Neighbourhood watch: multiple alarm callers communicate directional predator movement in Richardson’s ground squirrels, Spermophilus richardsonii Amy B. Thompson * , James F. Hare 1 Department of Biological Sciences, University of Manitoba, Canada article info Article history: Received 4 August 2009 Initial acceptance 10 December 2009 Final acceptance 14 April 2010 Available online 7 June 2010 MS. number: A09-00516R2 Keywords: alarm calling communication network indirect detection Richardson’s ground squirrel Sciuridae sociality 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. Ó 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. As is the case for many group-living species (Hass & Valenzuela 2002; Uetz et al. 2002), antipredator benefits have contributed substantially to the evolution and maintenance of ground squirrel sociality (Blumstein 2007; Hare & Murie 2007). Enhanced predator detection (i.e. the ‘many eyes’ hypothesis) is one means by which such benefits accrue (Lima 1990; Uetz et al. 2002). With more eyes scanning the environment, larger groups detect predators sooner (Kenward 1978) and each individual within the group is able to devote less time to vigilance, and more time to foraging (Kildaw 1995; Lima 1995), where alarm signals communicate the pres- ence of a potential predatory threat to otherwise unwary individ- uals (Beauchamp & Ruxton 2007; Blumstein 2007). Beyond enhanced detection, alarm signals convey a wealth of information regarding the precise nature of the threat at hand. Davis (1984) reported that Richardson’s ground squirrel alarm calls are referential, with short, typically singular, frequency-modulated chirps being issued in response to avian predators, and longer duration, repeated, stable-frequency whistles being issued in the presence of terrestrial predators. Warkentin et al. (2001), however, noted that chirps and whistles were not associated uniformly with terrestrial and avian predators, respectively, but rather with the context and response urgency implicit in the encounter with a presumptive predator. Thus, Richardson’s ground squirrel alarm vocalizations appear to be context specific rather than functionally referential per se. In addition to that general contextual information, Richardson’s ground squirrels discriminate among individual alarm callers (Hare 1998a) and use this ability to adjust their response to the number of individuals calling (Sloan & Hare 2008), as well as the past reli- ability of individual alarm callers (Hare & Atkins 2001). Alarm calls also provide receivers with information on the location of the caller in space (Sloan et al. 2005), and with repeated whistles, the prox- imity of the predator to the caller (Warkentin et al. 2001). While changes in the rate of repeated calling by an individual caller does not communicate predator movement (Wilson & Hare 2003), the ability of receivers to discriminate among individual callers (Hare 1998a), to locate those callers in space (Sloan et al. * Correspondence and present address: A. B. Thompson, Department of Natural Resource Sciences, McGill University, Montreal, QC H9X 3V9, Canada. E-mail address: amy.thompson@mail.mcgill.ca (A.B. Thompson). 1 J. F. Hare is at the Department of Biological Sciences, Room 121, Machray Hall, University of Manitoba, Winnipeg, MB R3T 2N2, Canada. Contents lists available at ScienceDirect Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav 0003-3472/$38.00 Ó 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2010.04.028 Animal Behaviour 80 (2010) 269e275