Predator experience enhances giraffe vigilance to oxpecker alarm calls

Animals often benefit from the alarm calls of other species to detect danger, but how such cues are integrated into vigilance strategies remains unclear. Giraffes (Giraffa spp.) rely on early threat detection to avoid ambush and are known hosts of red-billed oxpeckers (Buphagus erythrorhynchus), which form mutualistic associations with large mammals by feeding on ectoparasites and emitting alarm calls in response to approaching threats. While these calls are thought to provide early-warning benefits, it remains unclear how giraffes interpret them, and whether their responses vary with prior exposure to predation risk.

We conducted playback experiments across three giraffe populations differing in predator presence to test whether giraffes adjust vigilance in response to oxpecker alarm calls. Individuals in the predator-inhabited reserve maintained vigilance longer than those in predator-free areas, suggesting that prior exposure enhances responsiveness to alarm calls. Acoustic analyses revealed that oxpecker alarm calls are characterized by low harmonic-to-noise ratios, consistent with harsh, broadband signals that are known to enhance attention and urgency perception in alarm contexts. However, call structure alone did not explain vigilance responses; instead responses were modulated by ecological context, specifically whether giraffes lived in areas with or without lions.

Our findings suggest that oxpeckers serve a sentinel-like function and that giraffes use their alarm calls as early-warning signals, with stronger responses observed in populations exposed to predators. This supports the idea that eavesdropping on heterospecific alarm calls can provide context-dependent benefits, with predator-experienced giraffes showing greater sensitivity to oxpecker alarms. By linking behavioral flexibility with ecological context, this study offers a framework for understanding how mutualistic communication systems adapt to changing predation pressures.