Dormice are getting lighter in spring and heavier in autumn

Every spring and autumn, licensed volunteers in England and Wales open dormouse nest boxes and weigh the animals they find. Their measurements are added to a national database that has grown since the late 1980s, gathering tens of thousands of weight records over 30 years. Until now, nobody had separated those records by season.

A new study has done just that. Over time, dormice weighed in spring have become lighter, while those weighed in autumn have become heavier. The trends run in opposite directions in the same woodlands and among the same animals. For three decades, the divide was hidden in plain sight within the volunteers’ data.

Dormice: two seasons, two stories

University of Exeter researchers drew on records from the National Dormouse Monitoring Programme, a long-running citizen science project that follows the animals at woodland sites throughout England and Wales.

The volunteers had not set out to weigh dormice for a climate study. Instead, weights were recorded year after year alongside each animal’s sex and age.

That persistence proved valuable. Orly Razgour, an Exeter biosciences researcher and the study’s senior author, worked with colleagues to analyse more than 23,000 weight records covering 31 years.

Dividing the measurements by season exposed a contrast that a simple average would have completely concealed.

In late spring, shortly after waking from hibernation, dormice have been getting lighter over time. In autumn, just before returning to hibernation, they have been getting heavier.

For the same animal in the same woodland, the direction of change depends on the month: weights are falling as dormice emerge from winter and rising as they prepare to enter it.

Lighter after winter

The decline in spring is the finding that concerns conservationists. A dormouse that comes out of hibernation in poor condition faces a difficult period, as breeding begins almost as soon as the animals wake. Lighter mothers tend to have fewer or weaker young.

Over the three decades, average spring body weight fell by about a gram – a significant loss for an animal weighing roughly 19 grams. During the same period, Britain warmed by around 0.6°C.

This is consistent with a broad pattern biologists have recorded in many warm-blooded species: body sizes tend to decrease as temperatures rise. According to one widely cited review, most birds and mammals studied show that trend.

The cause of the spring decline, however, remained difficult to identify. The researchers examined winter temperatures, winter rainfall and even the number of days with snow cover. None accounted for the loss in weight.

One possibility is that warmer, wetter winters wake hibernating dormice more frequently, using up their fat reserves, or drench their ground nests and draw away body heat.

Earlier spring flowering in Britain, recorded in a separate study, could also mean that hungry dormice wake after their food has already gone.

Heavier before sleep

Autumn tells the opposite story, and it is here that the study offers a new insight.

Before this research, nobody had followed long-term changes in dormouse weight ahead of hibernation. The animals are now putting on more weight before their winter sleep than their counterparts did 30 years ago.

Rain is the likely influence. When the researchers compared autumn weights with summer weather, summer rainfall emerged as the strongest signal: wetter summers were associated with heavier dormice.

Food is probably the link. Hazel trees need consistent moisture in summer to produce full, high-quality nuts, which dormice eat to build up weight before winter.

Warmer summers, unexpectedly, had the reverse effect. Hotter conditions were associated with lighter animals in autumn, with males affected more strongly than females.

Males travel over larger areas and compete with more neighbours for food, potentially making them more vulnerable when heat reduces local food supplies. This difference between the sexes is itself a first for the species.

The hedgerow clue

The hedgerow detail is unusually specific. Dormice were heavier in landscapes with plentiful hedgerows between about 1.5 and 6 metres tall. Where hedges had grown above that range, the animals were lighter.

A hedge allowed to spread without management loses its dense, shrubby middle layer – the tangle that provides dormice with food and cover, shelters them and lets them move safely between feeding places. Overgrown hedges become gappy and top-heavy, with mature trees and bare stems beneath them.

Hedges cut back too severely lose their flowers and berries as well. There is a Goldilocks height, and hedgerow management appears to fit squarely within it.

Another finding complicated the picture for woodland. At a fine scale, greater broadleaf woodland cover was associated with lighter rather than heavier dormice.

As woodland matures and its canopy closes, the shrubs and flowering plants underneath can become less abundant, potentially reducing the food available to dormice. Variety across the broader landscape mattered more than woodland cover alone.

What this opens up

The central finding is that climate affects this hibernating animal differently by season, a divide that an annual average would entirely obscure.

Spring and autumn weights are moving in opposite directions under different influences, and combining them hides both trends. For a species whose numbers in Britain have fallen by 70% since 2000, that distinction has practical importance.

The findings point conservationists towards measures they can take. Climate is difficult to manage woodland by woodland, but hedgerow height is not.

The research suggests that keeping hedges within the middle height range, with trimming on a longer rotation rather than severe annual cutting, could directly help dormice living in and around them.

It also changes what scientists should monitor. Declining spring body weight could be quietly undermining the breeding success of Britain’s disappearing dormice, making that relationship worth investigating next.

The work demonstrates what a large network of volunteers armed with nest boxes and notebooks can uncover over 30 years: a signal that no short study could have detected.