Squirrels Do the Math: Flight Trajectories in Eastern Gray Squirrels (Sciurus carolinensis)

Abstract

Animals are under strong selective pressures to make correct decisions when attempting to escape an approaching predator, and not surprisingly many studies have shown that animals adjust their flight initiation behavior in response to risk. However, we have a poor understanding of animals' capability to select an appropriate flight trajectory. We investigated whether eastern gray squirrels would adjust their flight trajectory based on the relative locations of the squirrel, the approaching threat, and potential refuges. We used a person running toward a focal squirrel (N = 122) as the threat and considered the three trees nearest the squirrel and taller than 8 m to be potential refuges. Squirrels were strongly affected by the angle (θ) formed by the locations of person, squirrel, and the three nearest trees. A squirrel was less likely to run to the nearest tree (Tree 1) when θ 1 was relatively acute, but also less likely to run to Tree 1 when θ 2 was obtuse, making Tree 2 a more attractive refuge. A squirrel was more likely to run to Tree 1 if it was close and if Tree 2 was relatively far. Subtle differences in the effects of θ 1 vs. θ 2 on squirrel refuge choice support the idea that squirrels prefer a nearby refuge. Squirrels were more likely to select Trees 2 and 3 rather than Tree 1 only when θ 2 was obtuse (105 •). In contrast, most squirrels chose to run to Tree 1 when θ 1 was >65 • ; thus squirrels were more likely to choose Tree 1 even when doing so required running at least partly toward the approaching threat. The decisions made by focal squirrels provide evidence that this species' assessment of risk is highly nuanced. A great deal of variation has been reported in responses to predators within species. While part of the variation may be due to strategic unpredictability on the part of the prey, part of it may also be due to differences in flight trajectory and refuge preferences that have not been well-studied.

Empirical studies of escape behavior find mixed support for the race for life model

“…Our results demonstrated that when compared with other variables typically reported to explain variation in FID, such as alert distance and level of human disturbance, Φ explained significant variation in FID. Our results are consistent with Kramer and Bonenfant’s (1997) original findings, as well as those in Cooper Jr.’s (2016) race for life model, and recent empirical findings reported by Eason et al (2019) . In all cases, as Φ decreased, FID decreased.…”

“…However, animals may not make as fine distinctions in angle when making escape decisions, and may use coarser assessments of predator risk due to orientation. Therefore, we parameterized a series of GLMs in which we binned our data into different categories comprising different angles ( Figure 4 ) which were based on the predictions of the models of Kramer and Bonenfant (1997) , and Cooper (2016) and empirical results reported by Eason et al (2019) . Using AIC to compare model fits, we then assessed which angle categorization scheme most effectively explained variation in our data.…”

“…Field data for woodchucks Marmota monax strongly supported the model ( Kramer and Bonenfant 1997 ), and has informed subsequent models of escape behavior that integrate multiple risk factors. Recent work by Eason et al (2019) demonstrates the importance of relative orientation of predator, prey, and to a potential refuge on FID. When presented with multiple refuge options, Eastern grey squirrels Sciuris carolinensis choose the refuge option that optimized the tradeoff between distance fled to a burrow and how directly prey must run toward an approaching predator.…”

Occurrence of Conspecific in the Neighboring Space Influence on Flight Initiation Distance in Cape Hare <i>Lepus capensis</i> under Human Stimuli

“…FID is an outstanding metric for measuring an individual's fearfulness under certain conditions [26], and flight response is also studied to recognize the areas where the species are not influenced by humans [27]. This index has been studied in several species from different taxa, particularly in birds [28], fishes or lizards [7] [29] [30], ungulates and small mammals [23] [31] [32] [33]. However, no study has been conducted so far on the cape hare, a species that occurs in a heavily hunted landscape in the Karakorum Range.…”

Innate heuristics and fast learning support escape route selection in mice

“…Next, we tested escape route selection in four different arenas where the length of the left arm was progressively increased while keeping constant the initial angle between each path and the threat platform (arenas 1-4, Figure 1D). In this experiment the relative value of the left hand-side path decreases between arenas 1 and 4 as mice should in principle escape along the shortest path to minimize exposure to danger [10][11][12]. The differences in geodesic distance (path length) between the threat and shelter platforms translated not only into differences in distance travelled but also into time taken to traverse each path during escape (Figure S1C, D).…”