red deer regularly enter daily bouts of much-reduced metabolic rate and substantially lowered peripheral body temperature in response to cold and energy stress during winter. The use of a temporary reduction in metabolic heat production and reversible hypothermia, similar to daily torpor in small mammals, is a significant mechanism for winter energy savings in red deer and possibly in other non-hibernating temperate endotherms. The proposed study will examine how changes in environmental conditions, such as food availability and protein content of food, interact with an endogenous seasonal rhythm, to trigger daily bouts of hypometabolism by red deer. We will manipulate food quantity and quality available to individual deer kept in a large enclosure under semi-natural conditions. Sixteen female red deer will receive an alternate monthly ration of either food ad libitum or reduced food quantity at an automated feed and weighing station. Furthermore, eight deer will receive a high protein diet, and the other group a low protein diet, with diets switched between the two years of the experiment. In addition to food intake at the station, we will estimate intake from natural vegetation using the n-alkane method, enabling us to quantify total volume and quality of food intake. We will estimate metabolic rate using transmitters residing in the reticulum of deer to detect heart beat vibrations, and also measure core body temperature. These data are stored and further transmitted to an automated computer logging station at the adjoining research institute by an electronic unit located in a collar, which also measures activity. We will use mixed effects models with `individual` as a random factor to examine the effects of food restriction, food protein content, daily climatic variables, body mass and season on heart rate (as a proxy for metabolic rate and energy expenditure) and body temperature of the deer. We will accordingly analyse the frequency, extent and interrelations of daily bouts of hypometabolism and temporary hypothermia by red deer. We expect our study to significantly improve our understanding of the governing cues and physiological mechanisms for energy conservation by large non- hibernating endotherms in harsh temperate climates.

Red deer (Cervus elaphus) live in temperate environments where they face a large seasonal fluctuation in food supply and thermal conditions. Like many temperate animals, red deer rely extensively on their accumulated body energy stores over winter and adaptations for energy savings are therefore paramount for survival. Indeed, northern ungulates show a strong seasonal change in metabolic rate, which is in red deer up to 60% lower in winter than spring and summer. As food digestion causes a subsequent increase in metabolic rate (heat increment of feeding), higher food intake contributes partly to a higher metabolic rate in summer. However, lower food intake in winter is not simply a function of lower food availability, but is also caused by an endogenous reduction in appetite. An endogenous circannual rhythm is well known to act via hormonal pathways in cueing numerous winter adaptations, such as pre-winter fattening, growing a thick pelage, and reducing activity. It was recently shown that red deer regularly enter daily bouts of much-reduced metabolic rate and substantially lowered peripheral body temperature in response to cold and energy stress during winter. The use of a temporary reduction in metabolic heat production and reversible hypothermia, similar to daily torpor in small mammals, is a significant mechanism for winter energy savings in red deer and possibly in other non-hibernating temperate endotherms. The proposed study will examine how changes in environmental conditions, such as food availability and protein content of food, interact with an endogenous seasonal rhythm, to trigger daily bouts of hypometabolism by red deer. We will manipulate food quantity and quality available to individual deer kept in a large enclosure under semi-natural conditions. Sixteen female red deer will receive an alternate monthly ration of either food ad libitum or reduced food quantity at an automated feed and weighing station. Furthermore, eight deer will receive a high protein diet, and the other group a low protein diet, with diets switched between the two years of the experiment. In addition to food intake at the station, we will estimate intake from natural vegetation using the n-alkane method, enabling us to quantify total volume and quality of food intake. We will estimate metabolic rate using transmitters residing in the reticulum of deer to detect heart beat vibrations, and also measure core body temperature. These data are stored and further transmitted to an automated computer logging station at the adjoining research institute by an electronic unit located in a collar, which also measures activity. We will use mixed effects models with "individual" as a random factor to examine the effects of food restriction, food protein content, daily climatic variables, body mass and season on heart rate (as a proxy for metabolic rate and energy expenditure) and body temperature of the deer. We will accordingly analyse the frequency, extent and interrelations of daily bouts of hypometabolism and temporary hypothermia by red deer. We expect our study to significantly improve our understanding of the governing cues and physiological mechanisms for energy conservation by large non- hibernating endotherms in harsh temperate climates.