Anticipatory Reproduction and Population Growth in Seed Predators

Abstract

Mast seeding, the intermittent, synchronous production of large seed crops by a population of plants, is a well-known example of resource pulses that create lagged responses in successive trophic levels of ecological communities. These lags arise because seed predators are thought capable of increasing reproduction and population size only after the resource pulse is available for consumption. The resulting satiation of predators is a widely cited explanation for the evolution of masting. Our study shows that both American and Eurasian tree squirrels anticipate resource pulses and increase reproductive output before a masting event, thereby increasing population size in synchrony with the resource pulse and eliminating the population lag thought to be universal in resource pulse systems.

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).…”

Survival costs of reproduction vary with age in North American red squirrels

“…This index is highly correlated with the total cones produced per tree (LaMontagne et al 2005) and it is indicative of the food available to squirrels because the cones are clipped and stored for later consumption over the winter and following spring (Smith 1968;Boutin et al 2006). We used the mean number of cones counted per tree (average of logtransformed data, Boutin et al 2006) as an index of the food available for reproduction in the subsequent spring (i.e.…”

“…In addition, the squirrels are part of a typical resourcepulse system , where the main food resource (seeds of white spruce, Picea glauca) exhibits yearly variation, spanning three orders of magnitude between failure and mast years (McAdam & Boutin 2003;Boutin et al 2006). This temporal variation provides a rare opportunity to determine whether the costs of reproduction vary with food conditions.…”

“…The reliance on previously stored food and the defence of individual caches means that the individual females have the potential to assess the level of stored food that is available for current reproduction. However, as female squirrels do not adjust the size of their litter according to the food available from the previous autumn (Humphries & Boutin 2000;Boutin et al 2006), reproduction is expected to be more costly during the years of low food availability.…”

“…Red squirrels cache unopened cones in hoarding sites called middens during late summer and autumn, and then husk these cones throughout the following winter and spring to extract the seed that provides most of their energy requirements. Middens are defended year round, and food available for reproduction in the spring is determined by the abundance of cones produced in the previous year (McAdam & Boutin 2003;Boutin et al 2006). The reliance on previously stored food and the defence of individual caches means that the individual females have the potential to assess the level of stored food that is available for current reproduction.…”