Territory acquisition mediates the influence of predators and climate on juvenile red squirrel survival

Abstract

1. Juvenile survival to first breeding is a key life-history stage for all taxa. Survival through this period can be particularly challenging when it coincides with harsh environmental conditions such as a winter climate or food scarcity, leading to highly variable cohort survival. However, the small size and dispersive nature of juveniles generally make studying their survival more difficult.2. In territorial species, a key life-history event is the acquisition of a territory. A territory is expected to enhance survival, but how it does so is not often identified. We tested how the timing of territory acquisition influenced the winter survival of juvenile North American red squirrels Tamiasciurus hudsonicus, hereafter red squirrels, and how the timing of this event mediated the sources of mortality.We hypothesized that securing a territory prior to when food resources become available would reduce juvenile susceptibility to predation and climatic factors overwinter.3. Using 27 years of data on the survival of individually marked juvenile red squirrels, we tested how the timing of territory acquisition influenced survival, whether the population density of red squirrel predators and mean temperature overwinter were related to individual survival probability, and if territory ownership mediated these effects. 4. Juvenile red squirrel survival was lower in the years of high predator abundance and in colder winters. Autumn territory owners were less susceptible to lynx Lynx canadensis and possibly mustelid Mustela and Martes spp., predation. Autumn territory owners had lower survival in colder winters, but surprisingly non-owners had higher survival in cold winters. 5. Our results show how the timing of a life-history event like territory acquisition can directly affect survival and also mediate the effects of biotic and abiotic factors later in life. This engenders a better understanding of the fitness consequences of the timing of key life-history events.

Decoupling the effects of food and density on life‐history plasticity of wild animals using field experiments: Insights from the steward who sits in the shadow of its tail, the North American red squirrel

“…Are the effects additive or interactive? We know that the abundance of terrestrial predators in Kluane is highly variable (Figure 1c) but previous experimental work shows that individual survival rates or population dynamics of red squirrels are not heavily influenced by the number of terrestrial predators (Boonstra et al., 2001; but see Studd et al., 2015; Hendrix et al., 2020). Nonetheless, GCs are likely affected by both predation risk and warmer than average ambient temperatures.…”

“…Yukon red squirrels experience tremendous fluctuations in ecological factors that can induce plasticity and act as ecological agents of selection, such as food, conspecific density and predation risk (Figure 1). However, we have mostly focused our attention on food and conspecific density as previous experimental studies that excluded terrestrial predators and our long‐term data analyses show that terrestrial predators seem to have minimal effects on overall squirrel densities (Boonstra et al, 2001; Figure 2), though it is likely that predators do influence the survival of individual juveniles (Hendrix et al., 2020; Studd et al., 2015). Using targeted experimental manipulations of food, density or the acoustic cues reflecting density, we have been able to unravel how changes in density and food induce plasticity in life‐history traits in natural populations.…”

Maternal glucocorticoids have minimal effects on HPA axis activity and behavior of juvenile wild North American red squirrels

“…It could be more beneficial for offspring to retain a sensitivity to their own environment during subsequent development that allows any maternal cues to be "overwritten" by the cues present in their own environment as they become more independent from their mother (Leimar and McNamara, 2015). In our study area, predation risk, resource availability, and conspecific competition fluctuate between an individual's birth and first breeding season (Dantzer et al, 2013;Hendrix et al, 2020;McAdam and Boutin, 2003;Taylor et al, 2014). Theoretically, in such variable environments, it may be more beneficial for offspring to be shaped by the parental environment rather than their own environment experienced during development (Leimar and McNamara, 2015).…”

Familiar Neighbors, but Not Relatives, Enhance Fitness in a Territorial Mammal

“…6 However, we found no evidence to suggest that kin provide fitness benefits for red squirrels (Table 1), and there were no significant interactions between relatedness and familiarity in any of our models. It is possible that the observed low relatedness among neighbors (average neighborhood relatedness = 0.05 ± 0.001), which likely occurs as the result of low overwinter juvenile survival, 37 could have constrained the potential for kin selection to act at this scale. However, our findings do not mean that neighbor relatedness has no biologically meaningful effects on red squirrel fitness, but simply that the scale at which we measured these effects did not exhibit any influence of kinship.…”