Neighbourhood watch: multiple alarm callers communicate directional predator movement in Richardson’s ground squirrels, Spermophilus richardsonii

Richardson’s ground squirrels are social animals that warn conspecifics of a predator’s presence through the production of alarm vocalizations. Their ability to discriminate among individual alarm callers and to identify the location of those callers may allow receivers to track predator movement from acoustic information in multiple-caller bouts. Observations of encounters with live terrestrial and avian predators revealed that squirrels were significantly more likely to produce a multiple-caller bout that tracked predator movement when avian predators were airborne than when predators remained on the ground. To test whether receivers perceived such differences, squirrels were presented with playbacks of multiple-caller bouts composed of either chirps (commonly issued in response to airborne predators), or whistles (commonly issued in response to predators on the ground) from callers that were either unfamiliar or familiar to the receiver. In response to unfamiliar chirps, but not unfamiliar whistles, receivers were significantly more vigilant when call bouts progressively increased in proximity than when call bouts progressively decreased in proximity. Thus, Richardson’s ground squirrels use multiple alarm callers to track airborne avian but not terrestrial predators, presumably owing to the more immediate threat that airborne predators pose, but also because of the relative paucity of directional information in bouts of whistle calls associated with potential threats on the ground. Squirrels were more responsive to calls from familiar neighbours, however, and despite our relatively small sample of familiar caller playbacks, receivers showed limited evidence of differential response to approaching versus receding bouts of whistle calls when they were familiar with callers.