Evolutionary theory expects social, communicative species to eavesdrop most on other species’ alarm calls [e.g., 1, 2] but also that solitary-living species benefit most from eavesdropping [3, 4]. Examples of solitary species responding to the alarm calls of other species, however, are limited and unconvincing [3–5]. The Swahili name for the red-billed oxpecker (Buphagus erythrorynchus) is Askari wa kifaru, the rhinos’ guard [6]. Black rhino (Diceros bicornis) are a solitary-living, non-vocal species and are critically endangered through hunting. We searched Hluhluwe-iMfolozi Park, South Africa, for rhinoceros for 27 months with and without the aid of radio telemetry and conducted 86 experimental, unconcealed approaches to 11 rhino, without or with varying numbers of resident oxpecker. Oxpeckers enabled rhinos to evade detection by us in 40% to 50% of encounters. Alarm-calling by oxpeckers significantly improved the rate and distance that rhinos detected our approach from 23% to 100% and 27 ± 6 m to 61 ± 4 m, respectively. Every additional oxpecker improved detection distance by 9 m. Rhinos alerted by oxpeckers’ alarm calls never re-oriented in our direction but moved to face downwind. Thus, oxpeckers’ calls communicate only threat proximity, not direction, and rhinos assume the hunter is stalking from downwind. We confirm that oxpeckers guard rhinos and the importance of depredation, not sociality, in the evolution of eavesdropping [4, 7]. Conservationists should consider reintroducing oxpeckers to rhino populations, reinstating their anti-human sentinel [8]. Video Abstract: [Figure presented] Predation threat alone, not sociality, might be the main driver of eavesdropping behavior between animal species, but remains difficult to test in situ. Plotz and Linklater show experimentally that black rhino, a solitary, non-vocal species vulnerable to human predation, use oxpecker alarm calls to better detect and avoid humans.
