PubMed Health⌕ Search

Biomedical subjects

Keith A Hobson

Publications and source records attributed to Keith A Hobson.

At least 19 recordsLinked to original sources
PubMed2006

Temporal trends of mercury in marine biota of west and northwest Greenland.

Temporal trends in mercury concentrations ([Hg]) during the last two to three decades were determined in liver of shorthorn sculpin, ringed seal and Atlantic walrus from northwest Greenland (NWG, 77 degrees N) and in liver of shorthorn sculpin and ringed seal from central west Greenland (CWG, 69 degrees N) during the last decade. Stable-nitrogen (delta(15)N) and carbon (delta(13)C) isotope values were determined in muscle of ringed seals to provide insight into potential trophic level changes through time. Log-linear regressions on annual median [Hg] did not reveal any temporal trend in shorthorn sculpin from CWG and NWG and walrus from NWG. In ringed seals from NWG, an increase in [Hg] of 7.8% per year was observed. When based on delta(15)N-adjusted [Hg] this rate increased to 8.5% but was still non-significant. In ringed seal from CWG no trend was found in [Hg] during the period 1994-2004. However, during the last part of the period (1999-2004) the [Hg] increased significantly. Including tissue delta(15)N values as a covariate had a marked effect on these results. The annual changes in delta(15)N-adjusted [Hg] was estimated to -5.0% for the whole period and 2.2% during the last 5 years compared to -1.3% and 12.4%, respectively, for the non-adjusted [Hg].

Animals↗
PubMed2006

Stable isotope evidence of diverse species-specific and individual wintering strategies in seabirds.

Although there is increasing evidence that climatic variations during the non-breeding season shape population dynamics of seabirds, most aspects of their winter distribution and ecology remain essentially unknown. We used stable isotope signatures in feathers to infer and compare the moulting (wintering) habitat of subantarctic petrels breeding at two distant localities (South Georgia and Kerguelen). Petrels showed species-specific wintering habitat preferences, with a similar pattern of latitudinal segregation for all but one taxon. At both localities, delta13C values indicated that blue petrels (Halobaena caerulea) moult in Antarctic waters, South Georgian diving petrels (Pelecanoides georgicus) in the vicinity of the archipelagos and/or in the Polar Frontal Zone and Antarctic prions (Pachyptila desolata) in warmer waters. In contrast, common diving petrels (Pelecanoides urinatrix) showed divergent strategies, with low and high intrapopulation variation at South Georgia and Kerguelen, respectively. Birds from Kerguelen dispersed over a much wider range of habitats, from coastal to oceanic waters and from Antarctica to the subtropics, whereas those from South Georgia wintered mainly in waters around the archipelago. This study is the first to show such striking between-population heterogeneity in individual wintering strategies, which could have important implications for likely demographic responses to environmental perturbation.

Animal Migration↗
PubMed2006

Stable-hydrogen isotope heterogeneity in keratinous materials: mass spectrometry and migratory wildlife tissue subsampling strategies.

Stable-hydrogen isotope measurements (deltaD) of biological tissues have gained widespread acceptance in wildlife and forensic studies, especially in tracking geographical movements of birds and other species. Continuous-flow isotope-ratio mass spectrometry enables high-throughput deltaD analyses to be conducted on tissue samples as small as 0.15 mg, compared with conventional offline analyses that require 7-10 mg. This reduction in sample size has raised concerns regarding intra-sample hydrogen isotopic variance due to potential biological heterogeneities that could exceed interpretations of geospatial origin. To help resolve this, feathers were obtained from captive birds to examine isotopic variance expected due to sample size, location, and heterogeneity factors, and from selected wild birds to examine isotopic variance due to these and to additional dietary or location changes during feather growth. Captive bird feathers were sub-sampled along the vane on either side of a single feather at masses of 0.25, 0.35, 0.45, 0.6, 1.0 and 2.0 mg, and along the rachis. The results showed consistency of feather deltaD measurements across a wide range of sample masses. Within-feather deltaD isotopic variance for captive and some wild birds was as low as +/-3 per thousand for vane material, which corresponds to a geospatial resolution of about 1 degree of latitude in central North America. Intra-sample variance for the rachis was +/-5 per thousand, with lower deltaD values for both wild and captive birds. However, given the extraordinary intra-feather deltaD variance observed in some wild species, we recommend researchers first carefully assess the degree of intra- and inter-sample hydrogen isotopic variation in the selected tissue growth period for the species of interest before geospatial interpretations of origin are attempted.

