Saving energy is important for humans and animals alike when resources are limited. Scientists at the Research Institute of Wildlife Ecology of the University of Veterinary Medicine, Vienna, found out that although higher-ranked red deer gain privileged access to patches of food, they also have higher metabolic rates and thus use more energy. This can be a serious disadvantage in winter when red deer rely largely on their limited stored body fat to survive.
Energy budget adjustments
Energy is the currency of life, and a central topic of wildlife ecological research is to understand how animals regulate their energy budgets with respect to its limited supply in the environment. Chris Turbill and colleagues set out to test the hypothesis that high rank, i.e. social dominance might be associated with higher metabolic rate. They measured heart rate and body temperature (proxy indicators for metabolic rate) using minimally invasive rumen transmitters in a herd of female red deer (Cervus elaphus) during winter. Red deer have a highly hierarchical herd structure. Before the experiment, behavioural observations were used to determine the social status of individuals in the group. The animals were allowed to move freely in the approx. 45 hectare large natural enclosure of the Research Institute. Owing to the poor natural forage available over winter, the deer were also fed pellets at an automated feeding station, which enabled the researchers to periodically restrict food rations and test the physiological impact of dominance in situations of food scarcity.
Dominance is costly
Red deer must withstand a negative energy balance over the winter season and maintenance of their body mass is critical to survival and reproduction. In lean times red deer, and other northern ungulates, save energy by drastically reducing their metabolism over cold winter nights – an energy-saving mechanism scientists refer to as "hypometabolism". Socially dominant animals, however, are less good at this. The experimental results found that dominant individuals do have faster heart rates and higher body temperatures during winter compared to subordinates. Consequently, dominant individuals suffered higher rates of body mass loss during periods of low food availability than subordinates. Because the normally disadvantaged lower-ranked individuals do better in keeping their head low, slowing down, and chilling out, they seem to withstand severe winter conditions better.
Average values hide part of the picture
Physiological ecologists have long been surprised by the observation that individuals often exhibit large differences in physiological characteristics, such as metabolic rate, even under identical conditions. Why should they differ? A recent surge of interest in animal personalities has proposed a logical explanation: individuals exhibit particular combinations of behavioural, physiological and life-history traits that together characterise an individual 'pace-of-life syndrome'. Walter Arnold, Head of the Research Institute of Wildlife Ecology, emphasizes: "Until recently physiologists have mostly been concerned with mean values of traits within wildlife populations. Our results show that there can be significant individual differences in physiological processes within one species, and these should not be ignored. After all, evolution builds on individual differences as the units of natural selection."
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Turbill, C. et al. Social dominance is associated with individual differences in heart rate and energetic response to food restriction in female red deer, Physiological and Biochemical Zoology. www.jstor.org/discover/10.1086/672372