Field warming increases body size of nocturnal slugs by extending their daily activity time in an alpine meadow

Although global warming is known to shift animal phenology, its effects on daily activity remain poorly understood. We used long-term field warming with large open-top chambers (15 × 15 × 2.5 m), which increased mean annual air temperature by 0.42°C from 2017–2023, to test whether warming alters daily activity time and body size of the nocturnal slug Agriolimax agrestis in an alpine meadow on the eastern Tibetan Plateau. Warming increased slug activity time by 37.17 min (+ 24.86%) in 2022 and 49.55 min (+ 29.16%) in 2023, and increased body size by 64.51% and 57.11%, respectively, without changing growth phenology. A complementary short-term experiment using small open-top chambers (2 × 2 × 2 m) elevated temperature by 0.44°C and similarly increased activity time by 180.18% and body size by 35.63% under controlled feeding. Slug abundance increased by ca.500% in warmed plots from 2019 to 2023, likely driven by enhanced body size and survival. Overall, warming substantially extends daily activity time, thereby enlarging slug body size and promoting population growth, highlighting an overlooked mechanism by which climate warming affects animal traits and demographics in cold regions.