Behavioral shifts of reintroduced Milu deer Elaphurus davidianus in East Dongting lake of China.

Reintroduction is an important method for recovering endangered and rare species in their natural habitats. Understanding how these species adapt to new environments is essential for their conservation and population recovery. However, behavioral adaption (particularly circadian activity patterns) of reintroduced Milu deer (Elaphurus davidianus), a special species disappeared in the wild for centuries, remains poorly understood during their initial release into historical range from captivity. We investigated the circadian behavioral rhythms of Milu reintroduced to the East Dongting Lake, using data transmitted by the GPS collars from winter 2020 to autumn 2022. The results showed that Milu's diurnal activity predominated over nocturnal activity (generally crepuscular pattern), with 1-4 activity peaks within a 24-hour period. From winter 2020 to autumn 2022, there were significant sexual differences in activity rhythms during winter 2020, summer 2021, summer 2022, and autumn 2022 (P < 0.05). Except for summer 2022 of male Milu (U2 = 0.09, P > 0.05), the deer exhibited pronounced activity concentration trends in the eight quarters. Moreover, human disturbance also influenced the circadian rhythm and activity levels of Milu. Our study indicated that Milu gradually adjusted their circadian activity rhythms after reintroduction to their historical habitat and revealed the remarkable behavioral plasticity of Milu. The reintroduction of Milu into nature reserves underscores the significant role of combining ex situ conservation with in situ conservation for the protection of endemic endangered species.