01.07.2020: Animals may aggregate for various reasons, such as spatial resource distribution, for reasons of sexual selection and mating opportunities, or to lower individual predation risk. Edible dormice tend to remain entirely solitary while foraging for food, but frequently share sleeping sites. A current study by scientists at the Research Institute of Wildlife Ecology of the Vetmeduni Vienna, who have been studying a dormouse population in the Vienna Woods for thirteen years, found that the animals huddle together during acute cold spells, but prefer solitary lifestyles in reproductive years when their food consumption is high.

When to huddle?

One of the principal reasons why animals sit or sleep close together is to brave cold ambient temperatures. Huddling together helps them save on the energy needed for maintaining a steady body temperature.  

Thomas Ruf and Claudia Bieber from the Research Institute of Wildlife Ecology of the University of Veterinary Medicine in Vienna analyzed the grouping behaviour of more than 4000 marked dormice, captured in a 13-year study in nest boxes in the Vienna Woods. The dormice use the nest boxes as sleeping sites in lieu of natural tree holes. Their aim was to clarify whether social thermoregulation, i.e. minimizing heat loss, is the primary cause for group formation, and which factors affect group size and composition.

The researchers were able to confirm their assumption that dormice temporarily form both mixed and single-sex groups in response to acute cold ambient temperatures – engaging in thermoregulatory huddling. This is especially important for individuals with small body mass. Huddling is avoided—except for conditions of severe cold load—in years of full mast seeding of deciduous trees, when they engage in reproduction and high foraging activity.

Loners by choice, huddlers out of necessity

“It is remarkable that entire populations of dormice switch between predominantly solitary lives in reproductive years to social behaviour in non-reproductive years. This means that there is a cost associated with huddling,” says lead author Thomas Ruf. The researchers surmise that the competition for local food resources, even when food is generally abundant, is what makes dormice prefer to stay away from their conspecifics in “good” years.

The frequency and size of groups changes seasonally and peaks in midsummer. It is not unusual for dormice to also use torpor (a kind of daily resting phase to save energy) in the active season, and to combine it with huddling with others. “It is possible that there are synergies between huddling and torpor behaviour,” says co-author of the study Claudia Bieber. “We will have to investigate how huddling and torpor work together in future studies.”

Siblings like to snuggle

Group sizes can range anywhere from 2 to 16 individuals, with smaller groups being a lot more common than large ones. Group size seems to depend on the mean body mass of group members, the composition. In the study population group size significantly increased with the proportion of related (sibling) individuals in the group. Although larger group sizes come with costs in terms of food competition, in the study such costs were mitigated by the fact that resources were also commonly shared with kin, which should lead to indirect fitness benefits. Since competition for resources will strongly increase with the number of individuals present, it seems highly adaptive that larger groups of dormice contained not just a constant fraction, but increasing proportions of relatives.

The researchers also noted a significant male-bias among the huddling dormice. The impact of competition was mitigated by this biased sex ratio, as it avoids sharing of food resources with related females. Importantly, dormice preferentially huddle in male biased groups with litter mates from previous years.

I like you, I like you not

Dormice are unusual among rodents in that their sociability varies with resource availability. At present, it seems clear that dormice massively switch from high-energy turnover, continuously high body temperature and intense foraging in reproductive years to an energy-saving mode in non-reproductive years that makes extensive use of huddling, as well as reduced foraging activity, and short torpor.

The article "Use of Social Thermoregulation Fluctuates With Mast Seeding and Reproduction in a Pulsed Resource Consumer" by Thomas Ruf and Claudia Bieber was published in Oecologia.