Reduced size in a montane butterfly at its warm range boundaries

Variation in insect size is often related to temperature during development and may affect the persistence of populations under future climate warming if smaller individuals have reduced fitness. Montane species are particularly vulnerable to climate‐driven local extinctions due to range retractions at their warm range margins, and so we examined spatial and temporal variation in body size in the butterfly Erebia epiphron (Lepidoptera: Nymphalidae) in the United Kingdom, where it is restricted to two montane regions in England and Scotland. We examined spatial and temporal variation in body size in relation to temperature. We sampled 19 populations (6–15 individuals per population) in England and Scotland between 2018 and 2019 spanning elevations from 380 to 720 m and examined museum specimens collected between 1890 and 1980. We examined individual body size (forewing length) and its relationship with the local temperature of sites, as well as temporal variation in body size over the last century in relation to the temperature during larval development. The forewing lengths of field‐collected individuals in England were on average 7%–8% smaller than in Scotland (England, mean = 14.9 mm, Scotland, mean = 15.9 mm), and warmer sites also had smaller individuals (0.13 mm reduction in wing length per 1°C increase in local site mean temperature). However, we found no effect of temporal temperature variation on body size changes during larval development. The observed smaller body size in English populations could have impacts on fecundity and dispersal ability. Future work should seek to understand the life‐cycle lengths, genetics and phenotypic plasticity of these two populations to evaluate potential explanations for regional differences.