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

Abstract

SUMMARY 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(fH) 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 (Ts) 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 Ts during winter contributed considerably to the reduction in fH. In addition to thermoregulation, fH 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.

Thyroid hormones correlate with field metabolic rate in ponies,Equus ferus caballus

“…As previously shown for wild horses (Arnold et al, 2006), cold ambient temperatures led to reduced energy expenditure, f H and activity in our ponies in winter compared with summer. THs, as a proxy for BMR (Elliott et al, 2013;Welcker et al, 2013) and also as an indicator of FMR, were affected by season.…”

“…red squirrels (Humphries et al, 2005) and least weasels (Zub et al, 2009)]. Large mammals, with the exception of bears, normally lack the ability to enter torpor (Arnold et al, 2006). However, recent studies on wild Przewalski horses, Alpine ibex, red deer and Shetland ponies all indicate that these species overwinter by employing some form of a hypometabolism (Arnold et al, 2004(Arnold et al, , 2006Brinkmann et al, 2014;Signer et al, 2011).…”

Dietary experience modifies horses' feeding behavior and selection patterns of three macronutrient rich diets1

“…When the nutrient content of the available plant matter declines, the horse alters its selection patterns to increase intake or adopt energy saving strategies such as decreased activity and seeking shelter or both (Gill, 1987;Arnold et al, 2006). This indicates that the horse is sensitive to the changing nutrient or energy content of the available plant matter.…”

Physiological and life history strategies of a fossil large mammal in a resource-limited environment

“…Nevertheless, evidence is accumulating that certain ungulates significantly reduce endogenous heat production to cope with energetically challenging situations (food shortage, harsh climatic conditions) (32). Thus, seasonal fluctuations in metabolic rate and in body temperature (heterothermy) have been described for ungulates with a winter nadir in northern species (32)(33)(34) and with a summer nadir in desert species (35,36). Taking into account these recent advances in ungulate physiology, the zonal bone of Myotragus quite likely reflects seasonal fluctuations in metabolic rate and/or body temperature over an extended juvenile period in response to fluctuating resource conditions on the island.…”