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

Abstract

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.

Mandible shape variation and feeding biomechanics in minks

“…Note, however, that actual MA values are calculated using moment arms (not lever arms, as in this study), that is the perpendicular distance between the mandibular condyle and the vector running from the muscle origin to its insertion. Since lever arms have been used extensively in the literature to provide reasonable MA estimates 72 , and can be calculated independently of the cranium, they were preferred in this study.…”

Comparative morphology of the dormouse skull and the influence of size and ecology

“…The mechanical advantage (MA) of three masticatory muscles is estimated for all dormouse genera (Casanovas‐Vilar & van Dam, 2013; Cox et al, 2020). The MA is calculated based on the ratio of in‐ and out‐levers (Figure 2).…”

“…With respect to the biomechanical analysis, a number of caveats should be noted. Firstly, the use of lever arms is a rather simplistic way of estimating the MA, whereas the use of moment arms would be more appropriate (Cox et al, 2020). Secondly, as this study uses 3D landmarks, the linear distance between the condyle and muscle insertion also include lateral and medial displacement.…”

Interactions between native and invasive species: A systematic review of the red squirrel-gray squirrel paradigm

“…In coniferous forests fat accumulation is likely to be less pronounced than in mixed deciduous woods because autumn and winter food supplies are more predictable, and maneuverability to feed on cones in the canopy is important (Lurz and Lloyd, 2000;Wauters et al, 2007;Tranquillo et al, 2022). The different body shapes and interspecific differences in fat accumulation capacity are most likely genetically determined, since studies on size and weight variation in red squirrels over a variety of habitat types suggest local selection for smaller or larger size, based on quality and quantity of primary food resources (tree seeds) (Wauters et al, 2007;Cox et al, 2020;Tranquillo et al, 2022).…”