In order to monitor bat population trends, an annual census is performed of all known underground hibernacula in Europe. During these censuses, bats are sometimes found to show signs of arousal, presumably from non-tactile stimuli caused by the observer, e.g. air currents, sound, light or an increase in temperature. We assume that heat and/or light from a torch can play a role in awaking hibernating bats. Observers use different light sources, which produce different amounts of heat. We experimentally tested the heat produced by three commonly used torches: two incandescent torches (krypton and halogen) and one LED torch. We performed the experiment on 28 January 2007 in an old brick kiln in Windesheim (Overijssel Province, the Netherlands), which is used by hibernating bats. The results show that temperatures in a crevice significantly increased when using the 'hot' incandescent torch types. The effect of these torches on both air and surface temperature was significant after both 10 and 30 seconds. Under the assumption that an increase of the ambient temperature by 5 degrees C or more can cause a bat to arouse from torpor, we conclude that using 'hot' torches such as incandescent (halogen) lights poses a risk to the animals under study. To minimise heat disturbance from light sources, we recommend LED torches as the best available alternative.
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