Three Decades of Data Reveal Climate Change is Altering Seasonal Body Mass Trends of Britain’s Hazel Dormice

A 30-year study finds Britain's dormice are getting lighter in spring and fatter in autumn as climate change alters hibernation and feeding patterns.

By: AXL Media

Published: Mar 25, 2026, 5:14 AM EDT

Source: Information for this report was sourced from University of Exeter

The Physiological Impact of a Warming Archipelago

New research spanning thirty years of biological data has uncovered a striking shift in the physical condition of Britain’s native hazel dormice. Scientists from the University of Exeter, working alongside conservation trusts, have documented a consistent trend where these small mammals are becoming lighter in the spring but significantly fatter by the autumn. This discovery, based on records from 1993 to 2023, suggests that the changing British climate is forcing a rapid physiological adaptation. While hazel dormice typically weigh around 20g, even a 1g fluctuation represents a substantial change in their total body mass and overall energy reserves.

The Winter Energy Drain and Spring Emergence

The decline in average spring weight is particularly concerning for conservationists monitoring the species' post-hibernation health. Researchers suggest two primary theories for why dormice are emerging 1g lighter in May and June than in previous decades. First, smaller body sizes may offer a thermoregulatory advantage in warmer temperatures, as smaller frames lose heat more efficiently. Alternatively, the trend may be the result of a more taxing hibernation period, where warmer, wetter winters cause dormice to wake more frequently. These interruptions force the rodents to consume vital fat stores prematurely, leaving survivors in a physically diminished state as they enter the breeding season.

Autumn Weight Gain and Rainfall Patterns

In contrast to the spring decline, dormice have been found to be roughly 1g heavier during their pre-hibernation "scramble" in October and November. This increase in autumn body mass is likely linked to increased summer rainfall, which can paradoxically boost the availability of hazelnuts, berries, and seeds. While this extra weight may seem beneficial for winter survival, experts warn that unpredictable and extreme weather patterns still pose a significant threat. The increased mass suggests the animals are capitalizing on short-term feeding opportunities to compensate for the higher energy demands of a volatile environment.