With six heatwaves from May to August, and temperatures as high as 38.2C in parts of Britain, the summer of 2026 was the UK’s hottest on record. The impact on British wildlife is not yet understood, but with such seasonal extremes likely to become more common it is more important than ever that we build a clear picture of the impact of such extended periods of hot weather on species and ecosystems.

There are concerns especially for small mammals, such as the UK’s smallest rodent, the harvest mouse, which can grow up to 7cm and weighs about the same as a single sheet of A4 paper. While most of our small mammals (voles, mice and shrews) seek refuge from extreme heat (and predators) in underground burrows, the harvest mouse stays above ground and weaves spherical nests from living grass steps in which to sleep and breed.

Small mammals are often especially affected by drought and extended hot spells, and a much larger-than-average number of dead shrews and moles were found by Mammal Society volunteers in the summer months of this year, having expired from heat exhaustion and/or dehydration. Harvest mice could be especially vulnerable to heat since they sleep and breed above ground. The heat over summer has caused grasses and vegetation to dry and die back early, which could greatly impact the nesting behaviour and reproduction of harvest mice.