Food and seasonal adaptation in red deer

Herbivores of the boreal and temperate zones like red deer (Cervus elaphus) are exposed to strong seasonal variations of both abiotic environmental conditions and food availability. In order to cope with these conditions, northern ungulates have evolved a number of adaptations, e.g. seasonality of reproduction, growth and fattening, and a well insulating winter fur. These reactions are known to be under photoperiodic control though evidence is growing that additional cues like the availability and quality of food may govern fine-tuning and ensure flexibility. However, our knowledge about the relative importance of various environmental signals and their integrated processing is rather limited. In addition, it is still a matter of debate which mechanisms contribute to considerably lower metabolic rates of northern ungulates during winter. We found in a preliminary study that red deer are able to substantially decrease metabolic rate by undergoing nocturnal episodes of peripheral hypothermia to a so far unknown extent. Nevertheless, our results also indicated that annual changes in metabolic rate are primarily due to seasonal changes of the quality of natural forage. We therefore intend to investigate experimentally to which extent protein and energy content of food (1) act as zeitgeber for seasonal adaptions, (2) influence seasonal changes of metabolic rate, and (3) affect the occurrence of episodes of peripheral hypothermia. We propose a three year study on free-ranging red deer kept in a large enclosure under semi-natural conditions. Differential supplemental feeding ad lib. of two spatially not separated groups via a computer controlled feeding station with three sorts of pellets reflecting seasonal maximum and minimum in energy and protein content of natural forage, respectively, will reveal the influence of food quality on seasonal adaptation. Deer will be equipped with especially designed telemetry transmitters for continuous and automatic recording of heart rate as an indicator of metabolic activity, of subcutaneous temperature, and of locomotor activity. Furthermore, natural herbage intake, digestibility, energy intake and climate conditions will be measured. The results of this study will provide fundamental insights into mechanisms of seasonal changes of metabolic rates and cues involved in their regulation. Furthermore, the results will be relevant for wildlife management since feeding of free-ranging red deer during winter is common. Possible disturbances of the reduction of energy requirements during winter by inadequate supplemental food may importantly contribute to high damages of forest vegetation by red deer.

Herbivores of the boreal and temperate zones are exposed to strong seasonal variations of both ambient temperatures and food availability. We found in a previous study that red deer are able to substantially decrease metabolic rate by undergoing nocturnal episodes of peripheral hypothermia to a so far unknown extent. Such reactions are known to be under photoperiodic control though evidence is growing that additional cues like the availability and quality of food may govern fine-tuning and ensure flexibility. We therefore investigated experimentally to which extent availability and protein content of food influence seasonal changes of metabolic rate and affect the occurrence of episodes of peripheral hypothermia and other seasonal adaptations. Despite ad libitum feeding, our 16 experimental adult female red deer showed pronounced seasonal changes of heart rate, subcutaneous temperature, voluntary food intake, body mass, and locomotor activity supporting the assumption of food-independent predominance of photoperiodic control. However, comparison of these data with the results from our previous study when deer received only restricted supplemental feeding revealed that bouts of peripheral cooling were by far less intense and less frequent under the ad lib. feeding regime. This result provides further support for our view that hypometabolism with the consequence to temporarily abandon high body temperature is a ubiquitous reaction of endothermic animals in response to energetic challenges. In addition, secretion of cortisol, measured as concentration of glucocorticoid metabolites in faeces, increased during winter only half as much under ad lib. feeding. Apparently, the mobilization of fat reserves, for which cortisol is supposed to be important, is under ad lib. feeding by far less intense. Another important result of this study was that, in contrast to previous views, seasonal changes of food intake and its protein content do not sufficiently explain the seasonal changes of metabolic rate. However, whether a higher protein content of food alone is sufficient to prevent pronounced seasonal adaptations remains open to question. Our results suggest that such an effect is only to expect when overall food supply is limited. The results of this study are not only important for a better understanding of the function of seasonal adaptation and the mechanisms involved but also for the common practice in Europe of feeding red deer during winter in order to prevent damage to natural vegetation. In addition to this practical application of our research results we achieved other methodical innovations in the course of this project. A telemetry transmitter with a completely new design has been developed and the n-alkane technique, a relatively new method for determining amount and composition of food intake by herbivores has been improved.