Going Bald — The Hairy Affair of Timing in Telemetry Studies: Moulting Activity in European Bat Species

Telemetry is a widely used method for radio-tracking bats. To minimize negative effects of attaching a tag, like bald spots during hibernation and nursery period, narrowing down the timing of the moult of bats is important. In this study, we investigated the moulting of 11 European bat species with a focus on Myotis daubentonii, Myotis myotis and Pipistrellus pipistrellus. Telemetry data were taken from a 20 years data base. For moulting activity bats were caught with mist nests during their activity period in 2019 and 2020. During data acquisition the condition of the hair was categorized using two different methods: visual hair growth and shed hair pulled off the bats' pelage. Both methods revealed the same results. In general, bats moulted once a year in mid-June (first moult: 17.06) until mid-August (last moult: 13.08), with a significant difference between adult males, reproductive females and non-reproductive females. Adult males (n = 197) and non-reproductive females (n = 60) moulted from mid-June (first moult: 17.06) to end of July (last moult: 27.07), while reproductive females (n = 216) moulted from beginning of July (first moult: 02.07) to mid-August (last moult: 17.08). This variance is probably caused by sex-specific energetic processes like pregnancy, lactation and spermatogenesis. When tagged before moulting season bats could regrow fur in the same year. As seen in recaptured bats (n = 31), tagged in or after this period bats stayed bald during hibernation and reproduction period until the following moulting season. Further studies are needed to specifically assess the impact of bald spots on hibernation and reproductive success. A preceding cost-benefit analysis of the expected knowledge gain versus the potential negative impact on individual and colony scale is recommended.