African elephant rumbles differ between populations and sympatric social groups: possible consequences of vocal learning?

Vocal production learning, the ability to modify vocalizations in response to sounds made by others, was a critical prerequisite for the evolution of human speech but is rare among mammals. Elephants have exhibited this ability in captivity, yet its function in wild elephants remains unknown. Female African savannah elephants (Loxodonta africana) live in large societies with nested tiers of association in which vocal signatures of group identity could facilitate recognition of distant social affiliates. Vocal production learning allows the formation of such group signatures in many species and can also cause vocal differentiation between populations. However, the existence of vocal signatures of social group or population in elephants was unexplored. We recorded multiple social groups of wild elephants in two Kenyan populations (Samburu and Amboseli) and used random forest models to determine if calls could be assigned to individual callers, family groups, bond groups (collections of family groups), or populations based on acoustic structure. Calls were assigned by a random forest model to individual callers and populations with better-than-chance accuracy, demonstrating population-level divergence in vocalization structure. While random forest models failed to accurately assign calls to family or bond group, calls from the same family or bond group were significantly more similar (higher proximity scores) than calls from different groups, suggesting the existence of group signatures as well. We discuss possible drivers of this differentiation and argue that vocal learning is the most likely explanation for population- and group-level variation in elephants. The existence of group signatures suggests recognition of large numbers of individuals as a possible adaptive function for vocal production learning in elephants.

发声学习（vocal production learning）是指个体依据他人发出的声音调整自身发声的能力，它是人类言语演化的关键前提，但在哺乳动物中极为罕见。大象在圈养环境中已被证实具备该能力，然而野生大象中该能力的功能仍未明确。雌性非洲草原象（Loxodonta africana）栖息于具有多层嵌套社交联结层级的大型社会群体中，群体身份的发声标识或可帮助个体识别远距离的社交同伴。发声学习可促使诸多物种形成此类群体身份发声标识，同时也能引发不同种群间的发声分化。但此前尚未有研究探讨大象是否存在社交群体或种群的发声标识。我们在肯尼亚的两个种群（桑布鲁与安博塞利）中记录了多群野生大象的社交群体，并基于声学结构采用随机森林模型，判断叫声是否可被归类至单个发声个体、家族群体、联结群体（即家族群体的集合）乃至种群层级。随机森林模型可将叫声以高于随机猜测的准确率归类至单个发声个体与种群，这表明大象的发声结构存在种群水平的分化。尽管随机森林模型无法准确将叫声归类至家族或联结群体，但同一家族或联结群体的叫声相较于不同群体的叫声，其相似性显著更高（相似性得分也更高），这同样提示群体身份发声标识的存在。我们探讨了引发该发声分化的潜在驱动因素，并提出发声学习是解释大象种群与群体水平发声变异的最合理解释。群体身份发声标识的存在表明，对大量个体的识别或许是大象发声学习的一项适应性功能。