Richardson's ground squirrels (Spermophilus richardsonii) do not communicate predator movements via changes in call rate

The call rate of repetitive alarm calls produced by Richardson's ground squirrels (Spermophilus richardsonii) conveys the extent of threat during predator encounters. It remains unknown, however, whether changes in call rate communicate predator movements. That is, does an increasing call rate indicate an approaching predator and a decreasing rate the opposite? We presented free-living squirrels with moving predator models and recorded their responses. Vigilant behaviour increased more when predators approached versus retreated, suggesting that squirrels recognize the changes in threat associated with predator movements. Squirrels rarely produced alarm calls during these encounters, however, suggesting that squirrels do not rely entirely on alarm vocalizations to assess the threat posed by potential predators. Receivers of manipulated calls did not respond differentially to alarm calls containing an increasing or decreasing rate of syllable production. Thus, while rate may encode information about the extent of threat, Richardson's ground squirrels do not communicate predator movements via changes in call rate.