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First published online November 1, 2006
Journal of Experimental Biology 209, 4566-4573 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02536

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Seasonal adjustment of energy budget in a large wild mammal, the Przewalski horse (Equus ferus przewalskii) II. Energy expenditure

Walter Arnold^*, Thomas Ruf and Regina Kuntz

Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna, Savoyenstraße 1, 1160 Vienna, Austria

^* Author for correspondence (e-mail: walter.arnold{at}vu-wien.ac.at)

Accepted 8 September 2006

Many large mammals show pronounced seasonal fluctuations of metabolic rate (MR). It has been argued, based on studies in ruminants, that this variation merely results from different levels of locomotor activity (LA), and heat increment of feeding (HI). However, a recent study in red deer (Cervus elaphus) identified a previously unknown mechanism in ungulates - nocturnal hypometabolism - that contributed significantly to reduced energy expenditure, mainly during late winter. The relative contribution of these different mechanisms to seasonal adjustments of MR is still unknown, however. Therefore, in the study presented here we quantified for the first time the independent contribution of thermoregulation, LA and HI to heart rate (f_H) as a measure of MR in a free-roaming large ungulate, the Przewalski horse or Takhi (Equus ferus przewalskii Poljakow).

f_H varied periodically throughout the year with a twofold increase from a mean of 44 beats min^-1 during December and January to a spring peak of 89 beats min^-1 at the beginning of May. LA increased from 23% per day during December and January to a mean level of 53% per day during May, and declined again thereafter. Daily mean subcutaneous body temperature (T_s) declined continuously during winter and reached a nadir at the beginning of April (annual range was 5.8°C), well after the annual low of air temperature and LA.

Lower T_s during winter contributed considerably to the reduction in f_H. In addition to thermoregulation, f_H was affected by reproduction, LA, HI and unexplained seasonal variation, presumably reflecting to some degree changes in organ mass. The observed phase relations of seasonal changes indicate that energy expenditure was not a consequence of energy uptake but is under endogenous control, preparing the organism well in advance of seasonal energetic demands.

Key words: hypometabolism, hypothermia, metabolic rate, winter adaptation, body temperature regulation.

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