Physiological limits to nergy turnover in mammals

Traditionally, animals were considered to be energetically limited mainly by food availability in their environment. However, studies on mammals and birds over the past two decades have clearly demonstrated that, even when food is abundant, animals may encounter endogenous physiological limitations of energy turnover during periods of high nutrient demands, such as lactation. It is still not fully understood whether these maximum sustainable metabolic rates (SusMR) are due to "peripheral" limits, i.e. the capacity of energy expending organs (skeletal muscles, mammary glands) or to "central" limits, namely the capacity of the alimentary tract. Although there is currently more evidence - derived mainly from experiments on laboratory mice - in favour of a peripheral limitation, there are still indications for central limits from other studies. Moreover, it is unclear whether the same type of constraint is responsible for energetic limitations in all mammals. We propose to investigate sustainable energy turnover in a herbivorous mammal, the European brown hare, and to address the question of central vs. peripheral limitation during lactation in this species. Hares provide an excellent model for these studies, mainly for two reasons: A central limitation, if it exists, will likely occur in herbivorous mammals, such as the hare, which require large gastrointestinal tracts. Moreover, in hares energy expenditure for milk production can be determined both easily and accurately. Because hares nurse their offspring only once a day for several minutes, daily milk output can be measured directly by weighing juveniles just before and after suckling, and by determining milk energy content. Thus, lactational performance can be assessed directly, rather than from juvenile growth, which can be affected by a multitude of variables. Experimentally, we will approach our aims by feeding females with high energy requirements (with a constant litter size of three young) two diets of different energy content during lactation. We hypothesize that, if central rates of energy assimilation are limiting, lactating females will be unable to fully compensate for a low-energy diet by increasing their food intake and/or gastrointestinal tract. Specifically, we hypothesize that females offered a low- energy diet may be unable to increase the size and capacity of their alimentary organs rapidly enough to reach optimal milk production. We feel that these studies will significantly enhance our understanding of the causes and consequences of limits to sustainable energy turnover in mammals. These insights may help to improve current methods in sports-medicine, animal feeding practices, and methods of meat production.

Traditionally, animals were considered to be energetically limited mainly by food availability in their environment. However, studies on mammals and birds over the past two decades have clearly demonstrated that, even when food is abundant, animals may encounter endogenous physiological limitations of energy turnover during periods of high nutrient demands, such as lactation. It is still not fully understood whether these maximum sustainable metabolic rates (SusMR) are due to "peripheral" limits, i.e. the capacity of energy expending organs (skeletal muscles, mammary glands) or to "central" limits, namely the capacity of the alimentary tract. Although there is currently more evidence - derived mainly from experiments on laboratory mice - in favour of a peripheral limitation, there are still indications for central limits from other studies. Moreover, it is unclear whether the same type of constraint is responsible for energetic limitations in all mammals. We propose to investigate sustainable energy turnover in a herbivorous mammal, the European brown hare, and to address the question of central vs. peripheral limitation during lactation in this species. Hares provide an excellent model for these studies, mainly for two reasons: A central limitation, if it exists, will likely occur in herbivorous mammals, such as the hare, which require large gastrointestinal tracts. Moreover, in hares energy expenditure for milk production can be determined both easily and accurately. Because hares nurse their offspring only once a day for several minutes, daily milk output can be measured directly by weighing juveniles just before and after suckling, and by determining milk energy content. Thus, lactational performance can be assessed directly, rather than from juvenile growth, which can be affected by a multitude of variables. Experimentally, we will approach our aims by feeding females with high energy requirements (with a constant litter size of three young) two diets of different energy content during lactation. We hypothesize that, if central rates of energy assimilation are limiting, lactating females will be unable to fully compensate for a low-energy diet by increasing their food intake and/or gastrointestinal tract. Specifically, we hypothesize that females offered a low- energy diet may be unable to increase the size and capacity of their alimentary organs rapidly enough to reach optimal milk production. We feel that these studies will significantly enhance our understanding of the causes and consequences of limits to sustainable energy turnover in mammals. These insights may help to improve current methods in sports-medicine, animal feeding practices, and methods of meat production.