Amino acid δ15N underestimation of cetacean trophic positions highlights limited understanding of isotopic fractionation in higher marine consumers

Compound‐specific stable isotope analysis (CSIA) of amino acids (AAs) has been rapidly incorporated in ecological studies to resolve consumer trophic position (TP). Differential ¹⁵N fractionation of “trophic” AAs, which undergo trophic ¹⁵N enrichment, and “source” AAs, which undergo minimal trophic ¹⁵N enrichment and serve as a proxy for primary producer δ¹⁵N values, allows for internal calibration of TP. Recent studies, however, have shown the difference between source and trophic AA δ¹⁵N values in higher marine consumers is less than predicted from empirical studies of invertebrates and fish. To evaluate CSIA‐AA for estimating TP of cetaceans, we compared source and trophic AA δ¹⁵N values of multiple tissues (skin, baleen, and dentine collagen) from five species representing a range of TPs: bowhead whales, beluga whales, short‐beaked common dolphins, sperm whales, and fish‐eating (FE) and marine mammal‐eating (MME) killer whale ecotypes. TP estimates (TP_CSIA) using several empirically derived equations and trophic discrimination factors (TDFs) were 1–2.5 trophic steps lower than stomach content‐derived estimates (TP_SC) for all species. Although TP_CSIA estimates using dual TDF equations were in better agreement with TP_SC estimates, our data do not support the application of universal or currently available dual TDFs to estimate cetacean TPs. Discrepancies were not simply due to inaccurate TDFs, however, because the difference between consumer glutamic acid/glutamine (Glx) and phenylalanine (Phe) δ¹⁵N values (δ¹⁵N_Glx‐Phe) did not follow expected TP order. In contrast to pioneering studies on invertebrates and fish, our data suggest trophic ¹⁵N enrichment of Phe is not negligible and should be examined among the potential mechanisms driving “compressed” and variable δ¹⁵N_Glx‐Phe values at high TPs. We emphasize the need for controlled diet studies to understand mechanisms driving AA‐specific isotopic fractionation before widespread application of CSIA‐AA in ecological studies of cetaceans and other marine consumers.     

Comparative trophic impacts of two globally invasive cyprinid fishes reveal species‐specific invasion consequences for a threatened native fish

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Predicting leaf wax n‐alkane 2H/1H ratios: controlled water source and humidity experiments with hydroponically grown trees confirm predictions of Craig–Gordon model

The extent to which both water source and atmospheric humidity affect δ²H values of terrestrial plant leaf waxes will affect the interpretations of δ²H variation of leaf waxes as a proxy for hydrological conditions. To elucidate the effects of these parameters, we conducted a long‐term experiment in which we grew two tree species, Populus fremontii and Betula occidentalis, hydroponically under combinations of six isotopically distinct waters and two different atmospheric humidities. We observed that leaf n‐alkane δ²H values of both species were linearly related to source water δ²H values, but with slope differences associated with differing humidities. When a modified version of the Craig–Gordon model incorporating plant factors was used to predict the δ²H values of leaf water, all modelled leaf water values fit the same linear relationship with n‐alkane δ²H values. These observations suggested a relatively constant biosynthetic fractionation factor between leaf water and n‐alkanes. However, our calculations indicated a small difference in the biosynthetic fractionation factor between the two species, consistent with small differences calculated for species in other studies. At present, it remains unclear if these apparent interspecies differences in biosynthetic fractionation reflect species‐specific biochemistry or a common biosynthetic fractionation factor with insufficient model parameterization.     

Compound‐specific 15N analysis of amino acids: A tool to estimate the trophic position of tropical seabirds in the South China Sea

