Body mass modulates huddling dynamics and body temperature profiles in rabbit pups

Patterns and predictors of inter-litter differences in rabbit pup locomotor activity, based on an automatized quantification method

Individual-level sibling interactions in the litter huddle have been studied extensively, especially in the domestic rabbit (Oryctolagus cuniculus). However, little is known about inter-litter differences in pup activity patterns during early postnatal life, in particular regarding the drivers of such variation. In our study on 2-3-day-old rabbit pups, we predicted lower locomotor activity in litters with lower mean body masses on the day of birth (starting body mass) and with lower daily milk intake per pup, possibly constituting a behavioral strategy of pups to cope with associated energetic constraints. For an automatized assessment of pup locomotor activity in the litter huddle, we successfully developed and validated a method based on the quantification of dissimilarities between consecutive frames of video footage. Using this method, we could confirm a U-shaped time course of litter-level locomotor activity, with maximum values shortly before and after the once-daily nursing typical for the rabbit. As predicted, between-litter variation in mean starting body mass and in daily milk intake affected the degree of locomotor activity in the litter huddle, in an interactive way. That is, in litters with heavier starting body masses, pup locomotor activity was greater in pups with an initially higher milk intake, suggesting that only pups with better body condition and a higher energy intake could afford higher levels of activity. This interaction was exclusively apparent during the middle phase of the 24 h inter-nursing interval, when litter activity was low. Shortly before nursing, when pups show higher levels of locomotor behavior in anticipation of the mother's arrival, and shortly after nursing when the pups were more active possibly due to adjustments of their positions in the huddle, activity levels were decoupled from pups' starting body mass and previous milk intake. Our findings highlight the importance of pup body mass and daily energy intake, two parameters known to be related to maternal characteristics, in shaping inter-litter differences in pup locomotor activity.

The immunomodulatory effects of social isolation in mice are linked to temperature control

Living in isolation is considered an emerging societal problem that negatively affects the physical wellbeing of its sufferers in ways that we are just starting to appreciate. This study investigates the immunomodulatory effects of social isolation in mice, utilising a two-week program of sole cage occupancy followed by the testing of immune-inflammatory resilience to bacterial sepsis. Our results revealed that mice housed in social isolation showed an increased ability to clear bacterial infection compared to control socially housed animals. These effects were associated with specific changes in whole blood gene expression profile and an increased production of classical pro-inflammatory cytokines. Interestingly, equipping socially isolated mice with artificial nests as a substitute for their natural huddling behaviour reversed the increased resistance to bacterial sepsis. Together these results suggest that the control of body temperature through social housing and huddling behaviour are important factors in the regulation of the host immune response to infection in mice and might provide another example of the many ways by which living conditions influence immunity.

Individual differences in early body mass affect thermogenic performance and sibling interactions in litter huddles of the house mouse

We asked whether within-litter differences in early body mass are associated with differences in house mouse pups' thermogenic performance and whether such variation predicts individual differences in competitive interactions for thermally more advantageous positions in the huddle. We explored pups' thermogenic performance in isolation by measuring changes in (maximal) peripheral body temperatures during a 5-min thermal challenge using infrared thermography. Changes in peripheral body temperature were significantly explained by individual differences in body mass within a litter; relatively lighter individuals showed an overall quicker temperature decrease leading to lower body temperatures toward the end of the thermal challenge compared to heavier littermates. Within the litter huddle, relatively lighter pups with a lower thermogenic performance showed consistently more rooting and climbing behavior, apparently to reach the thermally advantageous center of the huddle. This suggests that within-litter variation in starting body mass affects the pups' thermal and behavioral responses to environmental challenges.

Huddling Conserves Energy, Decreases Core Body Temperature, but Increases Activity in Brandt's Voles (Lasiopodomys brandtii)

Huddling as social thermoregulatory behavior is commonly used by small mammals to reduce heat loss and energy expenditure in the cold. Our study aimed to determine the effect of huddling behavior on energy conservation, thermogenesis, core body temperature (T_b) regulation and body composition in Brandt's voles (Lasiopodomys brandtii). Adult captive-bred female Brandt's voles (n = 124) (~50 g) in 31 cages with 4 individuals each were exposed to cool (23 ± 1°C) and cold (4 ± 1°C) ambient temperatures (T_a) and were allowed to huddle or were physically separated. The cold huddling (Cold-H) groups significantly reduced food intake by 29% and saved digestible energy 156.99 kJ/day compared with cold separated groups (Cold-S); in cool huddling groups (Cool-H) the reduction in food intake was 26% and digestible energy was saved by 105.19 kJ/day in comparison to the separated groups (Cool-S). Resting metabolic rate (RMR) of huddling groups was 35.7 and 37.2% lower than in separated groups at cold and cool T_as, respectively. Maximum non-shivering thermogenesis (NSTmax) of huddling voles was not affected by T_a, but in Cold-S voles it was significantly increased in comparison to Cool-S. Huddling groups decreased wet thermal conductance by 39% compared with separated groups in the cold, but not in the cool T_a. Unexpectedly, huddling voles significantly decreased T_b by 0.25 - 0.50°C at each T_a. Nevertheless, activity of Cold-H voles was higher than in Cold-S voles. Thus, huddling is energetically highly effective because of reduced metabolic rate, thermogenic capacity and relaxed T_b regulation despite the increase of activity. Therefore, Brandt's voles can remain active and maintain their body condition without increased energetic costs during cold exposure. This study highlights the ecological significance of huddling behavior for maintenance of individual fitness at low costs, and thus survival of population during severe winter in small mammals.

Modelling the emergence of rodent filial huddling from physiological huddling

Huddling behaviour in neonatal rodents reduces the metabolic costs of physiological thermoregulation. However, animals continue to huddle into adulthood, at ambient temperatures where they are able to sustain a basal metabolism in isolation from the huddle. This 'filial huddling' in older animals is known to be guided by olfactory rather than thermal cues. The present study aimed to test whether thermally rewarding contacts between young mice, experienced when thermogenesis in brown adipose fat tissue (BAT) is highest, could give rise to olfactory preferences that persist as filial huddling interactions in adults. To this end, a simple model was constructed to fit existing data on the development of mouse thermal physiology and behaviour. The form of the model that emerged yields a remarkable explanation for filial huddling; associative learning maintains huddling into adulthood via processes that reduce thermodynamic entropy from BAT metabolism and increase information about social ordering among littermates.