個體可透過聚集防禦來減少面對補食者與競爭者給予的風險，但部分避險行為可能讓部分個體付出受傷或死亡等高昂成本，而避險行為的代價與好處可能會隨著群大小有所變化，進一步影響群體避險策略的使用。穴居蝙蝠常於棲所形成大群集，目前尚不清楚群居是否能降低蝙蝠於棲所內的風險與影響叫聲的使用。本研究藉由模擬臺灣小蹄鼻蝠(Rhinolophus monoceros)於洞穴中在受到人類干擾，來了解群大小對於其避險策略的影響，並初步探討社會叫聲與避險行為的關係。結果顯示，當入侵者試圖捕捉蝙蝠時，臺灣小蹄鼻蝠展現了偵測、接觸飛行與飛離等避險行為，並發出11種社會叫聲，但以社會定位叫聲與噪音型叫聲使用最多。隨著蝙蝠所處的子群集大小增加，蝙蝠接觸飛行行為增加而飛離行為減少，推測臺灣小蹄鼻蝠在子群集較大時，傾向以聚集之方式加強稀釋效應。此外當子群集大小增加時，臺灣小蹄鼻蝠會增加兩類主要社會叫聲的數量，但僅社會定位叫聲和對接觸飛行次數增加有顯著關係，未來需使用回播實驗進一步驗證社會叫聲功能。本研究結果顯示臺灣小蹄鼻會根據群集內的子群集大小調整避險行為，推測穴居蝙蝠可能利用於棲所成大群體，進而降低穴居蝙蝠在棲所內的捕食壓力，同時，並推測他們在黑暗中可使用聲音訊號進行溝通，來協調群體避險策略的調整。

Individuals can mitigate risks from predators and competitors through aggregation-based defense mechanisms. However, some behavioral responses to risk may impose significant costs, such as injury or death, on certain individuals. The costs and benefits of these behaviors often vary with group size, influencing the adoption of collective risk-mitigation strategies. Cave-dwelling bats commonly form large aggregations at roosts, but it remains unclear whether grouping reduces roost-specific risks and how it influences the use of vocalizations.This study investigated the effects of group size on strategies to risk in Formosan lesser horseshoe bats (Rhinolophus monoceros) under simulated human disturbance and explored the relationship between social calls and defensive behaviors. When intruders attempted to capture the bats, they exhibited a range of defensive behaviors, including vigilance, contact flight, and take flight, while producing 11 distinct types of social calls. Among these, socialocation and noise calls were the most frequently used.As cluster size increased, bats displayed more contact flight behavior and fewer taking flights, suggesting a preference for clustering to enhance the dilution effect in larger cluster. Additionally, the use of social calls increased with cluster size, but statistical analysis revealed that only socialocation calls were significantly associated with the frequency of contact flight, while noise calls showed no significant relationship. These findings imply that socialocation calls may facilitate coordination during contact flight, whereas noise calls likely do not serve as recruitment signals for mobbing, contrary to prior assumptions.This study demonstrates that Formosan lesser horseshoe bats adjust their defensive behaviors based on cluster size, leveraging aggregation to reduce predation risk within roosts. It further highlights the potential role of vocal communication in the dark, enabling bats to coordinate group responses and optimize their risk-mitigation strategies. Future playback experiments are needed to validate the functional roles of social calls in these defensive behaviors.

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