Compound‐specific ¹⁵N analysis of amino acids (AAs) is a powerful tool to determine the trophic position (TP) of organisms. However, it has only been used in a few studies of avian ecology because the AA patterns in the consumer‐diet nitrogen trophic discrimination factor (TDF_Glu‐Phe = ?¹⁵N_Glu??¹⁵N_Phe) were unknown in birds until recently, and tropical seabirds have never been investigated with this methodology. Here, we explore the application of this method to tropical seabirds. In this study, we recovered the fossilized bones of tropical seabirds from ornithogenic sediments on two coral islands in the Xisha Islands, South China Sea, as well as the bones and muscle of their predominant food source, flying fish (Exocoetus volitans). Compound‐specific ¹⁵N and ¹³C analyses of AAs in both seabird and fish bone collagen were conducted. The TP of flying fish was calculated based on a widely used single TDF_Glu‐Phe approach. We then calculated the TP of tropical seabirds in three different ways: (a) according to the composition of their diet; (b) based on the single TDF_Glu‐Phe approach; and (c) using a multi‐TDF_Glu‐Phe approach. The results of the multi‐TDF_Glu‐Phe approach were much closer to the results based on the composition of the seabird diet than the results of the single TDF_Glu‐Phe approach, confirming its applicability for tropical seabirds. For seabird bone samples of different ages, TP determined from the multi‐TDF_Glu‐Phe approach was most similar to that of bulk δ¹⁵N of bird collagen, with seabirds occupying higher TPs during the Little Ice Age, as previously shown. In addition, the ¹³C Suess effect was reflected in the AAs δ¹³C in our samples. This study applied a compound‐specific ¹⁵N analysis of AAs to determine the TP of tropical seabirds that has potential to extend to all tropical seabirds many of which are widely distributed and play a key role in the evolution of coral island ecosystems.     

Evidence for a multi‐peak adaptive landscape in the evolution of trophic morphology in terapontid fishes

Using the Australian marine‐freshwater terapontid fishes as a model system, we examined the role of dietary phenotypic optima in an adaptive macro‐evolutionary landscape. Comparative modelling relying on both a priori and data‐driven identification of selective regimes suggested multi‐peak models as best describing much of the dietary phenotypic landscape of terapontids. Both approaches identified common phenotypic optima for different lineages of marine and freshwater herbivores, and minimal differentiation between carnivores and omnivores, irrespective of their phylogenetic relationships, as the model best describing morphological evolution. Significant correlations also existed between these phenotypic axes and proportions of non‐animal dietary items in species’ diets. While simulation results provided evidence for a multi‐peak adaptive landscape in the evolution of trophic morphology in terapontids, they could not rule out chance convergence in these adaptive peaks. However, they do provide scope for identifying areas for more detailed, functionally specific study of phenotypic convergence in herbivorous terapontid trophic habits. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 623–634.     

Lake size and fish diversity determine resource use and trophic position of a top predator in high‐latitude lakes

Prey preference of top predators and energy flow across habitat boundaries are of fundamental importance for structure and function of aquatic and terrestrial ecosystems, as they may have strong effects on production, species diversity, and food‐web stability. In lakes, littoral and pelagic food‐web compartments are typically coupled and controlled by generalist fish top predators. However, the extent and determinants of such coupling remains a topical area of ecological research and is largely unknown in oligotrophic high‐latitude lakes. We analyzed food‐web structure and resource use by a generalist top predator, the Arctic charr Salvelinus alpinus (L.), in 17 oligotrophic subarctic lakes covering a marked gradient in size (0.5–1084 km²) and fish species richness (2–13 species). We expected top predators to shift from littoral to pelagic energy sources with increasing lake size, as the availability of pelagic prey resources and the competition for littoral prey are both likely to be higher in large lakes with multispecies fish communities. We also expected top predators to occupy a higher trophic position in lakes with greater fish species richness due to potential substitution of intermediate consumers (prey fish) and increased piscivory by top predators. Based on stable carbon and nitrogen isotope analyses, the mean reliance of Arctic charr on littoral energy sources showed a significant negative relationship with lake surface area, whereas the mean trophic position of Arctic charr, reflecting the lake food‐chain length, increased with fish species richness. These results were supported by stomach contents data demonstrating a shift of Arctic charr from an invertebrate‐dominated diet to piscivory on pelagic fish. Our study highlights that, because they determine the main energy source (littoral vs. pelagic) and the trophic position of generalist top predators, ecosystem size and fish diversity are particularly important factors influencing function and structure of food webs in high‐latitude lakes.     

Carbon and nitrogen isotope fractionation of amino acids in an avian marine predator,the gentoo penguin (Pygoscelis papua)

Compound‐specific stable isotope analysis (CSIA) of amino acids (AA) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns in AA isotope fractionation in birds. We conducted a controlled CSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individual AA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ¹³C_C‐D and Δ¹⁵N_C‐D, respectively). We found that essential AA δ¹³C values and source AA δ¹⁵N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessential AA Δ¹³C_C‐D values reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDF_Glu‐Phe = 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ¹⁵N_C‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDF_Glu‐Phe equation with the avian‐specific TDF_Glu‐Phe value from our experiment provided estimates that were more ecologically realistic than estimates using a single TDF_Glu‐Phe of 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use of CSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.     

