Scientists Discover a Clue in a Small Dormouse That Reveals How Climate Is Changing Wildlife

Three Decades of Citizen Data Reveal a Hidden Phenomenon

Every fall and spring, certified volunteers across England and Wales open dormouse nest boxes to weigh the animals they find inside. This valuable data feeds into a national database as part of the National Dormouse Monitoring Program, a citizen science effort that has been growing since the late 1980s. Over time, tens of thousands of weight records have been accumulated, carefully noted alongside the animals’ age and sex, yet no one had thought to sort them by season.

This is precisely what a team of researchers from the University of Exeter set out to do in a new study published in the journal Scientific Reports. Initially, none of these records were intended to support climate research. However, this patience has paid off handsomely for the scientific community, revealing completely unexpected dynamics.

Orly Razgour, a biosciences researcher at the University of Exeter and the study’s lead author, analyzed more than 23,000 weight measurements spanning 31 years with her colleagues. By sorting this data by month, they discovered a split trend that calculating a simple annual average had completely masked: hazel dormice are getting lighter and lighter as winter ends, but heavier and heavier before winter begins—all within the same woodlands.

An increasingly difficult spring awakening

The decline in spring weight is the aspect that most concerns environmentalists. In late spring, just after the end of hibernation, muscardins have shown a steady decline in weight over the decades. Emerging from this long sleep in poor physical condition foreshadows a challenging season, as the breeding season begins almost immediately after waking. Scientists point out that lighter mothers generally give birth to fewer or more vulnerable offspring.

Over this three-decade period, spring body weight has dropped by about one gram on average. This loss is particularly significant for an animal whose total weight is generally around 19 grams. At the same time, Great Britain has experienced a warming of about 0.6 degrees Celsius over the same period. This phenomenon is part of a broader pattern documented by biologists in many warm-blooded species, whose bodies tend to shrink as temperatures rise, as confirmed by a widely cited scientific review.

The exact cause of this weight loss, however, remains elusive. Researchers examined winter temperatures, winter precipitation, and the number of days with snow cover, but none of these factors explains this spring weight loss. Hypotheses point to warmer, wetter winters that might wake the dormice more often, depleting their fat reserves, or that might flood their ground nests, thereby robbing them of body heat. Furthermore, earlier spring flowering across the country, documented in a separate study, could mean that these hungry rodents wake up to find their pantry already empty.

Fall weight gain influenced by summer weather

In the fall, the picture is completely reversed, and it is precisely on this point that the study breaks new ground. Prior to this research, no one had tracked the long-term changes in the weight of dormice before hibernation. The data show that animals in the pre-hibernation phase now accumulate more reserves than their predecessors did 30 years ago.

The main driver of this weight gain appears to be rain. When the team cross-referenced fall weights with summer weather data, summer precipitation emerged as the strongest indicator. Wetter summers result in heavier hazel dormice. The reason most likely lies in their diet: hazel trees require consistent summer moisture to produce plump, high-quality hazelnuts, which the dormice gorge on to build up fat reserves before winter.

Curiously, hotter summers had the opposite effect. Hotter conditions were associated with lighter animals in the fall, and this impact affected males more severely than females. Males travel greater distances and compete with more neighbors for food, which potentially leaves them more vulnerable when heat reduces local resources. This sex-related difference is, in fact, the first observation of its kind for this species.

The Crucial Indicator: Hedge Density

The study also provides highly precise details regarding the ideal habitat for these small mammals. The researchers discovered that hazel dormice were heavier in landscapes rich in hedgerows measuring between 1.5 and 6 meters in height. Conversely, where hedgerows exceeded this height, the animals weighed less than average.

The mechanism behind this finding is simple: a hedge left to grow unchecked loses its dense, shrubby middle layer. It is precisely this tangle of vegetation and food that the hazel dormouse relies on for shelter and to move safely between its foraging areas. Overgrown hedgerows become sparse, heavy at the top, and dominated by mature trees with bare trunks at the base. On the other hand, if they are pruned too severely, they lose their flowers and berries. There is therefore an ideal height, and hedgerow management appears to need to adhere strictly to it.

A surprise finding has added nuance to this understanding of wooded environments. At a finer scale, greater coverage of deciduous forests has been associated with lighter—not heavier—dormice. As forests age and their canopies close in, the shrubs and flowering plants growing beneath them may become scarcer, potentially reducing the food vital to the species. The study concludes that variety across the surrounding landscape matters far more than the mere density of forest cover.

New Perspectives for Conservation

The key finding of this research is that climate affects this hibernating species along a seasonal divide that an annual average would simply erase. The impacts of spring and fall are moving in opposite directions, driven by distinct forces. For an animal whose population in Great Britain has fallen by 70% since the year 2000, this distinction is much more than just an academic finding.

This discovery offers environmentalists concrete avenues for action. While it is difficult to manage the climate on a woodlot-by-woodlot basis, hedge height is entirely controllable. The research suggests that maintaining hedges within this intermediate height range—with pruning carried out on a longer rotation rather than a drastic annual cut—could directly support the dormice that live in and around them.

Scientists must now turn their attention to other factors. A spring body that is losing weight could quietly erode reproductive success, thereby partly explaining the disappearance of British dormice—a line of inquiry that deserves to be explored in future research. Ultimately, this breakthrough demonstrates what an army of volunteers—armed with notebooks and monitoring nest boxes—can uncover over 30 years: a crucial signal that no short-term study could ever have detected.

Source: earth.com

Scientists Discover a Clue in a Small Dormouse That Reveals How Climate Is Changing Wildlife