Abstract. 1. Three life-history traits of stag beetle imagines were studied in the valley of the River Moselle (Alf, Bullay, Rhineland-Palatinate, Germany): activity period (first observed emergence from the larval habitat until last sighting, 2000–2005), condition (residuals of regression body mass vs. body length, 2003–2005), and dispersal distance (line-of-sight distance between release site and last location, 2003–2005). We used radio telemetry to monitor dispersal of 56 free-ranging individuals and field observations of 266 imagines to estimate the overall duration of the activity period. A total of 330 estimates of conditions were obtained from 171 individuals.
2. The duration of activity period was inversely related to average air temperature during adult activity. In 2003, the activity period lasted 38 days in males and 55 days in females at 21.8 °C air temperature on average; in 2004, 82 days in males and 111 days in females at 17.6 °C.
3. Condition decreased continuously from emergence to death, but the rate of mass loss was 3–4 times greater during the warm summer of 2003 than in the more temperate ones of 2004 and 2005. Replicate measurements in 51 individuals showed that the among-years differences in loss rate were positively related to ambient air temperature during the activity period, but were independent from sex and size.
4. The average dispersal distance was significantly larger in males (802 m) than in females (263 m), but did not differ among the years.
5. The responses of stag beetles to among-years temperature variation suggest that climate change may increase vulnerability of local populations by reducing the activity period and increasing metabolic cost for thermoregulation.