Squirrelpox and other mortalities in Scotland’s red squirrels (Sciurus vulgaris), 2010 to 2024

Abstract Native red squirrels ( Sciurus vulgaris L. 1758) are considered endangered in Great Britain with disease, most prominently squirrelpox (SQPx), and competition mediated replacement by the invasive Eastern grey squirrel ( Sciurus carolinensis G. 1788 ) being the greatest threats. This study aimed to establish the principal causes of Scottish red squirrel mortality and investigate spatiotemporal and biological factors of predisposition. We conducted 594 post-mortem examinations from opportunistically sampled red squirrels, between 2010 and 2024. Seventy SQPx cases were also diagnosed (70 of 594, 11.8%), 69 of which were within South Scotland, including five significant spatiotemporal clusters within this area. SQPx was most frequently diagnosed during the spring and summer litters, accounting for 41.34% and 35.71% of all SQPx diagnoses respectively. The most common cause of mortality was trauma (46.1%), of which 76.4% were related to road traffic accidents. These traumatic deaths were most common in the Lowlands with two significant spatiotemporal clusters of road mortalities identified; adult mortalities were more likely to be diagnosed as a traumatic death compared to < 16-week-old squirrel mortalities. Pneumonia was diagnosed in 11.8% of cases, with greater frequency in < 16-week-old squirrels than adults and greater frequency in females compared to males. This study constitutes an in-depth assessment of causes of death in Scottish red squirrels and also highlights potential risk factors to inform future conservation decision making.