Long‐term variation in the littoral food web of an acidified mountain lake

Evidence of chemical recovery from acidification in European freshwaters has emerged in recent years, with many previously damaged systems responding to decades of reduced acidifying emissions. Biological recovery, however, has often lagged behind, and this has been ascribed to several possible mechanisms, including inertia in the food web. We examined two decades of change in hindcasted food webs for Lochnagar, a Scottish mountain lake, to make inferences about the potential dynamical stability of the system and to assess the prospects for future biological recovery. Although community composition tracked temporal changes in acidity, this was neither sustained nor directional, and mainly manifested as shifts in relative abundances rather than the establishment of more acid‐sensitive species. The food web was highly interconnected and reticulate, especially in years when species richness was low, and subsidized by external inputs of detritus. Among the primary consumers, generalist herbivore–detritivores maintained feeding links with the scant algal resources, which appeared insufficient to support viable populations of specialist grazers. Together, these characteristics, which are shared with many other acidified freshwaters, are likely to make the community dynamically stable and resistant to invasions of potential new colonists, thereby slowing the pace of future biological recovery.     

High‐resolution food webs based on nitrogen isotopic composition of amino acids

Food webs are known to have myriad trophic links between resource and consumer species. While herbivores have well‐understood trophic tendencies, the difficulties associated with characterizing the trophic positions of higher‐order consumers have remained a major problem in food web ecology. To better understand trophic linkages in food webs, analysis of the stable nitrogen isotopic composition of amino acids has been introduced as a potential means of providing accurate trophic position estimates. In the present study, we employ this method to estimate the trophic positions of 200 free‐roaming organisms, representing 39 species in coastal marine (a stony shore) and 38 species in terrestrial (a fruit farm) environments. Based on the trophic positions from the isotopic composition of amino acids, we are able to resolve the trophic structure of these complex food webs. Our approach reveals a high degree of trophic omnivory (i.e., noninteger trophic positions) among carnivorous species such as marine fish and terrestrial hornets.This information not only clarifies the trophic tendencies of species within their respective communities, but also suggests that trophic omnivory may be common in these webs.     

The influence of lipid‐extraction and long‐term DMSO preservation on carbon (δ13C) and nitrogen (δ15N) isotope values in cetacean skin

Stable isotope analysis (SIA) has rapidly become a useful tool to study the ecology of wild animal populations, especially for elusive, wide‐ranging predators like marine mammals. The development of projectile biopsy techniques resulted in the collection of thousands of cetacean tissue samples that were archived in a dimethyl sulfoxide (DMSO) solution for long‐term, multidecadal preservation. Here we examine the influence of DMSO preservation on carbon (δ¹³C) and nitrogen (δ¹⁵N) values by comparing a set of paired delphinid skin samples stored frozen without preservative and in DMSO for up to 22 yr. Treatment of paired frozen and DMSO‐preserved skin in a 2:1 chloroform:methanol solution yielded similar δ¹³C and δ¹⁵N values, revealing that DMSO and lipid contamination have similar isotopic effects on skin, and that these effects can be removed using routine lipid‐extraction methods. Further, amino acid concentrations in DMSO‐preserved and frozen skin tissue were similar, providing independent evidence of minimal protein alteration due to preservation. Access to a rich archive of skin samples preserved in DMSO will expand our ability to examine temporal and spatial variability in the isotope values of cetaceans, which will aid our understanding of how their ecology has been influenced by historical changes in environmental conditions.     

