<b>Vocal complexity constrains the dear enemy effect: a comparative study of coal tits and green-backed tits</b>

This study was conducted in the Liupanshan National Nature Reserve (35°15′－35°41′ N, 106°09′－106°30′ E), Ningxia Hui Autonomous Region, northwestern China. The reserve covers approximately 67,800 hectares and features an elevation range of 1,935 to 2,371 meters above sea level with forest coverage exceeding 70%. The climate is mid-temperate, transitioning semi-humid to semi-arid, characterized by a mean annual temperature of 5.8℃ and average annual precipitation ranging of 600-800 mm. Vegetation is diverse, primarily consisting of secondary coniferous forest, broadleaf mixed forests, and meadow grasslands. Key tree species include the Chinese red pine (<i>Pinus tabuliformis</i>) and Asian white birch (<i>Betula platyphylla var. japonica</i>) (Shi et al. 2009).The “dear enemy effect”, wherein territorial animals reduce aggression toward familiar neighbors while maintaining heightened responses to strangers, is a widespread strategy to minimize energy expenditure on territory defense. However, its expression across species with varying vocal complexities remains poorly understood. This study investigated neighbor-stranger discrimination in two sympatric tit species exhibiting contrasts in song repertoire complexity: coal tits (<i>Periparus ater</i>) with large, complex repertoires (21 distinct song types) and green-backed tits (<i>Parus monticolus</i>) with limited, simple repertoires (4 song types). Using playback experiments, we assessed behavioral responses to songs of familiar neighbors versus strangers. Coal tits exhibited robust discrimination, responding with significantly greater aggression (e.g., increased flight frequency within 5-meter radius of the nest box and closer approach distance to the speaker) to strangers, aligning with the dear enemy effect. In contrast, green-backed tits showed minimal and undifferentiated responses to both songs of familiar neighbors and strangers, indicating a lack of discrimination. These results demonstrate that the dear enemy effect is not universal and that vocal repertoire complexity plays a critical role in shaping individual recognition abilities. Our study underscores the importance of bioacoustics constrains in territorial behaviors and highlights how simplified vocal signals may limit the expression of complex social behaviors such as neighbor-stranger discrimination.本研究在中国西北部宁夏回族自治区六盘山国家级自然保护区（北纬35°15′－35°41′，东经106°09′－106°30′）开展。该保护区总面积约67800公顷，海拔范围为1935至2371米，森林覆盖率超过70%。气候属于中温带半湿润向半干旱过渡类型，年平均气温5.8℃，年平均降水量600~800毫米。区内植被类型多样，以次生针叶林、针阔混交林和草甸草原为主，主要树种包括油松（Pinus tabuliformis）和日本白桦（Betula platyphylla var. japonica）（Shi等，2009）。 “亲爱的敌人效应（dear enemy effect）”是指领域性动物会降低对熟悉邻居的攻击行为，同时维持对陌生个体的增强反应，以此减少领地防御过程中的能量消耗。该效应是一种广泛存在的策略，但在鸣唱复杂度各异的物种中的具体表现仍有待深入研究。 本研究选取两种鸣唱曲目复杂度差异显著的同域分布山雀类物种，开展邻域-陌生个体识别相关研究：鸣唱曲目丰富复杂、包含21种独特鸣唱类型的煤山雀（Periparus ater），以及鸣唱曲目有限且简单、仅含4种鸣唱类型的绿背山雀（Parus monticolus）。通过回放实验，我们评估了受试个体对熟悉邻居与陌生个体鸣唱的行为反应。 结果显示，煤山雀表现出显著的识别能力：对陌生个体鸣唱的攻击反应显著更强，例如在巢箱5米半径范围内的飞行频次增加、更接近扬声器，这一结果符合“亲爱的敌人效应”。与之相反，绿背山雀对熟悉邻居与陌生个体的鸣唱均表现出微弱且无差异的反应，表明其缺乏相应的识别能力。 本研究结果表明，“亲爱的敌人效应”并非普遍存在于所有物种中，而鸣唱曲目复杂度在个体识别能力的塑造中发挥关键作用。本研究强调了生物声学约束对领域行为的重要性，并揭示了简化的鸣唱信号可能会限制诸如邻域-陌生个体识别这类复杂社会行为的表达。