Animal Migration↗
PubMed2006

Effect of diet quality on carbon and nitrogen turnover and isotopic discrimination in blood of a New World nectarivorous bat.

Diet composition of carbon and nitrogen (C:N) could affect diet-tissue isotopic discrimination and elemental turnover rate in consumers but studies that test the nature of these changes are scarce. We compared carbon and nitrogen isotopic discrimination and turnover rates in individuals of Pallas' long-tongued bats Glossophaga soricina fed diets with protein soya isolate or amaranth grains as their main source of protein. Diets were of similar protein biological value but the soya diet had higher nitrogen content (2.2%N) and lower C:N ratio (39.6) than the amaranth diet (1.3%N, C:N=40.5). Most bats on the soya diet gained body mass whereas most bats on the amaranth diet lost body mass. Half-lives of carbon (24.3+/-3.8 days) and nitrogen (25.6+/-4.4 days) in bats switched to the soya diet were very similar. In contrast, in the bats switched to the amaranth diet, carbon half-life (39.7+/-3.4 days) was longer than that of nitrogen (25.0+/-6.0 days). The enrichment in 15N between diet and blood was higher when bats were fed the amaranth diet (4.4+/-0.2 per thousand) than when they were fed the soya diet (3.3+/-0.2 per thousand). Similarly, bats on the amaranth diet had higher 13C enrichment (2.0+/-0.2 per thousand) than bats on the soya diet (0.1+/-0.1 per thousand). Our results support recent hypotheses of the effect of nutrition on diet-tissue isotopic discrimination and turnover rate, and further shows that blood stable isotope analysis is an adequate approach to track seasonal dietary shifts in wild bats.

Amaranthus↗
PubMed2005

Intrapopulation variation in gray wolf isotope (delta(15)N and delta(13)C) profiles: implications for the ecology of individuals.

Trophic relationships among organisms in terrestrial boreal ecosystems define ecological communities and are important in determining dynamics of energy flow and ecosystem function. We examined trophic relationships between the gray wolf (Canis lupus) and 18 mammalian species from the boreal forest of central Saskatchewan, Canada, using delta(13)C and delta(15)N stable isotope values measured in guard hair samples. Variance in isotope values for wolves and other carnivores was investigated as a proxy for variation in diet among individuals. Isosource, an isotopic source partitioning model, quantified the relative range in proportions of five most-likely prey items in the diets of wolves. The distribution of feasible contributions from each source was dominated by elk (Cervus elaphus; mean: 48%, range:11-75%), followed by white-tailed deer (Odocoileus virginianus; mean: 21%, range: 0-54%), moose (Alces alces; mean:14%, range: 0-41%), beaver (Castor canadensis; mean: 8%, range:0-25%) and snowshoe hare (Lepus americanus; mean: 8%, range: 0-24%). Despite social foraging, our results indicate highly variable diets among individuals and we discuss this in terms of individual versus group ecology of boreal wolves.

Animals↗
PubMed2005

Stable isotopes, beaks and predators: a new tool to study the trophic ecology of cephalopods, including giant and colossal squids.

Cephalopods play a key role in the marine environment but knowledge of their feeding habits is limited by lack of data. Here, we have developed a new tool to investigate their feeding ecology by combining the use of their predators as biological samplers together with measurements of the stable isotopic signature of their beaks. Cephalopod beaks are chitinous hard structures that resist digestion and the stable isotope ratios of carbon (delta13C) and nitrogen (delta15N) are indicators of the foraging areas and trophic levels of consumers, respectively. First, a comparison of delta13C and delta15N values of different tissues from the same individuals showed that beaks were slightly enriched in 13C but highly impoverished in 15N compared with lipid-free muscle tissues. Second, beaks from the same species showed a progressive increase in their delta15N values with increasing size, which is in agreement with a dietary shift from lower to higher trophic levels during cephalopod growth. In the same way, there was an increase in the delta15N signature of various parts of the same lower beaks in the order rostrum, lateral walls and wings, which reflects the progressive growth and chitinization of the beaks in parallel with dietary changes. Third, we investigated the trophic structure of a cephalopod community for the first time. Values of delta15N indicate that cephalopods living in slope waters of the subantarctic Kerguelen Islands (n=18 species) encompass almost three distinct trophic levels, with a continuum of two levels between crustacean- and fish-eaters and a distinct higher trophic level occupied by the colossal squid Mesonychoteuthis hamiltoni. delta13C values demonstrated that cephalopods grow in three different marine ecosystems, with 16 species living and developing in Kerguelen waters and two species migrating from either Antarctica (Slosarczykovia circumantarctica) or the subtropics (the giant squid Architeuthis dux). The stable isotopic signature of beaks accumulated in predators' stomachs therefore revealed new trophic relationships and migration patterns and is a powerful tool to investigate the role of the poorly known cephalopods in the marine environment.

