BACKGROUND:We have developed a new clinical research approach for the quantification of cellular proliferation in human infants to address unanswered questions about tissue renewal and regeneration. The approach consists of oral 15N-thymidine administration to label cells in S-phase, followed by Multi-isotope Imaging Mass Spectrometry for detection of the incorporated label in cell nuclei. To establish the approach, we performed an observational study to examine uptake and elimination of 15N-thymidine. We compared at-home label administration with in-hospital administration in infants with tetralogy of Fallot, a form of congenital heart disease, and infants with heart failure. METHODS:We examined urine samples from 18 infants who received 15N-thymidine (50 mg/kg body weight) by mouth for five consecutive days. We used Isotope Ratio Mass Spectrometry to determine enrichment of 15N relative to 14N (%) in urine. RESULTS/FINDINGS:15N-thymidine dose administration produced periodic rises of 15N enrichment in urine. Infants with tetralogy of Fallot had a 3.2-fold increase and infants with heart failure had a 4.3-fold increase in mean peak 15N enrichment over baseline. The mean 15N enrichment was not statistically different between the two patient populations (p = 0.103). The time to peak 15N enrichment in tetralogy of Fallot infants was 6.3 ± 1 hr and in infants with heart failure 7.5 ± 2 hr (mean ± SEM). The duration of significant 15N enrichment after a dose was 18.5 ± 1.7 hr in tetralogy of Fallot and in heart failure 18.2 ± 1.8 hr (mean ± SEM). The time to peak enrichment and duration of enrichment were also not statistically different (p = 0.617 and p = 0.887). CONCLUSIONS:The presented results support two conclusions of significance for future applications: (1) Demonstration that 15N-thymidine label administration at home is equivalent to in-hospital administration. (2) Two different types of heart disease show no differences in 15N-thymidine absorption and elimination. This enables the comparative analysis of cellular proliferation between different types of heart disease.

The great shearwater (Ardenna gravis) is a common pelagic bird with a distribution that spans almost the entire Atlantic basin, which in conjunction with its relatively high abundance, makes great shearwaters an effective bio indicator. We compared δ13C and δ15N values from the feathers, red blood cells (RBCs), and plasma of great shearwaters collected in 2014 and 2015 from the waters off Massachusetts and Cape Cod. The δ13C and δ15N values of RBCs were quite constant between sampling periods and years, suggesting a generally stable food web over that time period. However, the δ13C of plasma indicates a small seasonal change in diet between July and September for both years, with plasma δ15N values suggesting a slight increase in trophic level late in summer. Comparison of the δ15N of RBCs and plasma indicates that great shearwaters experienced a diet shift during the first few weeks of summer 2014, but not in 2015. Comparisons with other studies suggest that these shearwaters feed at a lower trophic level than great shearwaters sampled in the Bay of Fundy and that there is a decrease in δ13C with increasing latitude, which could indicate a more pelagic diet in northern waters. Stable isotope analysis of the sixth primary feathers provided evidence that these feathers are molted in the Northern Hemisphere and that the diet of great shearwaters shortly after arrival was different in 2014 and 2015. This study demonstrates that within species comparisons of tissue isotopic signatures over time and comparisons of isotopic signatures of tissues with different turnover rates, can detect changes in diet and be used as a tool to monitor for changes in marine food webs over time and space. The relevant signals remain informative even in the absence of species-specific data on tissue-diet discrimination factors, tissue turnover rates, or knowledge of dietary components and their stable isotopic signatures, suggesting dietary changes indicative of a corresponding change in the food web.

