Abstract
Simple SummaryThe social complexity hypothesis (SCH) for communication predicts that species with complex social systems exhibit complex communication systems. Testing the SHC in a broad range of species can contribute to a better understanding of human evolution because a co-evolutionary runaway process between social and vocal complexity may have shaped human language. Here we compare patterns of vocal complexity between the two species of African elephants: the savanna elephant exhibiting a complex social organization and the forest elephant exhibiting a simpler social organization. We review the existing literature and present novel insights into the vocal communication system of the elusive forest elephant, along with a first direct comparison with savanna elephants. Our findings suggest that the African elephants may contradict the SCH, as well as other factors potentially shaping patterns of vocal complexity across species. A better understanding of vocal complexity in the two species of African elephants will depend on continuing advancements in remote data collection technologies to overcome the challenges of observing forest elephants in their dense rainforest habitat, as well as the availability of comparable data quantifying both structural and contextual variability in the vocal production of both species of African elephants.The social complexity hypothesis (SCH) for communication states that the range and frequency of social interactions drive the evolution of complex communication systems. Surprisingly, few studies have empirically tested the SHC for vocal communication systems. Filling this gap is important because a co-evolutionary runaway process between social and vocal complexity may have shaped the most intricate communication system, human language. We here propose the African elephant Loxodonta spec. as an excellent study system to investigate the relationships between social and vocal complexity. We review how the distinct differences in social complexity between the two species of African elephants, the forest elephant L. cyclotis and the savanna elephant L. africana, relate to repertoire size and structure, as well as complex communication skills in the two species, such as call combination or intentional formant modulation including the trunk. Our findings suggest that Loxodonta may contradict the SCH, as well as other factors put forth to explain patterns of vocal complexity across species. We propose that life history traits, a factor that has gained little attention as a driver of vocal complexity, and the extensive parental care associated with a uniquely low and slow reproductive rate, may have led to the emergence of pronounced vocal complexity in the forest elephant despite their less complex social system compared to the savanna elephant. Conclusions must be drawn cautiously, however. A better understanding of vocal complexity in the genus Loxodonta will depend on continuing advancements in remote data collection technologies to overcome the challenges of observing forest elephants in their dense rainforest habitat, as well as the availability of directly comparable data and methods, quantifying both structural and contextual variability in the production of rumbles and other vocalizations in both species of African elephants.
Highlights
The various forms of animal social organization constitute adaptive strategies that enable individuals to maximize their fitness in the face of ecological constraints imposed on their survival and reproduction [1]
Even though assumed to be a driving force behind the evolution of human language, how the degree of complexity of the hierarchical structure characteristic for fission-fusion societies relates to patterns of vocal complexity has not been investigated in a comparative perspective
Within-species comparisons of acoustic variation demonstrate that call types involved in coordinating often socially more complex close-range interactions exhibit high variation compared to those used for long-distance communication [53,54,55,56], suggesting social complexity plays a role in shaping the degree of gradation within vocal repertoires
Summary
The various forms of animal social organization constitute adaptive strategies that enable individuals to maximize their fitness in the face of ecological constraints imposed on their survival and reproduction [1]. The underlying notion is that animals living in highly variable and complex social environments (e.g., individuals interacting frequently and in diverse ways), need to convey a broader range of information to coordinate their social interactions Such increased expressiveness can be achieved via various mechanisms, including an increase in vocal repertoire size and acoustic variation within and between call types, as well as the combination of call types into larger utterances. Even though already mentioned by Darwin [6], surprisingly few studies have empirically tested the social complexity hypothesis for vocal communication systems (e.g., [4,7,8,9]) Some of those have offered mixed results and leave many aspects of social and vocal complexity unexplored [10]. How the occurrence of analogs of syntax and formant modulation relates to variation in social complexity across non-human species remains little understood
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