Animal Migration↗
PubMed2005

Mercury and other trace elements in a pelagic Arctic marine food web (Northwater Polynya, Baffin Bay).

Total mercury (THg), methylmercury (MeHg) and 22 other trace elements were measured in ice algae, three species of zooplankton, mixed zooplankton samples, Arctic cod (Boreogadus saida), ringed seals (Phoca hispida) and eight species of seabirds to examine the trophodynamics of these metals in an Arctic marine food web. All samples were collected in 1998 in the Northwater Polynya (NOW) located between Ellesmere Island and Greenland in Baffin Bay. THg and MeHg were found to biomagnify through the NOW food web, based on significant positive relationships between log THg and log MeHg concentrations vs. delta15N muscle and liver . The slope of these relationships for muscle THg and MeHg concentrations (slope=0.197 and 0.223, respectively) were similar to those reported for other aquatic food webs. The food web behavior of THg and delta15N appears constant, regardless of trophic state (eutrophic vs. oligotrophic), latitude (Arctic vs. tropical) or salinity (marine vs. freshwater) of the ecosystem. Rb in both liver and muscle tissue and Zn in muscle tissue were also found to biomagnify through this food web, although at a rate that is approximately 25% of that of THg. A number of elements (Cd, Pb and Ni in muscle tissue and Cd and Li in seabird liver tissue) were found to decrease trophically through the food web, as indicated by significantly negative relationships with tissue-specific delta15N. A diverse group of metals (Ag, Ba, La, Li, Sb, Sr, U and V) were found to have higher concentrations in zooplankton than seabirds or marine mammals due to bioconcentration from seawater. The remaining metals (As, Co, Cu, Ga, Mn, Mo and Se in muscle tissue) showed no relationship with trophic position, as indicated by delta15N values, although As in liver tissue showed significant biomagnification in the seabird portion of the food web.

Animals↗
PubMed2005

Regional differences in collagen stable isotope and tissue trace element profiles in populations of long-tailed duck breeding in the Canadian Arctic.

Adult long-tailed ducks (Clangula hyemalis) were collected from nine locations across their breeding grounds in northern Canada and measurements of stable isotopes of carbon (delta(13)C), nitrogen (delta(15)N) and sulfur (delta(34)S) in bone collagen were used to investigate if relative use of freshwater habitats such as the Great Lakes (with expected depleted stable isotope profiles) compared with coastal marine environments (with expected enriched stable isotope foodweb profiles) could explain tissue trace element profiles. Contrary to expectation, all three stable isotopes did not covary in our sample, suggesting that mechanisms other than simple freshwater vs. marine isotopic gradients were involved among populations. All three stable isotopes varied significantly with collection location and both delta(15)N and delta(13)C values varied significantly between sexes suggesting that males exploit either a different food base or occur in different geographic areas than females for at least part of the year. The delta(34)S data, in particular, suggested that many of the birds breeding in the western Canadian Arctic probably overwinter in the Great Lakes along with many of the birds breeding in Hudson Bay. Males at the majority of collection locations had higher concentrations of hepatic Hg (1.1-8 microg/g dw), Cu (25-40 mug/g dw), Se (7.3-27 mug/g dw) and renal Cd (33-129 microg/g dw) than females. Concentrations of Hg, Cu and Cd were well below toxicological threshold levels found in the literature. However, hepatic Se concentrations in 64% of the females exceeded 10 mug/g dw and concentrations in 8% of the birds measured exceeded 33 microg/g dw suggesting levels of potential concern.

Animals↗
PubMed2005

Global application of stable hydrogen and oxygen isotopes to wildlife forensics.

