Interplay between group size, huddling behavior and basal metabolism: an experimental approach in the social degus

Abstract

Mammals exposed to low temperatures increase their metabolic rate to maintain constant body temperature and thus compensate for heat loss. This high and costly energetic demand can be mitigated through thermoregulatory behavior such as social grouping or huddling, which helps to decrease metabolic rate as function of the numbers of individuals grouped. Sustained low temperatures in endothermic animals produce changes over time in rates of energy expenditure, by means of phenotypic plasticity. However, the putative modulating effect that huddling exerts on the flexibility of the basal metabolic rate (BMR) due to thermal acclimation remains unknown. We determined BMR values in Octodon degus, an endemic Chilean rodent, after being acclimated to either 15 or 30°C during 60 days, both alone and in groups of three and five individuals. At 15°C, BMR of huddling individuals was 40% lower than that of animals housed alone. Moreover, infrared thermography revealed a significant increase in local surface temperatures in huddled animals. Furthermore, individual thermal conductance was lower in individuals acclimated to 15°C than to 30°C, but no differences were observed between single and grouped animals. Our results indicate that huddling prevents an increase in BMR when animals are acclimated to cold conditions and that this effect is proportional to the number of animals grouped. KEY WORDS: Acclimation, Basal metabolic rate, Huddling, Octodon degus, Phenotypic plasticity INTRODUCTIONEndothermy is defined as the ability to produce endogenous heat, allowing individuals to maintain a positive temperature differential with the environment and remain in homeothermic condition (Hill et al., 2004). Heat production may be modulated through behavioral and physiological changes at different scales and across a wide range of ambient temperatures (Gilbert et al., 2010). For example, animals exposed to temperatures below the thermoneutral zone (TNZ) must compensate for heat losses by increasing their metabolic rate in order to remain homeothermic (Canals, 1998). Thus survival of small mammals at low temperatures may depend on their ability to reduce heat loss and/or to increase metabolic rate, which in many cases involves a large energy cost (Kauffman et al., 2003).To compensate for the increased energy expenditure caused by exposure to low temperatures, individuals may exhibit behavioral responses such as social grouping or huddling (Canals, 1998 et al., 2010). Recently, Gilbert et al. (Gilbert et al., 2012) documented that local heating is crucial in reducing the extent of the cold challenge in huddling rabbit pups. Through thermal images, they demonstrated that at 14°C, the mean surface temperature of the huddle was higher than the mean temperature of isolated pups. Their study demonstrated that local heating when huddling provided each pup with an ambient 'public warmth' in the cold. Thus, huddling behavior reduces energy costs by reducing the metabolic rate and average thermal conductance of each individual in the grou...

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“…When temperatures drop, animals start to metabolize and "spend" energy to warm up through for example through shivering. Shivering counts as one of the most energy-costly bodily processes and thus is expensive (King & Farner, 1961); it may then also not come as a surprise that animals seek to economize thermoregulation, so that body thermoregulation is done as efficiently as possible (Nuñez-Villegas et al, 2014). …”

“…Experimental rodent research has suggested a first hint to this energy-saving hypothesis for homeotherms: The Octodon Degus' (a Chilean rodent) energy usage at 15 degrees Celsius is 40% lower and surface temperature higher when housed with three or five (vs. alone; Nuñez-Villegas et al, 2014). This causes social thermoregulation to be part of homeotherms' essence, as one article reviewing rat research shows that social thermoregulation is one core reason for why homeotherms live in groups (Ebensberger, 2001).…”

Synchronous activity lowers the energetic cost of nest escape for sea turtle hatchlings

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