Study shows late-born hibernators grow more rapidly than early-born counterparts

Dormice born late in the summer undergo accelerated growth, enabled by increased food intake and reduced activity duration, as a compensatory strategy for a late start.
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Juvenile hibernators born late in the reproductive season grow and reproduce faster than their early-born counterparts, but might have a shorter lifespan, says new research in eLife.

The study in garden dormice (Eliomys quercinus) could help us better understand how seasonal animal species respond to early-life conditions, and what impact these responses may have on adulthood and possibly future generations.

“Investing enough resources in growth and pre-hibernation fattening is crucial for hibernators if they are to survive the winter,” explains first author Britta Mahlert from the Research Institute of Wildlife Ecology, Austria. “However, this can present a challenge when the time before hibernation is shortened, as for young individuals born late in the summer. Typical responses demonstrated by late-born individuals include growing at higher-than-normal rates and employing energy-saving strategies, such as torpor, to cope with unfavourable conditions, including environments where there is little food to forage.”

Torpor is a state of reduced metabolic rate, which leads to a decrease in body temperature and other physiological activities. Hibernation is a state of prolonged torpor which lasts more than 24 hours, whereas animals can also undergo daily torpor. But very little is known about the use of torpor during development in young hibernators, and how it differs between early and late-born individuals.

To help fill in this knowledge gap, Mahlert and her team conducted the first study of whether early and late-born juvenile hibernators differ, according to food availability, in their rates of growth and pre-hibernation fattening, their use of energy-saving strategies such as torpor, and their reproductive success in the spring following hibernation.

The team studied 36 female juvenile garden dormice from a breeding colony kept at the Research Institute of Wildlife Ecology. These included 18 animals born in May (early born) and 18 born in August (late born).

Their observations revealed that those born in August grew and fattened twice as fast as those born in May. Late-born juveniles showed greater torpor use and reached similar body sizes but lower fat content than early-born individuals prior to hibernation. Torpor use was low during growth, suggesting that torpor is not compatible with this stage of development, but increased later to promote fattening and consolidate pre-hibernation fat stores.

“Our findings also revealed that female dormice born late in the season had a higher proportion of breeders during the following year than early-born individuals,” says senior author Sylvain Giroud, Principal Investigator at the Research Institute of Wildlife Ecology. “Being born late in the season might therefore be associated with a fast life history: rapid growth and reproduction but likely a shorter lifespan.”

Taken together, these results suggest that growth acceleration, enabled by increased food intake and reduced activity duration, acts as a compensatory strategy for a late start in the active season. Giroud adds that the next steps will be to determine the impact of a late birth on the overall reproductive output and lifespan of dormice, to see how this compensatory strategy affects their fitness – and that of potentially other hibernators – in the long term.

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