We take advantage of a natural gradient of human exploitation and oceanic primary production across five central Pacific coral reefs to examine foraging patterns in common coral reef fishes. Using stomach content and stable isotope (δ15N and δ13C) analyses, we examined consistency across islands in estimated foraging patterns. Surprisingly, species within the piscivore–invertivore group exhibited the clearest pattern of foraging consistency across all five islands despite there being a considerable difference in mean body mass (14 g–1.4 kg) and prey size (0.03–3.8 g). In contrast, the diets and isotopic values of the grazer–detritivores varied considerably and exhibited no consistent patterns across islands. When examining foraging patterns across environmental contexts, we found that δ15N values of species of piscivore–invertivore and planktivore closely tracked gradients in oceanic primary production; again, no comparable patterns existed for the grazer–detritivores. The inter-island consistency in foraging patterns within the species of piscivore–invertivore and planktivore and the lack of consistency among species of grazer–detritivores suggests a linkage to different sources of primary production among reef fish functional groups. Our findings suggest that piscivore–invertivores and planktivores are likely linked to well-mixed and isotopically constrained allochthonous oceanic primary production, while grazer–detritivores are likely linked to sources of benthic primary production and autochthonous recycling. Further, our findings suggest that species of piscivore–invertivore, independent of body size, converge toward consuming low trophic level prey, with a hypothesized result of reducing the number of steps between trophic levels and increasing the trophic efficiency at a community level.

The trophic niche of species can vary spatially due to numerous natural and anthropogenic factors, yet separating these distinct drivers can be difficult. We examined the role of natural oceanographic variation in the trophic ecology and dietary niche breadth of 8 common coral reef fishes spanning multiple trophic guilds. These fishes were collected from the Southern Line Islands of Kiribati, a chain of 5 uninhabited islands spanning a strong gradient of oceanic primary production. A combination of stomach contents and stable isotope analyses (delta N-15, delta C-13) were used to elucidate spatial variation in diet composition, trophic niche width, and degree of individual dietary specialization. Across species, populations were generally characterized by larger dietary niche widths at the islands exposed to greater nearshore primary production, although patterns among species were variable. Estimates of niche width varied by fish guild as a function of methodology, with planktivores exhibiting stronger effects using metrics calculated from stomach contents, and carnivores and herbivores exhibiting stronger effects from metrics calculated with stable isotope data. At the island level, the trophic niche of the fish community expanded in isotopic space as a function of increasing nearshore production, reflecting increased multispecies dietary diversity at the most productive islands. These results highlight the importance of considering natural oceanographic variability when evaluating the trophic structure of coral reef ecosystems, and provide a foundation for future research on ecosystem functioning across oceanographic gradients.

Background and Trophic Diversity Study Lake Turkana is an understudied desert lake shared by Kenya and Ethiopia. This system is at the precipice of large-scale changes in ecological function due to climate change and economic development along its major inflowing river, the Omo River. To anticipate response by the fish community to these changes, we quantified trophic diversity for seven ecological disparate species (Alestes baremose, Hydrocynus forskalli, Labeo horie, Lates niloticus, Oreochromis niloticus, Synodontis schall, and Tilapia zillii) using stable isotopes. Based on their marked morphological differentiation, we postulated that dietary niches of these species would be similar in size but show little overlap. The degree of trophic diversity varied greatly among the species studied, refuting our hypothesis regarding dietary niche size. Oreochromis niloticus and L. niloticus had the highest trophic diversity and significantly larger dietary niches than T. zillii, A. baremose and H. forskalli. Low overlap among the dietary niches of the seven species, with the exception of the synodontid catfish S. schall, is consistent with our second hypothesis. Predicting Species' Vulnerability Breeding vulnerability was highest among those species with the lowest trophic diversity. We predict that in suffering two strikes against them, A. baremose, H. forskalli, T. zillii, and L. horie will be most affected by the highly altered Lake Turkana ecosystem and that O. niloticus, L. niloticus and S. schall will be least affected. Low vulnerability among O. niloticus and L. niloticus is promising for the future of the lake's fishery, but the third most important fishery species (L. horie) will be highly vulnerable to impending ecosystem change. T. zillii should be treated as separate from O. niloticus in the fishery given higher sensitivity and a different ecological role. We see potential for expansion of the fishery for S. schall but don't recommend the development of a fishery for A. baremose and H. forskalli.

