In many species, territory advertisement is thought to be one of the primary functions of acoustic communication. North American red squirrels are a territorial species in which ‘rattles’ have long been thought to be the principal signal communicating territory ownership. These vocalizations have been assumed to deter intruders, thus reducing energetic costs and the risk of injury associated with direct aggressive interactions. However, this hypothesis has not been directly tested. Here we used a speaker occupation experiment to test whether red squirrel rattles function to deter conspecific rivals. We studied 29 male squirrels and removed each individual from his territory twice in a paired design. During the experimental treatment, we simulated the owner’s presence after its removal by broadcasting the owner’s rattle from a loudspeaker at the centre of the territory once every 7 min. During the control treatment, the territory was left in silence following the temporary removal of the owner. We found that the presence of a speaker replacement reduced the probability of intrusion by 34% and increased the latency to first intrusion by 7%, providing support for the hypothesis that rattles play an active role in reducing intrusion risk. However, intrusions were not completely averted by the speaker replacement, indicating that for some individuals vocalizations alone are not a sufficient deterrent without other cues of the territory owner.
Catchpole, C.K. (1982). The evolution of bird sounds in relation to mating and spacing behavior. — In: Acoustic communication in birds (Kroodsma, D.E. & Miller, E.H., eds). Academic Press, London, p. 297-319.
Dantzer, B., Boutin, S., Humphries, M.M. & McAdam, A.G. (2012). Behavioral responses of territorial red squirrels to natural and experimental variation in population density. — Behav. Ecol. Sociobiol.66: 865-878.
Hayes, S.A., Kumar, A., Costa, D.P., Mellinger, D.K., Harvey, J.T., Southall, B.L. & Le Boeuf, B.J. (2004). Evaluating the function of the male harbour seal, Phoca vitulina, roar through playback experiments. — Anim. Behav.67: 1133-1139.
Pereira, R., Rismondo, S., Caiano, M., Pedroso, S.S., Fonseca, P.J. & Amorim, M.C.P. (2013). The role of agonistic sounds in male nest defence in the painted goby Pomatoschistus pictus. — Ethology120: 53-63.
Shonfield, J., Gorrell, J.C., Coltman, D.W., Boutin, S., Humphries, M.M., Wilson, D.R. & McAdam, A.G. (2017). Using playback of territorial calls to investigate mechanisms of kin discrimination in red squirrels. — Behav. Ecol.28: 382-390.