Individual variation in the dear enemy phenomenon via territorial vocalizations in red squirrels

in Behaviour
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Abstract

Territoriality arises when the benefits of resources exceed the costs of defending them. The dear enemy phenomenon, where familiar territorial neighbours refrain from intruding on one another and mutually reduce their defensive efforts, allows for reduction of these costs but requires discrimination between conspecifics. We hypothesized that territorial vocalizations in red squirrels (Tamiasciurus hudsonicus) are used for this discrimination. We performed a speaker replacement experiment where red squirrels (N=41) were temporarily removed from their territories and replaced with a speaker broadcasting their own call, an unfamiliar call, or silence. Contrary to our prediction, there were no differences in overall intrusion risk among our three playbacks, but the identity of intruders did vary. Existing variation in familiarity within territorial neighbourhoods should be considered, rather than the binary classification of familiar or stranger, when studying dear enemy effects. We also discuss the variable importance of silence in acoustic territorial populations.

Individual variation in the dear enemy phenomenon via territorial vocalizations in red squirrels

in Behaviour

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References

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Figures

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    Cox proportional hazard model of intrusion by neighbours, incorporating whether or not an intrusion occurred with the latency to that intrusion event. There were no differences in intrusion hazard between owner playback and silence (z=0.15, p=0.88) or owner and unfamiliar playbacks (z=1.48, p=0.14).

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    Proportion of trials across three playback treatments where a neighbour (black) or non-neighbouring (grey) squirrel intruded during a 2-hour temporary removal. There were no significant differences between playbacks when considering all trials, only neighbour intrusions, or only non-neighbour intrusions.

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    Relative hazard of intrusion by individual neighbouring squirrels during a temporary removal, under three audio treatments. Intrusion hazard is modelled using a mixed effects Cox proportional hazard model, accounting for the latency to an intrusion event and whether or not one occurred within the two-hour temporary removal; a high hazard corresponds to a shorter latency and higher probability of intrusion. There was a higher hazard of intrusion from relatively unfamiliar neighbours when broadcasting the owner’s rattle (β=1.05, z=2.82, p=0.005), whereas relative familiarity did not affect the hazard of intrusion during silence (β=0.16, z=0.49, p=0.63) or unfamiliar rattle playbacks (β=0.07, z=0.15, p=0.88).

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    Standardized familiarity (z-scores, relative to neighbourhood) of intruding neighbours during temporary removals when broadcasting the owner’s call (N=9), silence (N=14), or an unfamiliar call (N=6). Standardized familiarity for intruders on the owner’s rattle was different from zero (t=2.85, df = 23, p=0.009), but this was not the case for either of the other two groups (silence: t=0.84, df = 23, p=0.41; unfamiliar: t=0.03, df = 23, p=0.97).

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    Comparison of (a) local density and (b) familiarity within 130 m of focal squirrels used for temporary removal experiments in 2015 (N=55) and 2017 (N=41). With two further years of very low cone production since the 2014 mast event, population density has fallen with the lack of new food resources while average familiarity in 2017 has accrued as there was minimal recruitment of juveniles during this period.

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