Tropical rain forests harbour the most diverse plant and animal assemblages known to science, but our understanding of assemblage structure and species interactions is limited. Bats, as the only flying mammals, have the potential to exploit resources from all strata in forest communities. Thus, fruit-eating phyllostomid bats often have been categorized into canopy-, subcanopy- and understorey-foraging species, based largely upon the height at which they were most frequently captured. Here we challenge this classification and use stable carbon isotopes to assess foraging height of bat species at an Amazonian rain-forest site in Ecuador and at a Caribbean lowland rain-forest site in Costa Rica for comparison with data from mist-net captures. The proportion of the heavy stable carbon isotope C-13 in relation to the lighter C-12 isotope increases in plants from ground level to the canopy (0.12 parts per thousand m(-1)-0.18 parts per thousand m(-1)), and these differences in stable carbon isotope signatures are reflected in the body tissue of phytophagous bats. We used the stable carbon isotope ratio (delta C-13) of wing tissue to estimate the foraging heights of 54 phyllostomid species in two Neotropical bat assemblages. Based on stable isotope data, phyllostomid species exploit food resources at all vertical strata of the forest. Capture height was not a reliable predictor of foraging height and suggests that bats most likely use lower strata to commute between foraging sites to avoid predators. Vertical stratification is likely to be a key factor promoting niche partitioning, thus promoting high local species richness in many tropical animal assemblages.

Understanding wetland food webs is critical for effective habitat management, restoration and conservation. Microalgae are recognized as key food sources for marsh invertebrates but the importance of different groups under various conditions is rarely examined. We tested the hypothesis that faunal utilization of microalgae, and cyanobacteria in particular, is significant in Southern California created and natural salt marshes but varies with habitat type (creek bank versus marsh interior) and season (spring versus autumn). We used stable isotope analysis and mixing models (IsoSource) to compare food webs in adjacent young (created) and mature (natural) salt marshes. Isotopic values of some primary producers, macrofauna, epifauna, and fish demonstrated significant differences between the adjacent salt marshes. delta C-13 and delta S-34 values of the benthic microalgal community varied with taxonomic composition (diatoms versus cyanobacteria) and to a lesser extent with season. Depleted delta N-15 values of benthic diatoms and macroalgae indicated that N-2 fixed within algal mats was recycled within the benthic algal community. Marsh fauna, including most major macrofauna taxal, Cerithidea, and Fundulus, also exhibited seasonal differences in isotopic composition, and Cerithidea and selected macrofauna (oligochaetes, polychaetes) from the marsh interior were more enriched in C-13 and depleted in N-15 than those from the creek bank. In the young marsh, the cyanobacteria contributed a minimum of 17-100% of the primary production in food webs supporting macrofauna, and cyanobacteria contributed at least 40% of the primary production included in Cerithidea and Fundulus food webs. A wider range of primary producers contributed to food webs in the mature marsh. Cyanobacteria were a greater source of trophic support for macrofauna from the marsh interior than the creek bank, whereas Spartina was a more important food source for creek bank macrofauna in both marshes. Insect larvae largely consumed cyanobacteria, whereas polychaetes exhibited greater utilization of Spartina. Phytoplankton was the primary food resource for mussels in both marshes. Although the spatial and temporal complexity of food webs has traditionally been collapsed into the study of relatively simplified food webs, isotope signatures reveal fine-scale patterns in food web structure that may be used to make more accurate assessments of ecosystem state. Accurate interpretation of marsh trophic structure using natural abundance stable isotopes requires fine-scale resolution in space and time, a large number of samples, and a high level of taxonomic resolution.

Long-term changes in leaf net production of the seagrass Posidonia oceanica (L.) Delile, and in the irradiance arriving at the meadow canopy, were investigated in a NW Mediterranean meadow using a palaeoecological approach. We conducted in situ shade experiments to find the relationships between the carbon stable isotope ratio (δ13C) of plant tissues, leaf net production, and ambient irradiance. The relationships were highly significant and fitted the Michaelis–Menten equation and power functions. These functions were used to reconstruct light and net production using the δ13C of P. oceanica detritus as a palaeoecological proxy along a 150cm core of P. oceanica matte (a peat-like deposit formed by this endemic seagrass species). The δ13C values of sheath detritus along the core (i.e., the net leaf production) showed a weak but significant decrease towards the present time (R=0.308; P=0.02) probably as a result of (1) an increase in DIC availability, and/or (2) a progressive change in the carbon isotopic signature of DIC, both of which are consistent with a rise in anthropogenic atmospheric CO2. The canopy irradiance, reconstructed for the last 1200years, showed a mean value of 128µEm−2s−1 with a range of 12.5–280µEm−2s−1. The reconstruction of the net leaf production for the same period yielded a mean value of 2.5mg dwday−1shoot−1 with a range of 2.0–2.7mg dwday−1shoot−1. Both ranges are within the values reported in the literature for present day studies. The absence of significant fluctuations or sudden changes through time suggests remarkable ecosystem stability during the last millennium.

