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Regardless of whether populations of sika deer (Cervus nippon) are native or introduced, their distribution continues to expand, presenting new ecological threats in several regions of the world, especially Japan. Consequently, there is growing demand for precautionary measures against such expansion, which is primarily associated with males during the initial stage of invasion. Therefore, the present study introduces two novel approaches—passive acoustic monitoring (PAM) and active acoustic monitoring (AAM)—to detect males with high sensitivity, using their howls during the rut. This study verifies the feasibility of these approaches by comparing them with existing methods based on spotlights and camera traps at five sites that support different deer densities. To validate PAM, we set unattended sound recorders to evaluate the time and variation in howl frequency at different sites. In addition, we measured the …