Justine Sears

Bio: Justine Sears is an academic researcher from University of Alaska Fairbanks. The author has contributed to research in topics: Stable isotope ratio & Rhinoceros. The author has an hindex of 3, co-authored 3 publications receiving 247 citations.

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

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

Disentangling effects of growth and nutritional status on seabird stable isotope ratios

Abstract: A growing number of studies suggest that an individual’s physiology affects its carbon and nitrogen stable isotope signatures, obscuring a signal often assumed to be only a reflection of diet and foraging location. We examined effects of growth and moderate food restriction on red blood cell (RBC) and feather δ15N and δ13C in rhinoceros auklet chicks (Cerorhinca monocerata), a piscivorous seabird. Chicks were reared in captivity and fed either control (75 g/day; n = 7) or ~40% restricted (40 g/day; n = 6) amounts of high quality forage fish. We quantified effects of growth on isotopic fractionation by comparing δ15N and δ13C in control chicks to those of captive, non-growing subadult auklets (n = 11) fed the same diet. To estimate natural levels of isotopic variation, we also collected blood from a random sample of free-living rhinoceros auklet adults and chicks in the Gulf of Alaska (n = 15 for each), as well as adult feather samples (n = 13). In the captive experiment, moderate food restriction caused significant depletion in δ15N of both RBCs and feathers in treatment chicks compared to control chicks. Growth also induced depletion in RBC δ15N, with chicks exhibiting lower δ15N when they were growing the fastest. As growth slowed, δ15N increased, resulting in an overall pattern of enrichment over the course of the nestling period. Combined effects of growth and restriction depleted δ15N in chick RBCs by 0.92‰. We propose that increased nitrogen-use efficiency is responsible for 15N depletion in both growing and food-restricted chicks. δ15N values in RBCs of free-ranging auklets fell within a range of only 1.03‰, while feather δ15N varied widely. Together, our captive and field results suggest that both growth and moderate food restriction can affect stable isotope ratios in an ecologically meaningful way in RBCs although not feathers due to greater natural variability in this tissue.

Rhinoceros Auklet Developmental Responses to Food Limitation: An Experimental Study

Abstract: Seabirds may be particularly vulnerable to neonatal food restriction because their nestling periods tend to be long and parents may not increase foraging effort during times of prey shortage. We performed a captive study of Rhinoceros Auklet (Cerorhinca monocerata) nestlings to identify adaptations for coping with food shortages, as well as possible consequences of early diet restriction on subadult morphology. We tested effects of a ~50% caloric restriction on Rhinoceros Auklet morphological allocation and levels of the stress hormone corticosterone. Rhinoceros Auklets were reared in captivity and provisioned either ~441 kJ per day or ~227 kJ per day of high-quality forage fish until fledging (n = 13 for both treatment groups). Food-restricted Rhinoceros Auklets allocated energy heavily toward skeletal growth at the expense of mass reserves, resulting in fledglings that were proportioned very differently compared to nonrestricted birds—i.e., at 42 days of age, all birds had approximately the sam...

Recent Bayesian stable-isotope mixing models are highly sensitive to variation in discrimination factors.

Abstract: Stable isotopes are now used widely in ecological studies, including diet reconstruction, where quantitative inferences about diet composition are derived from the use of mixing models. Recent Bayesian models (MixSIR, SIAR) allow users to incorporate variability in discrimination factors (Δ13C or Δ15N), or the amount of change in either δ13C or δ15N between prey and consumer, but to date there has been no systematic assessment of the effect of variation in Δ13C or Δ15N on model outputs. We used whole blood from Common Terns (Sterna hirundo) and muscle from their common prey items (fish and euphausiids) to build a series of mixing models in SIAR (stable isotope analysis in R) using various discrimination factors from the published literature for marine birds. The estimated proportion of each diet component was affected significantly by Δ13C or Δ15N. We also use recently published stable-isotope data on the reliance of critically endangered Balearic Shearwaters (Puffinus mauretanicus) on fisheries discards ...

Beyond diet reconstruction: stable isotope applications to human physiology, health, and nutrition.

