Cory T. Williams

Effects of nutritional restriction on nitrogen and carbon stable isotopes in growing seabirds

When using stable isotopes as dietary tracers it is essential to consider effects of nutritional state on isotopic fractionation. While starvation is known to induce enrichment of 15N in body tissues, effects of moderate food restriction on isotope signatures have rarely been tested. We conducted two experiments to investigate effects of a 50–55% reduction in food intake on δ15N and δ13C values in blood cells and whole blood of tufted puffin chicks, a species that exhibits a variety of adaptive responses to nutritional deficits. We found that blood from puffin chicks fed ad libitum became enriched in 15N and 13C compared to food-restricted chicks. Our results show that 15N enrichment is not always associated with food deprivation and argue effects of growth on diet–tissue fractionation of nitrogen stable isotopes (Δ15N) need to be considered in stable isotope studies. The decrease in δ13C of whole blood and blood cells in restricted birds is likely due to incorporation of carbon from 13C-depleted lipids into proteins. Effects of nutritional restriction on δ15N and δ13C values were relatively small in both experiments (δ15N: 0.77 and 0.41‰, δ13C: 0.20 and 0.25‰) compared to effects of ecological processes, indicating physiological effects do not preclude the use of carbon and nitrogen stable isotopes in studies of seabird ecology. Nevertheless, our results demonstrate that physiological processes affect nitrogen and carbon stable isotopes in growing birds and we caution isotope ecologists to consider these effects to avoid drawing spurious conclusions.

Corticosterone levels of tufted puffins vary with breeding stage, body condition index, and reproductive performance

Corticosterone (CORT) levels in free-living animals are seasonally modulated and vary with environmental conditions. Although most studies measure total CORT concentrations, levels of corticosteroid binding globulin (CBG) may also be modulated, thus altering the concentration of CORT available for diffusion into tissues (free CORT). We investigated the seasonal dynamics of CBG, total CORT, and free CORT in breeding tufted puffins (Fratercula cirrhata) during 2 years characterized by high rates of nestling growth and survival. We then compared concentrations of total CORT in this population to levels in chick-rearing puffins at another colony during 2 years with low productivity. At the high productivity colony, levels of CBG, total baseline CORT, free baseline CORT, and total maximum CORT were all higher prior to egg-laying than during late incubation and late chick-rearing. Levels of CBG were positively correlated with body condition index (BCI) and free baseline CORT was negatively correlated with BCI. Total baseline levels of CORT during chick-rearing were two to four times higher at the colony with low rates of nestling growth and survival. Our results demonstrate the need for long-term datasets to disentangle seasonal trends in CORT levels from trends driven by changes in environmental conditions. Given the negative effects associated with chronic elevation of CORT, our results indicate the cost of reproduction may be higher during years characterized by low productivity.

USING FATTY ACIDS AS DIETARY TRACERS IN SEABIRD TROPHIC ECOLOGY: THEORY, APPLICATION AND LIMITATIONS

Analysis of fatty acids (FAs) is an increasingly utilized tool in studies of trophic ecology in marine ecosystems. This powerful technique has proved useful in delineating spatial and temporal variability in diets, identifying the consumption of key species, and providing quantitative estimates of diet composition. Although consumer FA signatures are undeniably influenced by diet, they can also be affected by other factors including life-history stage, diet quality, and physiological state. Here, we review how FAs are assimilated, deposited, and metabolized in birds, and the implications of these processes on the various tissues commonly sampled for FA analyses. We then examine the assumptions underlying FA signature analysis when used in studies of seabird trophic ecology and propose a direction for future laboratory experiments that are needed to refine the approach. The correct interpretation of FA data relies on accounting for factors that alter predator FA metabolism and controlling for variability in the lipid content and FA composition of prey. Efforts should also be made to incorporate uncertainty associated with predator metabolism into models designed for quantitative diet estimation.

Daily body temperature rhythms persist under the midnight sun but are absent during hibernation in free-living arctic ground squirrels.

