Species rewilding, as a key strategy for rescuing endangered species and rebuilding wild populations, fundamentally relies on the behavioral plasticity of specific wildlife species. Although most current rewilding initiatives select optimal habitats, research on behavioral adaptation mechanisms in more challenging, extreme environments remains lacking. The Milu (Elaphurus davidianus), a typical wetland and plain species, naturally inhabits the warm marshlands of the Yangtze and Yellow River basins. In this study, using GPS tracking data, we focused on a population of rewilding Milu on the Inner Mongolia Plateau to investigate behavioral plasticity in terms of home range area, activity rhythm, and movement distance, aimed to elucidate their survival adaptation strategies within mid-elevation and cold environments. The results indicated significant seasonal and sex-based differences in both home range and movement distance: home ranges contract and movement distances are minimized during winter, while spatial activity expands markedly in summer—and continues to increase year by year following rewilding. During the study period, the number of daily activity peaks per individual ranged from zero to four. Furthermore, peak timing exhibited clear seasonal variation, with crepuscular patterns—morning and evening activity peaks—predominant across most months. Approximately three months after release, the activity rhythms of both males and females stabilized. These findings reveal key behavioral adjustments of Milu translocated to a mountainous, cold-temperate environment outside its original distribution range, and provide a scientific basis for long-term management and for assessing the ecological adaptability of this introduced population.

Reintroduction has been recognized as a vital approach to protecting and restoring wild populations of rare and endangered species. Although numerous reintroduction programs have been launched worldwide, in-depth studies on population establishment, dynamics, distribution patterns, and challenges remain limited. The Milu (Elaphurus davidianus), endemic to China and once declared Extinct in the Wild by the IUCN, has become one of the most successful examples of global endangered species reintroduction since the initiation of its recovery program in 1985. Focusing on the reintroduction project in the Poyang Lake basin—China’s largest freshwater lake—this study analyzed 14 years of monitoring data to examine the processes of population re-establishment, demographic trends, and spatiotemporal distribution of the species, while also identifying major ongoing challenges. Findings reveal the following: (1) the acclimatization process was essential for 10 individuals first reintroduced in 2013, and then 30 individuals joined to the growing population, followed by the largest-scale release of 47 in 2018, which finally led to eight consecutive years of natural reproduction, with a theoretically estimated population of 89 individuals until August 2025; (2) among 1697 individual sightings in 238 monitoring events, 952 records can be distinguished between male and female individuals (sex ratio = 1:1.63), yielding a mean herd size of 7.28 ± 0.68 individuals per sighting event—although numbers rose slightly in 2024, a declining trend was observed from 2019 to 2023 (from 11.26 to 5.13); (3) the Milu have expanded their range across eastern Poyang Lake, demonstrating a behavioral patterns of migrating out of the lake zone during flood seasons and returning in dry season; (4) the restoration of the Milu population is a systematic endeavor, in which government support and the participation of conservation professionals, scientists, volunteers, media, and the public are critical factors; and (5) ongoing threats such as human disturbance (e.g., habitat degradation) and climate change continue to jeopardize long-term population viability. This study offers important scientific insights into the recovery of endangered wildlife globally and underscores the essential role of governmental commitment, long-term monitoring, and multi-stakeholder collaboration in species reintroduction efforts.