Stable isotopes are being increasingly used in wildlife forensics as means of determining the origin and movement of animals. The heavy isotope content of precipitated water and snow (deltaD(p), delta(18)O(p)) varies widely and systematically across the globe, providing a label that is incorporated through diet into animal tissue. As a result, these isotopes are potentially ideal tracers of geographic origin. The hydrogen and oxygen isotope tracer method has excellent potential where (1) spatial variation of precipitation isotopes exist, and (2) strong, mechanistic relationships link precipitation and isotope ratios in biological tissue. Here, we present a method for interpolation of precipitation isotope values and use it to create global basemaps of growing-season (GS) and mean annual (MA) deltaD(p) and delta(18)O(p). The use of these maps for forensic application is demonstrated using previously published isotope data for bird feathers (deltaD(f)) in North America and Europe. The precipitation maps show that the greatest potential for applying hydrogen and oxygen isotope forensics exists in mid- to high-latitude continental regions, where strong spatial isotope gradients exist. We demonstrate that deltaD(f)/deltaD(p) relationships have significant predictive power both in North America and Europe, and show how zones of confidence for the assignment of origin can be described using these predictive relationships. Our analysis focuses on wildlife forensics, but the maps and approaches presented here will be equally applicable to criminal forensic studies involving biological materials. These maps are available in GIS format at http://www.waterisotopes.org.

Animals↗
PubMed2004

Using stable hydrogen and oxygen isotope measurements of feathers to infer geographical origins of migrating European birds.

Successful application of stable-hydrogen isotope measurements (deltaDf) of feathers to track origins of migratory birds and other wildlife requires a fundamental understanding of the correlation between deltaDf and deuterium patterns in rainfall (deltaDp) over continental scales. A strong correlation between deltaDp and deltaDf has been confirmed for birds and insects in North America, but not yet for other continents. Here, we compare deltaDf data from resident European birds to new deltaDp basemaps for Europe. Three maps, representing growing-season and mean annual deltaDp estimates from an elevation-explicit, detrended interpolation model and growing-season deltaDp estimates from simple Kriging, all indicate that strong isotope gradients occur across Europe with a general depletion occurring in a northeast direction. The feather data, representing 141 individuals of 25 avian species from 38 sites, ranged from -131 to -38 per thousand. Regression analysis showed that strong correlations existed between both mean annual and growing-season deltaDp estimated by detrended interpolation and deltaDf of non-aquatic and non-corvid birds (r2=0.66 and 0.65, respectively). We also examined mean annual and growing-season delta18Op vs. delta18Of for our samples. Both oxygen regressions were similar (r2=0.56 and 0.57, respectively) but poorer than for deuterium. Our study reveals that deltaD measurements of feathers from migratory birds in Europe may be used to track their origin and movements, and so provide a powerful investigative tool for avian migration research in Europe.

Animals↗
PubMed2004

Heterogeneity in stable isotope profiles predicts coexistence of populations of barn swallows Hirundo rustica differing in morphology and reproductive performance.

Population studies assume that individuals belonging to a study population are homogeneous for natal and breeding origin, although this assumption is rarely tested. We tested for heterogeneity in stable-isotope profiles (delta15N, delta13C, deltaD) of feathers grown in the African winter quarters from a Danish breeding population of adult barn swallows, Hirundo rustica. Deuterium isotope values did not provide useful information on population segregation of wintering swallows in Africa. However, both delta15N and delta13C values showed a clearly bimodal distribution with 6% belonging to one category and the remaining birds belonging to another category, resulting in this population comprising three categories of birds. Adults belonging to the two categories of delta13C isotope profiles differed weakly in morphology for several different characters. The frequency and the size of second broods differed between categories of delta13C isotope profiles. Phenotypes of nestlings from the first brood in terms of tarsus length, body mass and T-cell response differed significantly between the two delta15N isotope categories, suggesting that conditions during winter carried over to the breeding season at least as late as the first brood. Polymorphism can be maintained only if fitness is similar for birds from categories of isotope profiles. We suggest that fluctuating selection or migration-selection balance may maintain the observed polymorphism.

Analysis of Variance↗
PubMed2004

Organochlorine contaminants in seven species of Arctic seabirds from northern Baffin Bay.