In indigenous arctic reindeer and ptarmigan, circadian rhythms are not expressed during the constant light of summer or constant dark of winter, and it has been hypothesized that a seasonal absence of circadian rhythms is common to all vertebrate residents of polar regions. Here, we show that, while free-living arctic ground squirrels do not express circadian rhythms during the heterothermic and pre-emergent euthermic intervals of hibernation, they display entrained daily rhythms of body temperature (Tb) throughout their active season, which includes six weeks of constant sun. In winter, ground squirrels are arrhythmic and regulate core body temperatures to within ±0.2°C for up to 18 days during steady-state torpor. In spring, after the use of torpor ends, male but not female ground squirrels, resume euthermic levels of Tb in their dark burrows but remain arrhythmic for up to 27 days. However, once activity on the surface begins, both sexes exhibit robust 24 h cycles of body temperature. We suggest that persistence of nycthemeral rhythms through the polar summer enables ground squirrels to minimize thermoregulatory costs. However, the environmental cues (zeitgebers) used to entrain rhythms during the constant light of the arctic summer in these semi-fossorial rodents are unknown.

Hibernation and Circadian Rhythms of Body Temperature in Free-Living Arctic Ground Squirrels

In mammals, the circadian master clock generates daily rhythms of body temperature (Tb) that act to entrain rhythms in peripheral circadian oscillators. The persistence and function of circadian rhythms during mammalian hibernation is contentious, and the factors that contribute to the reestablishment of rhythms after hibernation are unclear. We collected regular measures of core Tb (every 34 min) and ambient light conditions (every 30 s) before, during, and following hibernation in free-living male arctic ground squirrels. Free-running circadian Tb rhythms at euthermic levels of Tb persisted for up to 10 d in constant darkness after animals became sequestered in their hibernacula in fall. During steady state torpor, Tb was constant and arrhythmic for up to 13 d (within the 0.19°C resolution of loggers). In spring, males ended heterothermy but remained in their burrows at euthermic levels of Tb for 22–26 d; patterns of Tb were arrhythmic for the first 10 d of euthermia. One of four squirrels exhibited a significant free-running Tb rhythm ( h) before emergence; this squirrel had been briefly exposed to low-amplitude light before emergence. In all animals, diurnal Tb rhythms were immediately reestablished coincident with emergence to the surface and the resumption of surface activity. Our results support the hypothesis that clock function is inhibited during hibernation and reactivated by exposure to light, although resumption of extended surface activity does not appear to be necessary to reinitiate Tb cycles.

Hibernating above the permafrost: effects of ambient temperature and season on expression of metabolic genes in liver and brown adipose tissue of arctic ground squirrels

SUMMARY Hibernating arctic ground squirrels (Urocitellus parryii), overwintering in frozen soils, maintain large gradients between ambient temperature (Ta) and body temperature (Tb) by substantially increasing metabolic rate during torpor while maintaining a subzero Tb. We used quantitative reverse-transcription PCR (qRT-PCR) to determine how the expression of 56 metabolic genes was affected by season (active in summer vs hibernating), metabolic load during torpor (imposed by differences in Ta: +2 vs –10°C) and hibernation state (torpid vs after arousal). Compared with active ground squirrels sampled in summer, liver from hibernators showed increased expression of genes associated with fatty acid catabolism (CPT1A, FABP1 and ACAT1), ketogenesis (HMGCS2) and gluconeogenesis (PCK1) and decreased expression of genes associated with fatty acid synthesis (ACACB, SCD and ELOVL6), amino acid metabolism, the urea cycle (PAH, BCKDHA and OTC), glycolysis (PDK1 and PFKM) and lipid metabolism (ACAT2). Stage of hibernation (torpid vs aroused) had a much smaller effect, with only one gene associated with glycogen synthesis (GSY1) in liver showing consistent differences in expression levels between temperature treatments. Despite the more than eightfold increase in energetic demand associated with defending Tb during torpor at a Ta of –10 vs +2°C, transcript levels in liver and brown adipose tissue differed little. Our results are inconsistent with a hypothesized switch to use of non-lipid fuels when ambient temperatures drop below freezing.

