Joint tail and vocal alarm signals of gray squirrels (Sciurus carolinensis)

in Behaviour
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Threat-specific vocalizations have been observed in primates and ground squirrels, but their contemporaneous usage with visible signals has not been experimentally analyzed for association with threat type. Here we examine the eastern gray squirrel, an arboreal squirrel that uses both vocal and tail signals as alarms. Squirrels were presented with cat and hawk models simulating natural terrestrial or aerial predator attacks and also with control objects that do not resemble predators but approach in a similar manner. Individuals responded with tail signals (twitches and flags) and vocalizations (kuks, quaas and moans), but only flags and moans are associated with predator type. Moans were elicited primarily by aerial stimuli and flags by terrestrial stimuli. Eastern gray squirrels use an alarm-signaling system in which signals in each modality potentially are associated with particular attributes of a threat or may be general alarms. Terrestrially-approaching stimuli yielded vocal and tail alarm signals regardless of whether the stimulus resembled a predator. With aerially-approaching stimuli, however, quaas were used more often when the stimulus resembled an aerial predator than when it did not. An approaching object’s physical appearance may therefore affect squirrels’ responses to aerial, but not terrestrial, objects. When the stimuli resembled real predators approaching in the natural manner (terrestrially or aerially), both tail flags and vocal moans were associated with predator type, so we also considered moans and flags together. The presence and absence of moans and flags in an alarm signaling bout yields a higher statistical index of predictive association as to whether the threat is aerial or terrestrial than does either component alone.

Joint tail and vocal alarm signals of gray squirrels (Sciurus carolinensis)

in Behaviour

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References

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Figures

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    The five objects (A–E) used as moving stimuli. Scale bars represent 10 cm, either horizontal distance at ground level in overhead views or vertical distance from the point in the scene at the lower end of the scale bar in other views. A and B show the two stimuli designed to resemble real predators. A1 and A2 show the model cat mounted on the radio-controlled motorized base, viewed from the front (A1) and a 3/4 front view (A2). B1 and B2 show the model hawk glider, viewed from below (B1) and above (B2). C and D show the ball used as a novel object, where the same object can approach either terrestrially or aerially. C1 and C2 show the ball mounted on the base, viewed from a 3/4 front view (C1) and from above (C2). D shows the ball alone, which was thrown by hand in the same manner as the hawk glider. E shows the base alone, which was included to test for specific effects of the ball and cat model. This figure is published in colour in the online edition of this journal, which can be accessed via http://booksandjournals.brillonline.com/content/journals/1568539x.

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    Proportion of all trials that elicited vocal signals. Some signaling bouts contained multiple call types and thus contributed to more than one category of call. (A) model cat (N=34) and model hawk trials (N=29); (B) terrestrial ball (N=29) and aerial ball (N=33) trials; (C) model cat (N=34) and terrestrial ball (N=29) trials; (D) model hawk (N=29) and aerial ball (N=33) trials. An asterisk denotes significant association (p0.05) of elicited signal with stimulus.

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    Proportion of all trials that elicited tail signals. Some signaling bouts contained multiple tail signal types and thus contributed to more than one category of tail signal. (A) Model cat (N=34) and model hawk trials (N=29); (B) terrestrial ball (N=29) and aerial ball (N=33) trials; (C) model cat (N=34) and terrestrial ball (N=29) trials; (D) model hawk (N=29) and aerial ball (N=33) trials. An asterisk denotes significant association (p0.05) of elicited signal with stimulus.

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