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The production of offspring by vertebrates is often timed to coincide with the annual peak in resource availability. However, capital breeders can extend the energetic benefits of a resource pulse by storing food or fat, thus relaxing the need for synchrony between energy supply and demand. Food-hoarding red squirrels (Tamiasciurus hudsonicus) breeding in the boreal forest are reliant on cones from a masting conifer for their nutrition, yet lactation is typically completed before the annual crop of cones is available for consumption such that peaks in energy supply and demand are not synchronized. We investigated the phenological response of red squirrels to annual variation in environmental conditions over a 20-year span and examined how intra- and inter-annual variation in the timing of reproduction affected offspring recruitment. Reproductive phenology was strongly affected by past resource availability with offspring born earlier in years following large cone crops, presumably because this affected the amount of capital available for reproduction. Early breeders had higher offspring survival and were more likely to renest following early litter loss when population density was high, perhaps because late-born offspring are less competitive in obtaining a territory when vacancies are limited. Early breeders were also more likely to renest after successfully weaning their first litter, but renesting predominantly occurred during mast years. Because of their increased propensity to renest and the higher survival rates of their offspring, early breeders contribute more recruits to the population but the advantage of early breeding depends on population density and resource availability.

Responses to proximate cues that directly affect fitness or cues directly released by selective agents are well-documented forms of phenotypic plasticity. For example, to reduce predation risk, prey change phenotype in response to light level (e.g., moon phase) when light affects predation risk from visual predators, and to chemical cues (kairomones) released by predators. Less well understood is the potential for organisms to perceive predation risk through "proxy cues": proximate cues that correlate with, but do not directly affect predation risk. Previous field studies indicate that body and spine length of an invasive cladoceran in Lake Michigan, Bythotrephes longimanus (the spiny water flea), increase during the growing season, coincident with a decrease in clutch size. Although the cause of seasonal trait changes is not known, changes are associated with warmer water temperature and increased predation risk from gape-limited fish (i.e., fish whose ability to consume Bythotrephes is limited by mouth size). Using a laboratory experiment, we found no effect of fish (Perca flavescens) kairomones on Bythotrephes morphology or life history. In contrast, higher water temperature led to longer absolute spine and body length, increased investment in morphological defense of offspring (measured as the ratio of spine-to-body length), and decreased clutch size and age at reproduction. These plastic responses are unlikely to be adaptive to temperature per se, but rather our findings indicate that temperature serves as a proxy cue of fish predation risk. Temperature correlates with risk of gape-limited fish predation due to growth of fish from larval stages incapable of consuming Bythotrephes early in the season, to larger sizes by midseason increasingly capable of consuming Bythotrephes, but limited by gape size to consuming smaller individuals. We argue that for Bythotrephes, temperature is a more reliable cue of predation risk than fish kairomones, because fish kairomones are present throughout the season due to continual presence of non-gape-limited adult fish, to which plastic response would have little effect. Organisms may, therefore, not only respond to changes in an environmental factor because the factor directly affects risk, but also when the environmental factor serves as a proxy signaling change in predation risk.

Interest in the evolution of invasive species has grown in recent years, yet few studies have investigated sources of variation in invasive species traits experiencing natural selection. The spiny water flea, Bythotrephes longimanus, is an invasive zooplankton in the Great Lakes that exhibits seasonal changes in tail spine and body length consistent with natural selection. Evolution of Bythotrephes traits, however, depends on the presence and magnitude of quantitative genetic variation, which could change within or across years. Clonal analysis of wild-captured Bythotrephes indicated that variance components for distal spine length were variable among but not within years. Spine length was always heritable but was not always influenced by maternal effects. In contrast, variance components for body length varied both within and among years, but likewise body length was always heritable and not always influenced by maternal effects. Results indicate that important Bythotrephes traits have heritable variation comparable to native species and other invasive species that would enable an evolutionary response to natural selection. This evolutionary capacity could contribute to the widespread success and dramatic effects of Bythotrephes invasion in systems with diverse biotic and abiotic conditions.

Predator satiation resulting from interannual reproductive synchrony has been widely documented in masting plants, but how reproductive synchrony within a year influences seed escape is poorly understood. We evaluated whether the intra-annual reproductive synchrony of individual white spruce trees (Picea glauca) increased seed escape from their primary predispersal seed predator, North American red squirrels (Tamiasciurus hudsonicus). Trees with cones that matured synchronously relative to those of other trees within red squirrel territories were significantly more likely to escape squirrel predation in years with both low and superabundant levels of cone production, generating a significantly positive linear selection differential for increasing intra-annual reproductive synchrony. Thus, this masting plant escapes seed predation in numbers through interannual synchrony in seed production and in time through intra-annual synchrony of seed availability.