Reproductive phenology of a food-hoarding mast-seed consumer: resource- and density-dependent benefits of early breeding in red squirrels.

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.

Light loggers reveal weather-driven changes in the daily activity patterns of arboreal and semifossorial rodents

Abstract Measuring daily and seasonal patterns of activity is useful for understanding the ecological and evolutionary drivers of behavior. We used collar-mounted light loggers to examine how nest attendance in arboreal squirrels and aboveground activity in semifossorial ground squirrels are affected by weather-driven changes in thermoregulatory conditions. Activity of lactating red squirrels (Tamiasciurus hudsonicus) had a diurnal pattern showing 3 daily peaks of activity with time spent outside the nest increasing with increasing ambient temperature, but decreasing with increasing relative humidity and wind. Despite the persistence of daylight during midsummer in the arctic environment, female arctic ground squirrels (Urocitellus parryii) also exhibited diurnal activity patterns with time spent above ground each day decreasing in response to precipitation but increasing with increasing ambient temperature and incident solar radiation. On cooler days, ground squirrels exhibited a unimodal activity pattern. However, on warm days, ground squirrels spent less time above ground when solar radiation and ambient temperature were both at their daily maxima, which resulted in a bimodal activity pattern. Our results highlight the utility of light loggers as a cost-effective means of addressing questions related to foraging behavior, parental care, thermoregulation, energetics, and timing of activity in arboreal and semifossorial small mammals.

Linking intraspecific variation in territory size, cone supply, and survival of North American red squirrels

Abstract In territorial species, competitive asymmetries can result in the uneven distribution of food resources as high-quality individuals force young or subordinates to occupy smaller or lower-quality sites, or both. However, spatiotemporal variation in the production of resources also can influence an individuals ability to monopolize resources and, consequently, affect survival. We examined how spatial and temporal variation in food supply affects the distribution of resources among territorial food-hoarding red squirrels (Tamiasciurus hudsonicus) by mapping cone production over 62.5 ha of white spruce (Picea glauca) forest and measuring the interrelationships between territory size, cone supply, and survival during 4 years of low cone abundance in Yukon, Canada. Territory size and cone production within a territory varied 10- and 520-fold, respectively, with juvenile squirrels occupying smaller territories with fewer cones. Because of small-scale heterogeneity in the distribution of cones, territory size explained low to moderate amounts of the variation in territory-wide cone production within (rs = 0.49–0.69) and across (rs = 0.31) years. Furthermore, spatial heterogeneity in cone production varied across years such that territory location was not useful for predicting relative food availability from one year to the next. Perhaps as a consequence of this heterogeneity, the number of cones within an individuals hoard was only weakly correlated with territory size (rs = 0.42) and not correlated with territory-wide cone production. Hoard size better predicted overwinter survival than did territory size, whereas cone production on a territory was not predictive of survival. We suggest that caution be used in equating territory size with food supply because small-scale spatial heterogeneity can weaken this relationship, particularly in food-hoarders. Spatiotemporal variation in food abundance, in turn, may cause high-quality territories in one year to be low-quality territories in other years, which will limit the ability of individuals to assess the long-term quality of territories at the time of settlement.

Persistence, Entrainment, and Function of Circadian Rhythms in Polar Vertebrates

Polar organisms must cope with an environment that periodically lacks the strongest time-giver, or zeitgeber, of circadian organization-robust, cyclical oscillations between light and darkness. We review the factors influencing the persistence of circadian rhythms in polar vertebrates when the light-dark cycle is absent, the likely mechanisms of entrainment that allow some polar vertebrates to remain synchronized with geophysical time, and the adaptive function of maintaining circadian rhythms in such environments.