McRae thinks it might not be an either/or situation, and the line between functionally referential and urgency-based alarm signals might be fuzzier than it's made out to be. Squirrel alarms could also be urgency-based, dependent on factors like how close the predator is and how rapidly it is approaching. (The classic examples of functionally referential alarm signals are those of vervet monkeys, who have specific calls for three different types of predator that each elicit a different and appropriate anti-predator behavior in other monkeys). For example, if a squirrel consistently gives a particular call when a cat is present and that call elicits anti-cat behavior in other squirrels, then the call functions as if it refers to the cat. The call is elicited by this object, and when other animals hear that call, they respond appropriately - as they would to that specific object. Functionally referential signals act as if the call refers to a specific object. If squirrels did respond differently to alarms that signaled terrestrial versus aerial predators, it might indicate that their alarm system is functionally referential. However, he doesn't yet know if squirrels respond in unique ways to the different types of alarm signals. McRae has shown that squirrels respond to other squirrels' alarm calls by becoming more vigilant and retreating up trees. Looking at whether or not tail flags were used reduces that error rate by about 17%, while looking at whether moans are involved in the signal reduces the error rate by about 20%. If terrestrial and aerial attacks are equally common, and a squirrel randomly guesses which type of threat is causing the alarm, it would be wrong about half the time. When McRae and Green looked at the two tail signals by themselves, the twitches were used in lots of different circumstances, sometimes even when there was no predator present.Ĭonsidering the vocal and tail alarms together drastically increased one's ability to predict what was eliciting the calls. Squirrels made quaas more often in response to terrestrial threats, but sometimes used them when an aerial threat approached, as well. "Whether or not they use kuks doesn't really tell you much about what's going on other than something is upsetting the squirrel," says McRae. Tail flags showed a moderate association with the terrestrially approaching model cat.Ĭonsidered by themselves, kuks served as a generic alarm signal.
Moans by themselves were specific to the aerially approaching model hawk.
Of the three vocalizations and two tail movements, only tail flags and moans were associated strongly with specific predators. Some, but not all, of the alarm signals were associated with predator type, and combinations of tail signals and vocalizations were more strongly associated with threats than either type of signal alone. Overall, McRae and Green found the squirrels have an alarm system with different degrees of specificity.
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