Age‐related variation in the trophic characteristics of a marsupial carnivore,the Tasmanian devil Sarcophilus harrisii

Age‐related changes in diet have implications for competitive interactions and for predator–prey dynamics, affecting individuals and groups at different life stages. To quantify patterns of variation and ontogenetic change in the diets of Tasmanian devils Sarcophilus harrisii, a threatened marsupial carnivore, we analyzed variation in the stable isotope composition of whisker tissue samples taken from 91 individual devils from Wilmot, Tasmania from December 2014 to February 2017. Both δ¹³C and δ¹⁵N decreased with increasing age in weaned Tasmanian devils, indicating that as they age devils rely less on small mammals and birds, and more on large herbivores. Devils <12 months old had broader group isotopic niches, as estimated by Bayesian standard ellipses (SEA_B mode = 1.042) than devils from 12 to 23 months old (mode = 0.541) and devils ≥24 months old (mode = 0.532). Devils <24 months old had broader individual isotopic niches (SEA_B mode range 0.492–1.083) than devils ≥24 months old (mode range 0.092–0.240). A decrease in δ¹⁵N from the older whisker sections to the more recently grown sections in devils <24 months old likely reflects the period of weaning in this species, as this pattern was not observed in devils ≥24 months old. Our data reveal changes in the isotopic composition of devil whiskers with increasing age, accompanied by a reduction in isotopic variation both among population age classes and within individuals, reflecting the effect of weaning in early life, and a likely shift from an initially diverse diet of small mammals, birds, and invertebrates towards increasing consumption of larger herbivores in adulthood.     

Back to the future: using palaeolimnology to infer long‐term changes in shallow lake food webs

1. Shallow lakes are often cited as classic examples of systems that exhibit trophic cascades but, whilst they provide good model systems with which to test general ecological theory and to assess long‐term community change, their food web linkages have rarely been resolved, so changes associated with the structure and dynamics of the ecological network as a whole are still poorly understood. 2. We sought to redress this, and to demonstrate the potential benefits of integrating palaeolimnological and contemporary data, by constructing highly resolved food webs and stable isotope derived measures of trophic interactions and niche space, for the extant communities of two shallow U.K. lakes from different positions along a gradient of eutrophication. The contemporary surface sediment cladoceran and submerged macrophyte assemblages in the less enriched site, Selbrigg Pond, matched the palaeolimnological assemblages of the more enriched site, Felbrigg Hall Lake, in its more pristine state during the 1920s. Thus, Selbrigg was a temporal analogue for Felbrigg, from which the consequences of long‐term eutrophication on food web structure could be inferred. These data represent the first steps towards reconstructing not only past assemblages (i.e. nodes within a food web), but also past interactions (i.e. links within a food web): a significant departure from much of the previous research in palaeolimnology. 3. The more eutrophic food web had far fewer nodes and links, and thus a less reticulate network, than was the case for the more pristine system. In isotopic terms, there was vertical compression in δ¹⁵N range (NR) and subsequent increased overlap in isotopic niche space, indicating increased trophic redundancy within Felbrigg. This structural change, which was associated with a greater channelling of energy through a smaller number of nodes as alternative feeding pathways disappear, could lead to reduced dynamic stability, pushing the network towards further simplification. These changes reflected a general shift from a benthic‐dominated towards a more pelagic system, as the plant‐associated subweb eroded. 4. Although these data are among the first of their kind, the palaeo‐analogue approach used here demonstrates the huge potential for applying food web theory to understand how and why these ecological networks change during eutrophication. Furthermore, because of the rich biological record preserved in their sediments, shallow lakes represent potentially important models for examining long‐term intergenerational dynamics, thereby providing a means by which models and data can be integrated on meaningful timescales – a goal that has long proved elusive in food web ecology.     

Trophic consequences of non‐native species for commercial fish in a backwater bay of the Three Gorges Reservoir,Southwest China

Trophic niche overlap in native and alien fish species can lead to competitive interactions whereby non‐native fishes outcompete indigenous individuals and eventually affect the viability of natural populations. The species Erythroculter mongolicus and Erythroculter ilishaeformis (belonging to the Culterinae), which are two commercially important fish species in the backwater bay of the Pengxi River in the Three Gorges Reservoir (TGR), were threatened by competition from the non‐native Coilia ectenes (lake anchovy). The latter is an alien species introduced into the lower reaches of the Yangtze River in China and now widespread in the TGR. The trophic consequences of non‐native lake anchovy invasion for E. mongolicus and E. ilishaeformis were assessed using stable isotope analysis (δ¹³C and δ¹⁵N) and associated metrics including the isotopic niche, measured as the standard ellipse area. The trophic niche of native E. mongolicus had little overlap (<15%) with the alien fish species and was significantly reduced in size after invasion by lake anchovy. This suggests that E. mongolicus shifted to a more specialized diet after invasion by lake anchovy. In contrast, the trophic niche overlap of native fish E. ilishaeformis with the alien fish species was larger (>50%) and the niche was obviously increased, implying that fish in this species exploited a wider dietary base to maintain their energetic requirements. Thus, marked changes for the native E. mongolicus and E. ilishaeformis were detected as the trophic consequences of invasion of non‐native lake anchovy.     