Abstract: Analysis of stable carbon and nitrogen isotopes from soft or mineralized tissues is a direct and widely-used technique for modeling diets. In addition to its continued role in paleodiet analysis, stable isotope analysis is now contributing to studies of physiology, disease, and nutrition in archaeological and living human populations. In humans and other animals, dietary uptake and distribution of carbon and nitrogen among mineralized and soft tissue is carried out with varying efficiency due to factors of internal biology. Human pathophysiologies may lead to pathology-influenced isotopic fractionation that can be exploited to understand not just skeletal health and diet, but physiological health and nutrition. This study reviews examples from human biology, non-human animal ecology, biomedicine, and bioarchaeology demonstrating how stable isotope analyses are usefully applied to the study of physiological adaptation and adaptability. Suggestions are made for future directions in applying stable isotope analysis to the study of nutritional stress, disease, and growth and development in living and past human populations.

Discrimination factors (Δ15N and Δ13C) in an omnivorous consumer : effect of diet isotopic ratio

Abstract: Summary 1Naturally occurring stable isotopes in resources and their consumer allow the estimation of nutritional flows between the two and have been much used to improve our understanding of the nutritional ecology of free-living animals. 2The difference in isotopic composition between an animal and its diet is represented by a discrimination factor. Carbon and nitrogen flows are estimated by calculating the discrimination factors in stable isotope ratios (δ15N and δ13C), which are presumed to be c. 3‰ and 1‰ heavier in the consumer tissues than those in their resources, respectively. 3The discrimination factor is known to vary according to species, tissue, age, growth rates and food quality, but the estimation of discrimination factors is difficult and a fixed discrimination factor is usually used in diet reconstruction. It has also been suggested that discrimination factors could vary linearly with the diet isotopic ratio. If this linear relationship could be demonstrated using regression, this would provide an adequate method for the estimation of discrimination factors. In order to understand how diet isotopic ratios affect the discrimination factor, we investigated the pattern of its change in nitrogen (Δ15N) and carbon (Δ13C) in different tissues (liver, muscle and hair) of an omnivore species, the rat Rattus rattus. We fed captive rats with diets of the same nutritional quality but on different isotopic ratios. 4First, discrimination factors for Δ15N and Δ13C showed great variability, ranging from –1·46‰ to 4·59‰ and from –8·79‰ to 0·64‰, respectively. Discrimination factors depended on both diet isotopic ratio and tissue. 5We also show that isotope ratios in shaved hairs showed a turnover during the first month, and then stabilized during the second month. Using shaved hairs has the potential to be an effective non-lethal method for determining resource shifts in non-specialist consumers. 6Finally, we demonstrated, for all tissues, a decrease of Δ15N and Δ13C with an increased values of δ15N and δ13C, respectively. These relationships allow us to propose a framework to estimate discrimination factors from diet isotopic ratios by means of regression models.

Short- and long-term effects of developmental corticosterone exposure on avian physiology, behavioral phenotype, cogni- tion, and fitness: A review

Abstract: A growing body of evidence from across taxa suggests that exposure to elevated levels of glucocorticoids during early development can have long-term effects upon physiological and behavioral phenotypes. Additionally, there is some, though limited, evidence that similar early exposure can also negatively impact cognitive ability. Following pioneering mammalian stud- ies, several avian studies have revealed that the responsiveness of the hypothalamo-pituitary-adrenal (HPA) axis as an adult can be explained by levels of corticosterone, the avian glucocorticoid, the individual experienced as a nestling or even as an embryo via yolk exposure. Studies also suggest that perinatal exposure to corticosterone can have effects upon avian 'personalities' or coping styles, and findings from mammalian studies suggest that these long-term effects are mediated epigenetically via altered expression of relevant DNA sequences. Although a consistent pattern across-species has yet to emerge, recent work in Florida scrub-jays Aphelocoma coerulescens found that baseline corticosterone levels in 11-day-old nestlings explained 84% of the varia- tion in 'personality' (bold vs. timid) when those individuals were tested approximately seven months later. Nestlings with ele- vated corticosterone levels were more timid than those individuals that as nestlings experienced relatively low corticosterone lev- els. Some researchers have suggested that parents might use such mechanisms to 'program' their offsprings' phenotype to best fit prevailing environmental conditions. This review will visit what is known about the links between stressful developmental condi- tions that result in exposure to elevated corticosterone and the short- and long-term effects of this steroid hormone upon central nervous system function and whether alterations thereof are beneficial, deleterious, or neutral. It will concentrate on examples from birds, although critical supporting studies from the mammalian literature will be included as appropriate (Current Zoology 57 (4): 514-530, 2011).