Current nest box designs may not be optimal for the larger forest dormice; pre-hibernation increase in body mass might lead to sampling bias in ecological data

Biologists commonly use nest boxes to study small arboreal mammals, including forest dormouse (Dryomys nitedula). Hibernating dormouse species often experience pronounced seasonal variations in body mass, which might lead to sampling biases if it is not taken into account when designing nest boxes. In our study of forest dormouse, we noticed that the entrance hole of nest boxes had been gnawed on. We hypothesized that this behavior was exhibited by individual dormice who had higher body mass and, therefore, were unable to pass through the entrance holes.To test our hypothesis, we categorized individual dormice present inside nest boxes based on their body mass; then compared the seasonal body mass dynamics with the timing of the gnawing behavior. We also compared nest box occupancy by forest dormouse before and after the gnawing behavior.Interestingly, we found that the gnawing behavior was displayed exclusively when part of the dormouse population increased considerably in body mass, which supports our hypothesis. Additionally, nest box occupancy decreased significantly from 20% before to 4.6% after the gnawing behavior.We suggest that researchers use nest boxes with entrance holes larger than 4 cm in future studies of forest dormouse to prevent the possible exclusion of the conspecifics that have higher body mass before hibernation. This type of sampling bias can probably happen in studies of other species, such as fat dormouse, that similarly show pronounced seasonal variations in body mass. We recommend that biologists consider the seasonal body mass dynamics of the target species when designing nest boxes to minimize bias in ecological data and improve management actions.Methods: I surveyed 65 nest boxes for dormice in North-western Iran in late-September and early-November 2019. I considered a nest box occupied only if one or more dormice were present in the nest box at the time of visit. 0=occupied, and 1=unoccupied.

生物学家通常利用巢箱（nest box）开展小型树栖哺乳动物（arboreal mammals）研究，其中包括森林睡鼠（forest dormouse，Dryomys nitedula）。冬眠型睡鼠物种的体重常出现显著的季节性波动，若在巢箱设计阶段未考量该特征，可能会引入抽样偏差（sampling bias）。在本项针对森林睡鼠的研究中，我们观察到巢箱的入口孔洞存在被啃咬的痕迹。我们提出假说：该啃咬行为由体重较高、无法穿过巢箱入口孔洞的睡鼠个体实施。为验证该假说，我们根据巢箱内捕获的睡鼠个体体重进行分类，随后将其季节性体重动态（body mass dynamics）与啃咬行为的发生时间开展对比。此外，我们还对比了啃咬行为发生前后，森林睡鼠对巢箱的占据率。
有趣的是，我们发现啃咬行为仅在睡鼠种群部分个体体重显著上升时发生，这一结果支持了我们的假说。此外，巢箱占据率从啃咬行为发生前的20%显著下降至发生后的4.6%。
我们建议，在未来针对森林睡鼠的研究中，研究者应使用入口孔洞直径大于4厘米的巢箱，以避免将冬眠前体重较高的同种个体（conspecifics）排除在外。此类抽样偏差同样可能出现在其他存在显著季节性体重波动的物种研究中，如肥睡鼠（fat dormouse）。我们建议生物学家在设计巢箱时，充分考量目标物种的季节性体重动态，以尽可能降低生态数据中的偏差，并优化相关管理措施。
研究方法：2019年9月下旬及11月上旬，我们在伊朗西北部对65个巢箱开展睡鼠调查。我们将调查时巢箱内存在1只或多只睡鼠的情况定义为巢箱被占据，其中0代表巢箱被占据，1代表巢箱未被占据。