Saving energy during hard times: energetic adaptations of Shetland pony mares

Abstract

Recent results suggest that wild Northern herbivores reduce their metabolism during times of low ambient temperature and food shortage in order to reduce their energetic needs. It is, however, not known whether domesticated animals are also able to reduce their energy expenditure. We exposed 10 Shetland pony mares to different environmental conditions (summer and winter) and to two food quantities (60% and 100% of maintenance energy requirement) during low winter temperatures to examine energetic and behavioural responses. In summer, ponies showed a considerably higher field metabolic rate (FMR; 63.4 +/- 15.0 MJ day(-1)) compared with food-restricted and control animals in winter (24.6 +/- 7.8 and 15.0 +/- 1.1 MJ day(-1), respectively). During summer, locomotor activity, resting heart rate and total water turnover were considerably elevated (P<0.001) compared with winter. Animals on a restricted diet (N=5) compensated for the decreased energy supply by reducing their FMR by 26% compared with control animals (N=5). Furthermore, resting heart rate, body mass and body condition score were lower (29.2 +/- 2.7 beats min(-1), 140 +/- 22 kg and 3.0 +/- 1.0 points, respectively) than in control animals (36.8 +/- 41 beats min(-1), 165 +/- 31 kg, 4.4 +/- 0.7 points; P<0.05). While the observed behaviour did not change, nocturnal hypothermia was elevated. We conclude that ponies acclimatize to different climatic conditions by changing their metabolic rate, behaviour and some physiological parameters. When exposed to energy challenges, ponies, like wild herbivores, exhibited hypometabolism and nocturnal hypothermia.