Stable isotopes as tracers of trophic interactions in marine mutualistic symbioses

Mutualistic nutritional symbioses are widespread in marine ecosystems. They involve the association of a host organism (algae, protists, or marine invertebrates) with symbiotic microorganisms, such as bacteria, cyanobacteria, or dinoflagellates. Nutritional interactions between the partners are difficult to identify in symbioses because they only occur in intact associations. Stable isotope analysis (SIA) has proven to be a useful tool to highlight original nutrient sources and to trace nutrients acquired by and exchanged between the different partners of the association. However, although SIA has been extensively applied to study different marine symbiotic associations, there is no review taking into account of the different types of symbiotic associations, how they have been studied via SIA, methodological issues common among symbiotic associations, and solutions that can be transferred from one type of association with another. The present review aims to fill such gaps in the scientific literature by summarizing the current knowledge of how isotopes have been applied to key marine symbioses to unravel nutrient exchanges between partners, and by describing the difficulties in interpreting the isotopic signal. This review also focuses on the use of compound‐specific stable isotope analysis and on statistical advances to analyze stable isotope data. It also highlights the knowledge gaps that would benefit from future research.     

Effect of historical land‐use and climate change on tree‐climate relationships in the upper Midwestern United States

Contemporary forest inventory data are widely used to understand environmental controls on tree species distributions and to construct models to project forest responses to climate change, but the stability and representativeness of contemporary tree‐climate relationships are poorly understood. We show that tree‐climate relationships for 15 tree genera in the upper Midwestern US have significantly altered over the last two centuries due to historical land‐use and climate change. Realised niches have shifted towards higher minimum temperatures and higher rainfall. A new attribution method implicates both historical climate change and land‐use in these shifts, with the relative importance varying among genera and climate variables. Most climate/land‐use interactions are compounding, in which historical land‐use reinforces shifts in species‐climate relationships toward wetter distributions, or confounding, in which land‐use complicates shifts towards warmer distributions. Compounding interactions imply that contemporary‐based models of species distributions may underestimate species resilience to climate change.     

Diets and trophic‐guild structure of a diverse fish assemblage in Chesapeake Bay,U.S.A.

Dietary habits and trophic‐guild structure were examined in a fish assemblage (47 species) of the Chesapeake Bay estuary, U.S.A., using 10 years of data from >25 000 fish stomachs. The assemblage was comprised of 10 statistically significant trophic guilds that were principally differentiated by the relative amounts of Mysida, Bivalvia, Polychaeta, Teleostei and other Crustacea in the diets. These guilds were broadly aggregated into five trophic categories: piscivores, zooplanktivores, benthivores, crustacivores and miscellaneous consumers. Food web structure was largely dictated by gradients in habitat (benthic to pelagic) and prey size. Size classes within piscivorous species were more likely to be classified into different guilds, reflecting stronger dietary changes through ontogeny relative to benthivores and other guilds. Relative to predator species and predator size, the month of sampling had negligible effects on dietary differences within the assemblage. A majority of sampled fishes derived most of their nutrition from non‐pelagic prey sources, suggesting a strong coupling of fish production to benthic and demersal food resources. Mysida (predominantly the opossum shrimp Neomysis americana) contributed substantially to the diets of over 25% of the sampled predator groups, indicating that this species is a critical, but underappreciated, node in the Chesapeake Bay food web.     

