Browsing by Author "Rendall, Drew"
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ItemAre the alarm calls of North American red squirrels (Tamiasciurus hudsonicus) functionally referential?(2009) Digweed, Shannon; Rendall, DrewNorth American red squirrels are a small-bodied and solitary-living species that faces a diversity of predators and produces two different variants of alarm calls in response to them. Recent studies have yielded conflicting interpretations of the predator-specific and functionally referential nature of these alarm call variants. We undertook a systematic set of playback experiments to quantify the responses of red squirrels to alarm calls produced by other squirrels during encounters with different predators. The experiment was designed to test a core requirement of functionally referential alarm calls, namely that different alarm call types induce distinct and functionally appropriate escape responses in listeners. Results indicated that squirrels registered and responded to alarm calls produced by others; however, their responses were not differentiated according to the type of alarm call they heard and thus did not provide evidence that the different alarm call variants hold any predator-specific, referential value. These outcomes are discussed in light of complementary work on alarm call production in red squirrels and broader aspects of this species’ life history in an effort to better understand the necessary and sufficient pressures promoting the evolution of referential call systems in animals. ItemPredator-associated vocalizations in North American red squirrels (Tamiasciurus hudsonicus): are alarm calls predator-specific?(2009) Digweed, Shannon; Rendall, DrewNorth American red squirrels are a small-bodied, solitary, territorial species that faces a diversity of predators. One report suggested that red squirrels produce two distinct vocalizations to aerial and ground predators: a tonal ‘seet’ and a broadband ‘bark’, respectively. This categorical mapping between alarm call variants and predator classes suggested that red squirrels might manifest a system of predator- specific, referential alarm calls. To test this hypothesis, we undertook a multi-year study of red squirrels in southern Alberta, Canada. We report details of alarm call production by red squirrels during natural predator encounters, in response to a series of predator simulation experiments, and during encounters with non-predatory species, including conspecific territorial intruders. The pattern of alarm call production was consistent across these conditions and involved two main call types, the tonal seet call and a more broadband ‘seet-bark’ call, that corresponded closely to the bark call identified previously. However, there was little evidence that call production was specific to particular predator classes. Instead, the two call types were produced together in mixed bouts to predators of all types as well as to non-predatory species and conspecific intruders. These outcomes contradict the hypothesis that alarm calls in red squirrels are referentially-specific. We suggest instead that calls might be directed primarily at the intruders themselves and function to announce their detection and possibly aid in deterring or repelling them. This possibility is consistent with a variety of other important features of the behavior and life history of red squirrels. ItemPredator-associated vocalizations in North American red squirrels (Tamiasciurus hudsonicus): to whom are alarm calls addressed and how do they function?(2009) Digweed, Shannon; Rendall, DrewAlarm vocalizations produced by prey species encountering predators can serve a variety of functions. North American red squirrels are a small-bodied mammal popularly known for producing loud, conspicuous alarm calls, but functional accounts of calling in this species are few and contradictory. We conducted research over a three-year period on a sample of 47 marked red squirrels in the foothills of the Canadian Rockies. We recorded the production of alarm calls during encounters with natural predators and in a series of simulated predator experiments. We tested for variation in call production patterns consistent with three traditional hypotheses concerning the conspecific warning functions of alarm calling: namely that they serve as warnings to kin, to potential mates, or to territorial neighbors with which callers have an established relationship. Patterns of calling did not provide clear support for any of these hypothesized functions. We consider several possible qualifications to our results. We also consider the possibility that conspicuous calls given by red squirrels during encounters with predators are directed at the predators themselves and function to announce their detection and possibly deter them. This possibility is consistent with additional life-history features of red squirrels including that they are a relatively solitary and territorial, food-hoarding species that produces the same conspicuous vocalizations in response to other squirrels intruding on their territory to steal cones. An important corollary of this account is that red squirrel alarm calls probably do not entail referentially-specific messages about different types of predator, as proposed previously. ItemWho's your neighbor? Acoustic cues to individual identity in red squirrel (Tamiasciurus hudsonicus) rattle calls(2012) Digweed, Shannon; Rendall, Drew; Imbeau, TeanaNorth American red squirrels (Tamiasciurus hudsonicus) often produce a loud territorial rattle call when conspecifics enter or invade a territory. Previous playback experiments suggest that the territorial rattle call may indicate an invader's identity as squirrels responded more intensely to calls played from strangers than to calls played from neighbors. This dear- enemy effect is well known in a variety of bird and mammal species and functions to reduce aggressive interactions between known neighbours. However, although previous experiments on red squirrels suggest some form of individual differentiation and thus recognition, detailed acoustic analysis of potential acoustic cues in rattle calls have not been conducted. If calls function to aid in conspecific identification in order to mitigate aggressive territorial interactions, we would expect that individual recognition cues would be acoustically represented. Our work provides a detailed analysis of acoustic cues to identity within rattle calls. A total of 225 calls across 32 individual squirrels from Sheep River Provincial Park, Kananaskis, AB, Canada, were analyzed with discriminant function analysis for potential acoustic cues to individual identity. Initial analysis of all individuals revealed a reliable acoustic differentiation across individuals. A more detailed analysis of clusters of neighbouring squirrels was performed and results again indicated a statistically significant likelihood that calls were assigned correctly to specific squirrels (55-75% correctly assigned); in other words squirrels have distinct voices that should allow for individual identification and discrimination by conspecifics.