Rival assessment by territorial southern white rhinoceros males via eavesdropping on the contact and courtship calls

Abstract

Eavesdropping on vocal signals allows animals to gather information about conspecifics. For males, eavesdropping can be a low-risk method for determining the dominance status (e.g. territorial or subordinate) and motivation (e.g. mate seeking) of rivals. Adult southern white rhino males, Ceratotherium simum, are territorial and, in addition to other vocalizations, use contact and courtship calls when communicating with females. Although male territories are exclusive, the owners may tolerate up to three resident subordinate males. However, rival males sometimes intrude. Moreover, these intrusions may lead to rivals interacting with females. We investigated whether territorial males eavesdrop on vocal signals directed towards females by intruding males to determine their dominance status (territorial or subordinate) and motivation (contact calling an anoestrous female or approaching a female in oestrus during courtship). To do this, we first recorded and analysed contact and courtship calls of adult southern white rhino males and determined that these calls signal the males' dominance status and motivation. Playback trials revealed that territorial males differentiated between the calls of subordinate and other territorial males, showing a shorter latency to approach and longer searching behaviour (i.e. walking and running) after the playback of a subordinate's call. The reason for this response could be that the intruding subordinate male might be looking to challenge the territorial male for his territory. However, subordinates probably pose the lowest risk to the resident male in terms of fighting ability. In contrast, the reactions of the territorial males to the calls of other territorial males suggest that the presence and not the motivation of an intruding male was the most important factor influencing the male's reaction. We conclude that territorial males eavesdrop on the acoustic signals of trespassing males to gather information about these rivals, which can help them to better defend their territory.