The tendency of females to mate with multiple males is often explained by direct and indirect benefits that could outweigh the many potential costs of multiple mating. However, behaviour can only evolve in response to costs and benefits if there is sufficient genetic variation on which selection can act. We followed 108 mating chases of 85 North American red squirrels (Tamiasciurus hudsonicus) during 4 years, to measure each female's degree of multiple male mating (MMM), and used an animal model analysis of our multi-generational pedigree to provide what we believe is the first estimate of the heritability of MMM in the wild. Female red squirrels were highly polyandrous, mating with an average of 7.0 ± 0.2 males on their day of oestrus. Although we found evidence for moderate levels of additive genetic variation (CV(A) = 5.1), environmental variation was very high (CV(E) = 32.3), which resulted in a very low heritability estimate (h(2) < 0.01). So, while there is genetic variation in this trait, the large environmental variation suggests that any costs or benefits associated with differences among females in MMM are primarily owing to environmental and not genetic differences, which could constrain the evolutionary response to natural selection on this trait.

Mothers often provide much of the early environment for their offspring. These maternal effects are predicted to result in unusual evolutionary dynamics in offspring traits if they are themselves heritable, but these important predictions have not previously, to our knowledge, been tested in the wild. Here, we quantified the responses of red squirrels (Tamiasciurus hudsonicus) to documented episodes of natural selection and found support for both of the fundamental predictions of models that describe maternal effect evolution. First, changes in juvenile growth rates across one generation of selection were five times greater than predicted by heritability (h2) alone, but were consistent with the additional contribution of maternal genetic effects. Second, responses to selection were influenced not only by the strength of selection in the current generation, but also by selection in the previous generation, indicating the presence of evolutionary momentum. These results were in agreement with predictions of a simple model including litter size as the only maternal effect, and provide, to our knowledge, the first empirical evidence for the importance of maternal effects to evolutionary dynamics in a natural population.

Selection will result in observable changes in traits only if it acts consistently in space and time, but few estimates of selection in natural populations have been temporally replicated. Here we estimate viability selection on nestling growth rates for 13 cohorts (1989-2001) of red squirrels (Tamiasciurus hudsonicus) from a natural population located in southwestern Yukon, Canada. Directional selection on nestling growth rates varied in magnitude and direction from one cohort to the next. The magnitude of directional selection was relatively weak in most years (median beta' = 0.24), but there were episodes of very strong viability selection (beta' > 0.5) in some cohorts. We found no evidence of significant stabilizing or disruptive selection on this trait. Examination of viability selection episodes over shorter time periods suggested that the strength of selection on juveniles in this population was positively related to the time scale over which selection was measured. Viability selection from birth to emergence from the natal nest (50 days of age) and from emergence to successful recruitment (100 days of age) were positively correlated, but were both independent of selection on nestling growth rates from recruitment to potential breeding age (one year). The strength of directional selection on growth rates prior to recruitment was negatively correlated with spring temperature whereas selection from recruitment to breeding was positively correlated with the abundance of spruce cones produced in the previous fall. Episodes of strong directional selection from birth to breeding age appear to be due to potentially rare combinations of environmental conditions. As a result, predicting the occurrence of very strong episodes of selection will be extremely difficult, but predicting the microevolutionary responses to observed selection on individual cohorts remains feasible.

Climate change is predicted to be most severe in northern regions and there has been much interest in to what extent organisms can cope with these changes through phenotypic plasticity or microevolutionary processes. A red squirrel population in the southwest Yukon, Canada, faced with increasing spring temperatures and food supply has advanced the timing of breeding by 18 days over the last 10 years (6 days per generation). Longitudinal analysis of females breeding in multiple years suggests that much of this change in parturition date can be explained by a plastic response to increased food abundance (3.7 days per generation). Significant changes in breeding values (0.8 days per generation), were in concordance with predictions from the breeder's equation (0.6 days per generation), and indicated that an evolutionary response to strong selection favouring earlier breeders also contributed to the observed advancement of this heritable trait. The timing of breeding in this population of squirrels, therefore, has advanced as a result of both phenotypic changes within generations, and genetic changes among generations in response to a rapidly changing environment.