Phyllostomid bats form some of the most speciose mammalian assemblages known, with more than 70 species estimated to co-occur on one site. These species encompass a dietary spectrum that ranges across several trophic levels, and many show morphological specializations for their dietary behaviour (e.g., long tongues in nectar-feeding bats). However, previously reported diets have varied among studies including assignments of species to feeding ensembles. In this study we present data that demonstrate that phyllostomid bats are opportunistic omnivores despite their specializations. We analyzed the diet of 67 phyllostomid bat species from the Neotropics based on both fecal analyses and nitrogen isotope ratios in wing tissue and found that most species complemented their primary diet with nutrients from many different food sources. From these new dietary data, we were unable to distinguish meaningful feeding ensembles of species based on diet. Thus, we argue that interspecific competition is not restricted to species within an ensemble but seems instead to occur across the entire assemblage. Our results suggest that phyllostomid species have specialized successively on distinct diets during their radiation without sacrificing their capability to exploit a variety of food types. The combination of morphological and behavioural specialization on the one hand and opportunistic omnivory on the other might promote the high diversity of phyllostomid bat assemblages.

We tested the usefulness of δ 15 N values in the organic matrix of whole shells from Merce- naria mercenaria as tracers of anthropogenic nitro- gen inputs to coastal ecosystems. Low and high strin- gency acidification methods were used to define parameters for reliable δ 15 N determination in shell material for comparison with δ 15 N values in soft tis- sues. δ 15 N values in shell from transplanted and native clams reflected %-wastewater contribution to estuaries, but were 2.3 to 2.5% lighter than δ 15 N values in soft tissues. Accuracy of δ 15 N values in shell material depended on recovering a sufficient quantity of organic N from shell (~70 μg) and was not altered by acidification method. Reliable δ 15 N values were obtained with as little as 80 mg of shell and using 100 μl of acid, but higher stringency methods (treating more shell with more acid for longer duration) typically yielded more N for sub- sequent stable isotope analysis. Conversely, higher concentrations of acid reduced N recovery. These results suggest that the content of N recovered was of greater concern to obtaining reliable δ 15 N values from shell material than acidification effects. Differ- ences between δ 15 N values in shell material and soft tissues likely reflected differences in N assimilation among tissues. In combination with other analyses, this method may be applied to refine modern and historical trophic assessments and discern natural from anthropogenic influences on coastal ecosystems

Omnivorous animals feed on several food items that often differ in macronutrient and isotopic composition. Macronutrients can be used for either metabolism or body tissue synthesis and, therefore, stable C isotope ratios of exhaled breath (δ13Cbreath) and tissue may differ. To study nutrient routing in omnivorous animals, we measured δ13Cbreath in 20-g Carollia perspicillata that either ate an isotopically homogeneous carbohydrate diet or an isotopically heterogenous protein-carbohydrate mixture. The δ13Cbreath converged to the δ13C of the ingested carbohydrates irrespective of whether proteins had been added or not. On average, δ13Cbreath was depleted in 13C by only ca. −2‰ in relation to the δ13C of the dietary carbohydrates and was enriched by +8.2‰ in relation to the dietary proteins, suggesting that C. perspicillata may have routed most ingested proteins to body synthesis and not to metabolism. We next compared the δ13Cbreath with that of wing tissue (δ13Ctissue) in 12 free-ranging, mostly omnivorous phyllostomid bat species. We predicted that species with a more insect biased diet—as indicated by the N isotope ratio in wing membrane tissue (δ15Ntissue)—should have higher δ13Ctissue than δ13Cbreath values, since we expected body tissue to stem mostly from insect proteins and exhaled CO2 to stem from the combustion of fruit carbohydrates. Accordingly, δ13Ctissue and δ13Cbreath should be more similar in species that feed predominantly on plant products. The species-specific differences between δ13Ctissue and δ13Cbreath increased with increasing δ15Ntissue, i.e. species with a plant-dominated diet had similar δ13Ctissue and δ13Cbreath values, whereas species feeding at a higher trophic level had higher δ13Ctissue than δ13Cbreath values. Our study shows that δ13Cbreath reflect the isotope ratio of ingested carbohydrates, whereas δ13C of body tissue reflect the isotope ratio of ingested proteins, namely insects, supporting the idea of isotopic routing in omnivorous animals.

