Behavioral responses of territorial red squirrels to natural and experimental variation in population density

Abstract

The relative scarcity of studies at the intersection of behavioral and population ecology is surprising given the presumed importance of behavior in density-dependent population regulation. Here we tested whether North American red squirrels (Tamiasciurus hudsonicus) adjust their behavior in response to local population density and whether they use rates of territorial vocalizations in their local neighborhood to assess density. We examined these relationships using 18 years of live trapping and 20 years of behavioral data that were collected across natural variation in local population density. To disentangle the effects of population density on behavior from those due to changes in per capita food abundance or changes in the frequency of antagonistic interactions with neighbors, we also experimentally manipulated population density with long-term food supplementation as well as perceived population density with longterm playbacks of territorial vocalizations. The frequency with which squirrels emitted territorial vocalizations was positively associated with local population density. In contrast, antagonistic physical interactions observed between squirrels and territorial intrusions were rare and the frequency of intrusions was weakly and negatively, not positively, associated with population density. Squirrels experiencing naturally and experimentally high density conditions spent less time in the nest and feeding but more time being vigilant. Similar density-dependent changes in behavior were observed in response to our manipulations of perceived population density, indicating that vocalization rates and not physical interactions or food abundance were the mechanism by which squirrels assessed and responded behaviorally to changes in local density.

Red squirrels use territorial vocalizations for kin discrimination

“…Territories are nonoverlapping, are often contiguous and tend to be stable throughout the year. However, their size varies among populations and years and tends to be larger among females than among males (Dantzer et al, 2012;LaMontagne et al, 2013;Price, Broughton, Boutin, & Sinclair, 1986). Each territory is defended by a single individual, but females will share their territories with their youngof-the-year, as well as with adult males during the 1 day of the year when the female is sexually receptive (Smith, 1968).…”

“…Yet, such physical altercations are rare, with most territorial disputes instead involving the production of territorial vocalizations known as 'rattles' (Dantzer, Boutin, Humphries, & McAdam, 2012;Gorrell et al, 2010;Lair, 1990; Fig. 1).…”

Error management theory and the adaptive significance of transgenerational maternal‐stress effects on offspring phenotype

“…Examples of such species include snowshoe hares ( Lepus americanus ) or North American red squirrels ( Tamiasciurus hudsonicus ) in the Yukon, Canada, that can experience extreme interannual fluctuations in the abundance of predators, food, or conspecifics. These fluctuations in predation risk for snowshoe hares occur in a regular 10‐year cycle (Krebs et al., 1995) whereas the fluctuations in food and density in red squirrels (Boutin et al., 2006; Dantzer, Boutin, Humphries, & McAdam, 2012; Dantzer et al., 2013) are episodic, occurring every 3–4 years. For both species, the environments faced by offspring are qualitatively different (i.e., either benign or very stressful) and remain so for the course of offspring development (i.e., for the purposes of offspring survival, the environments remain either benign or stressful).…”

Equipped for Life in the Boreal Forest: The Role of the Stress Axis in Mammals

“…Winter is a time of severe food limitation and extreme temperatures, and territorial defense of food stores permits winter survival. Throughout the boreal and northern forests of North America, both male and female red squirrels are highly aggressive in the non-breeding seasons in defense of food stores on their individual territories (Dantzer et al, 2012). We discovered that both sexes have moderately high concentrations of dehydroepiandrosterone (DHEA), an androgen precursor (Boonstra et al, 2008).…”

“…Red squirrels in southwestern Yukon live in a variable environment. Episodic pulses of their major food source, seed from white spruce trees (LaMontagne and Boutin, 2007;Fletcher et al, 2010), generates interannual variation in population density (Boutin et al, 2006;Dantzer et al, 2012). These changes in population density have important effects on the lifehistory traits of female red squirrels.…”