Eavesdropping in an African large mammal community: antipredator responses vary according to signaller reliability

Alarm calls can provide nontarget receivers with potentially life-saving information on predation risk. However, patterns of eavesdropping among species may be shaped by the reliability of the intercepted information, that is, the degree to which the alarm call represents a pertinent threat to the eavesdropping species ('relevance'). Prey are predicted to respond strongly to alarm calls from species that are attacked by the same predator guild, whereas species consumed by a larger or different subset of the carnivore community may act as a less reliable source of predator information. We used a playback experiment to examine whether the degree of antipredator responses to heterospecific alarm calls varied with the reliability of the calling species in three large African mammals: impala, Aepyceros melampus, common wildebeest, Connochaetes taurinus, and plains zebra, Equus quagga. Alarm calls of all three species were broadcast randomly to herds of their own species or to either of the other two species. In accordance with the reliability hypothesis, we found that all species reacted strongly to zebra alarm calls. Lions are the primary predator of zebra and represent a significant threat to all three prey species. In contrast, impala are consumed by a greater number of predators, and their alarm calls evoked weaker, mixed responses in the other two species.