Organochlorine contaminants (OCs) were determined in liver and fat of seven species of seabirds (Alle alle, Uria lomvia, Cepphus grylle, Rissa tridactyla, Pagophila eburnea, Larus hyperboreus, and Fulmaris glacialis) collected in May/June 1998 from the Northwater Polynya in northern Baffin Bay. OC concentrations ranged over an order of magnitude between seabird species and OC groups, with PCBs having the highest concentrations followed by DDT, chlordane, HCH and ClBz. Positive relationships between delta15N (estimator of trophic level) and OC concentrations (lipid basis) were found for all OC groups, showing that trophic position and biomagnification significantly influence OC concentrations in Arctic seabirds. Concentrations of a number of OCs in particular species (e.g., HCH in P. eburnean) were lower than expected based on delta15N and was attributed to biotransformation. P. eburnea and F. glacialis, which scavenge, and R. tridactyla, which migrate from the south, were consistently above the delta15N-OC regression providing evidence that these variables can elevate OC concentrations. Stable isotope measurements in muscle may not be suitable for identifying past scavenging events by seabirds. OC relative proportions were related to trophic position and phylogeny, showing that OC biotransformation varies between seabird groups. Trophic level, migration, scavenging and biotransformation all play important roles in the OCs found in Arctic seabirds.

Adipose Tissue↗
PubMed2004

From birds to butterflies: animal movement patterns and stable isotopes.

Establishing patterns of movement of wild animals is crucial for our understanding of their ecology, life history and behavior, and is a prerequisite for their effective conservation. Advances in the use of stable isotope markers make it possible to track a diversity of animal species in a variety of habitats. This approach is revolutionizing the way in which we make connections between phases of the annual cycle of migratory animals. However, researchers must exercise care in their application of isotopic methods. Here, we review stable isotope patterns in nature and discuss recent tracking applications in a range of taxa. To aid in the interpretation and design of effective and insightful isotope movement studies, we discuss a series of key issues and assumptions. This exciting field will advance rapidly if researchers consider these aspects of study design and interpretation carefully.

Journal Article↗
PubMed2003

Trophic partitioning in tropical rain forest birds: insights from stable isotope analysis.

Bird communities reach their highest taxonomic and trophic diversity in tropical rain forest, but the use of different foraging strategies to meet food requirements in such competitive environments is poorly understood. Conventional dietary analyses are poorly suited to investigate dietary patterns in complex systems. We used stable carbon ((13)C/(12)C) and nitrogen ((15)N/(14)N) isotope analysis of whole blood to examine avian trophic patterns and sources of diet in the tropical rain forest of Los Tuxtlas, Veracruz, Mexico. We used stable nitrogen isotope analysis to delineate trophic levels, and stable carbon isotope analysis to distinguish the relative contribution of C-3 and CAM/C-4 ultimate sources of proteins to diets. There was large inter- and intraspecific variation in whole blood delta(13)C and delta(15)N values in 23 species of birds. Stable nitrogen isotope analysis separated birds into several trophic levels, including species that obtained their dietary protein mostly from plants, insects or a combination of both food sources. Stable carbon isotope analysis showed that most birds fed on C3-based foods but Stub-tailed Spadebills (Platyrinchus cancrominus) included C-3- and C-4/CAM-specialist individuals. Our analyses provided insights into the nutritional contribution of plant and animal sources of protein and distinguish their photosynthetic origin over relatively long average time periods.

Animals↗
PubMed2003

Stable isotopes as indicators of altitudinal distributions and movements in an Ecuadorean hummingbird community.

Altitudinal migration and dispersal is an important component of the life history of several temperate and tropical birds but remains poorly understood due to the limited success of mark and recapture techniques. Stable isotopes of hydrogen (deltaD) in rainfall, and to a lesser extent, carbon (delta13C) in plants are known to change with altitude and hence may provide the basis of a technique for tracking the altitudinal movements in birds and other wildlife. We investigated the potential for this technique by measuring delta13C, deltaD, and delta15N values in tail feathers of eight species of hummingbirds ( Phaethornis malaris, P. syrmatophorus, P. guy, Adelomyia melanogenys, Coeligena torquata, C. lutetiae, Metallura baroni, M. williami) along an altitudinal gradient (300-3,290 m asl) in the Andes Mountains of Ecuador. Feather delta13C and deltaD values were correlated and each changed significantly with elevation above 400 m. In general, we found good agreement between feather deltaD values and those predicted from a generalized relationship of precipitation and surface water deltaD with altitude. Similarly, feather delta13C values showed an enrichment of approximately 1.5 per thousand per 1,000 m over the linear portion of the elevational response. Stable-nitrogen isotope values were variable, and so did not provide useful information on elevation in birds, apart from trophic effects. Overall there appears to be good potential for using the (deltaD, delta13C) stable isotope approach to track altitudinal movements and to elucidate previously unrecognized patterns of life history variation in both temperate and tropical species that migrate across elevational isotopic gradients.

Altitude↗