Interactive effects of elevated CO2 and nitrogen deposition on fatty acid molecular and isotope composition of above‐ and belowground tree biomass and forest soil fractions

Atmospheric carbon dioxide (CO₂) and reactive nitrogen (N) concentrations have been increasing due to human activities and impact the global carbon (C) cycle by affecting plant photosynthesis and decomposition processes in soil. Large amounts of C are stored in plants and soils, but the mechanisms behind the stabilization of plant‐ and microbial‐derived organic matter (OM) in soils are still under debate and it is not clear how N deposition affects soil OM dynamics. Here, we studied the effects of 4 years of elevated (¹³C‐depleted) CO₂ and N deposition in forest ecosystems established in open‐top chambers on composition and turnover of fatty acids (FAs) in plants and soils. FAs served as biomarkers for plant‐ and microbial‐derived OM in soil density fractions. We analyzed above‐ and belowground plant biomass of beech and spruce trees as well as soil density fractions for the total organic C and FA molecular and isotope (δ¹³C) composition. FAs did not accumulate relative to total organic C in fine mineral fractions, showing that FAs are not effectively stabilized by association with soil minerals. The δ¹³C values of FAs in plant biomass increased under high N deposition. However, the N effect was only apparent under elevated CO₂ suggesting a N limitation of the system. In soil fractions, only isotope compositions of short‐chain FAs (C₁₆₊₁₈) were affected. Fractions of ‘new’ (experimental‐derived) FAs were calculated using isotope depletion in elevated CO₂ plots and decreased from free light to fine mineral fractions. ‘New’ FAs were higher in short‐chain compared to long‐chain FAs (C_20?30), indicating a faster turnover of short‐chain compared to long‐chain FAs. Increased N deposition did not significantly affect the quantity of ‘new’ FAs in soil fractions, but showed a tendency of increased amounts of ‘old’ (pre‐experimental) C suggesting that decomposition of ‘old’ C is retarded by high N inputs.     

Repeated evolution of local adaptation in swimming performance: population‐level trade‐offs between burst and endurance swimming in Brachyrhaphis freshwater fish

Specialization is fundamentally important in biology because specialized traits allow species to expand into new environments, in turn promoting population differentiation and speciation. Specialization often results in trade‐offs between traits that maximize fitness in one environment but not others. Despite the ubiquity of trade‐offs, we know relatively little about how consistently trade‐offs evolve between populations when multiple sets of populations experience similarly divergent selective regimes. In the present study, we report a case study on Brachyrhaphis fishes from different predation environments. We evaluate apparent within/between population trade‐offs in burst‐speed and endurance at two levels of evolutionary diversification: high‐ and low‐predation populations of Brachyrhaphis rhabdophora, and sister species Brachyrhaphis roseni and Brachyrhaphis terrabensis, which occur in high‐ and low‐predation environments, respectively. Populations of Brachyrhaphis experiencing different predation regimes consistently evolved swimming specializations indicative of a trade‐off between two swimming forms that are likely highly adaptive in the environment in which they occur. We show that populations have become similarly locally adapted at both levels of diversification, suggesting that swimming specialization has evolved rather rapidly and persisted post‐speciation. Our findings provide valuable insight into how local adaptation evolves at different stages of evolutionary divergence.     

Foraging ecology and niche overlap in pygmy (Kogia breviceps) and dwarf (Kogia sima) sperm whales from waters of the U.S. mid‐Atlantic coast

A complementary approach of stomach content and stable isotope analyses was used to characterize the foraging ecology and evaluate niche overlap between pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales stranded on the U.S. mid‐Atlantic coast between 1998 and 2011. Food habits analysis demonstrated both species were primarily teuthophagous, with 35 species of cephalopods, and 2 species of mesopelagic fishes represented in their overall diets. Pianka's Index of niche overlap suggested high overlap between whale diets (O_n = 0.92), with squids from the families Histioteuthidae, Cranchidae, and Ommastrephidae serving as primary prey. Pygmy sperm whales consumed slightly larger prey sizes (mean mantle length [ML] = 10.8 cm) than dwarf sperm whales (mean ML = 7.8 cm). Mean prey sizes consumed by pygmy sperm whales increased with growth, but showed no trend in dwarf sperm whales. Significant differences were not detected in δ¹⁵N and δ¹³C values of muscle tissues from pygmy (10.8‰ ± 0.5‰, ?17.1‰ ± 0.6‰), and dwarf sperm whales (10.7‰ ± 0.5‰, ?17.0‰ ± 0.4‰), respectively. Isotopic niche widths also did not differ significantly and dietary overlap was high between the two species. Results suggest the feeding ecologies of the pygmy and dwarf sperm whales are similar and both species occupy equivalent trophic niches in the region.