Not so far: attenuation of low-frequency vocalizations in a rainforest environment suggests limited acoustic mediation of social interaction in African forest elephants

Abstract

Forest elephants Loxodonta cyclotis aggregate in large numbers in forest clearings. Whether they maintain contact as they move through the forest and are able to coordinate these aggregations, similar to the fission-fusion sociality of the well-studied savanna elephants Loxodonta africana, is currently unknown. Since sound attenuates faster in closed as compared to open habitats, the low-frequency rumble vocalizations of forest elephants may exhibit smaller detection ranges than measured for those of savanna elephants, which may restrict the ability of forest elephants to coordinate interactions between separated family units. Here, we modeled the attenuation of forest elephant rumbles using amplitude measurements of rumbles recorded in a rainforest in Gabon and estimated the distances at which elephants might be able to detect them under observed ambient sound conditions. Our results suggest an attenuation rate less than predictions of spherical spreading loss, suggesting that reflection of the sound waves within the forest results in constructive interference. Nevertheless, we found that forest elephant rumbles of average dominant frequency (31.07 Hz) under average ambient sound levels would not be detectable farther than 0.8 km. Moreover, for 50% of analyzed rumbles, the harmonic structure was completely attenuated at only 100 m. However, we estimated detection distances of up to 3.2 km for rumbles of average dominant frequency when ambient sound was at its lowest. Our findings suggest that long-distance communication to coordinate interactions among separated family units may be limited in forest elephants, with potentially important consequences for their social organization. The challenges associated with the extent of, and variation in, detection distances of long-distance vocalizations used by animals to mediate interactions between separated group members has rarely been investigated. While it has been suggested that forest elephants exhibit a fission-fusion sociality similar to savanna elephants, our results indicate shorter detection distances for forest elephant rumbles, suggesting a limited ability to mediate interactions between separated family units. However, under optimal ambient sound conditions, detection distances increased considerably. The long detection distances estimated for savanna elephants may reflect the optimal conditions under which the playback experiments were conducted. On average, savanna elephants may be much more limited in communication distance. Further studies on the constraints and opportunities that the different environments impose on these species’ communication capability may be critical to understanding potential differences in the social complexity they express.