Morphological and functional variation between isolated populations of British red squirrels (Sciurus vulgaris)

Isolation due to habitat fragmentation can lead to morphological and functional variation between populations, with the effect being well documented in rodents. Here, we investigated whether such morphological variation could be identified between British populations of the Eurasian red squirrel (Sciurus vulgaris). This species was once widespread across Great Britain, but suffered a severe population decline across the 20th century, leaving a highly fragmented distribution. The aim was to test for morphological and biomechanical variation of the mandible between the remaining British red squirrel populations, and between British and continental European red squirrels. Linear and geometric morphometric methods were used to analyse shape in a sample of over 250 red squirrel hemi‐mandibles from across Britain plus a sample from Germany representing the central European subspecies. Procrustes ANOVA identified significant shape variation between populations, with particularly distinct differences being noted between red squirrels from Germany and several British red squirrel populations, which may reflect their evolutionary history. Linear biomechanical measurements showed that the red squirrels from Formby and Jersey had a significantly lower mechanical advantage of the temporalis muscle than other British populations, suggesting they were less efficient at gnawing. This functional difference may be related to many factors, such as founder effect, potential inbreeding and/or past supplemental feeding with less mechanically resistant food items. The current distribution of red squirrels (Sciurus vulgaris) in Great Britain is highly fragmented. This study demonstrates significant variation in the mandible between British red squirrel populations, both in morphology and in biomechanical proxies. The squirrels at Formby on the Lancashire coast are particularly notable for their reduced mechanical advantage of the temporalis. We speculate that such variation may be driven by differences in evolutionary history and/or diet.