Communal nesting is the optimal strategy for heat conservation in a social marsupial: lessons from biophysical models

Abstract

Endothermy, understood as the maintenance of continuous and high body temperatures (TB) due to the combination of metabolic heat production and an insulative cover, is severely challenged in small endotherms inhabiting cold environments. As a response, social clustering combined with nest use (=communal nesting) is a common strategy for heat conservation. To quantify the actual amount of energy that is saved by this strategy, we studied the social marsupial Dromiciops gliroides (monito del monte), an endemic species of the cold forests of southern South America. It is hypothesized that sociability in this marsupial was driven by cold conditions, but evidence supporting this hypothesis is unclear. Here we used taxidermic models ("mannequins") to experimentally test the energetic benefits of clustering combined with nest use. To do this, we fitted and compared cooling curves of solitary and grouped mannequins, within and outside of a nest, at the typical winter ambient temperatures of their habitat (5°C). We found that the strategy that minimized euthermic cost of maintenance, was the combination of nest use and clustering, thus supporting communal nesting as a social adaptation to cope with the cold. Considering the basal metabolic rate of monitos, our estimates suggest that the savings represents almost half of energy consumption per day (in resting conditions). This study shows how simple biophysical models could help to evaluate bioenergetic hypotheses for social behavior in cold-adapted endotherms.

Climate change and population persistence in a hibernating marsupial

“…Indeed, the yearly schedule of activity in Dromiciops is marked by the hibernation cycle, which lasts 5 months (on average, 156 days), starting in autumn (middle April) and finishing in spring (middle September, see fig. 2 in [44]). During hibernation, animals reduce their metabolic rate by up to 95% and do not ingest food, thus relying completely on accumulated fat [45,46].…”

“…For estimating the duration of hibernation as the number of days where T A was below the torpor threshold (= T A below which 50% of animals are torpid, T 50 ) at four representative locations of the Dromiciops geographic range, we measured operative temperature as described in Nespolo et al . [44]. Originally, operative temperatures were intended to be measured in seven representative localities in the north, centre and south of the distribution of Dromiciops , covering coastal and Andean populations.…”

“…Data loggers were set for hourly measurements and were installed for 1 year (2020; installed on 5 January 2019, and removed 1 year later). Mannequins were manufactured, according to Nespolo et al [44], with skins of dead individuals obtained fresh (frozen) from museums. The animal's body was replaced with agar spheres, which are typically used to estimate operating temperatures since agar has a thermal coefficient similar to that of the living body [62].…”

“…These projections have a resolution of 500 m and every 1000 m of elevation bands, which we downloaded as raster layers and used with a resolution of 2.5 min (about 5 km per side). Given that projections for mean temperatures for different climate change scenarios are tabulated monthly, we assumed that during the 4-month period where Dromiciops is known to be at deep-torpor, from May to August [29,44], animals rely 100% on their fat stores. Thus, we also assumed that animals will survive the winter if the fat content is enough for these 120 days (using the energy consumption provided by equations (2.7) and (2.8)) or will die if fat is not enough.…”