Trophic ecology has benefitted from the use of stable isotopes for the last three decades. However, during the last 10 years, there has been a growing awareness of the isotopic biases associated with some pre-analytical procedures that can seriously hamper the interpretation of food webs. We have assessed the extent of such biases by: (1) reviewing the literature on the topic, and (2) compiling C and N isotopic values of marine invertebrates reported in the literature with the associated sample preparation protocols. The factors considered were: acid-washing, distilled water rinsing (DWR), sample type (whole individuals or pieces of soft tissues), lipid content, and gut contents. Two-level ANOVA revealed overall large and highly significant effects of acidification for both δ13C values (up to 0.9‰ decrease) and δ15 N values (up to 2.1‰ decrease in whole individual samples, and up to 1.1‰ increase in tissue samples). DWR showed a weak overall effect with δ13C increments of 0.6‰ (for the entire data set) or decrements of 0.7‰ in δ15 N values (for tissue samples). Gut contents showed no overall significant effect, whereas lipid extraction resulted in the greatest biases in both isotopic signatures (δ13C, up to −2.0‰ in whole individuals; δ15N, up to +4.3‰ in tissue samples). The study analyzed separately the effects of the various factors in different taxonomic groups and revealed a very high diversity in the extent and direction of the effects. Maxillopoda, Gastropoda, and Polychaeta were the classes that showed the largest isotopic shifts associated with sample preparation. Guidelines for the standardization of sample preparation protocols for isotopic analysis are proposed both for large and small marine invertebrates. Broadly, these guidelines recommend: (1) avoiding both acid washing and DWR, and (2) performing lipid extraction and gut evacuation in most cases.

Many animals in the tropics of Africa, Asia and South America regularly visit so-called salt or mineral licks to consume clay or drink clay-saturated water. Whether this behavior is used to supplement diets with locally limited nutrients or to buffer the effects of toxic secondary plant compounds remains unclear. In the Amazonian rainforest, pregnant and lactating bats are frequently observed and captured at mineral licks. We measured the nitrogen isotope ratio in wing tissue of omnivorous short-tailed fruit bats, Carollia perspicillata, and in an obligate fruit-eating bat, Artibeus obscurus, captured at mineral licks and at control sites in the rainforest. Carollia perspicillata with a plant-dominated diet were more often captured at mineral licks than individuals with an insect-dominated diet, although insects were more mineral depleted than fruits. In contrast, nitrogen isotope ratios of A. obscurus did not differ between individuals captured at mineral lick versus control sites. We conclude that pregnant and lactating fruit-eating bats do not visit mineral licks principally for minerals, but instead to buffer the effects of secondary plant compounds that they ingest in large quantities during periods of high energy demand. These findings have potential implications for the role of mineral licks for mammals in general, including humans.

Unlike most terrestrial mammals, female bats must supply their offspring with all required nutrients until pups achieve virtually adult size, at which time they are able to fly and become independent. Access to nutrients may be especially challenging for reproductively active females in mineral-poor landscapes such as tropical rainforests. We hypothesized that pregnant and lactating females from tropical landscapes acquire essential nutrients from locally-available mineral licks. We captured ten times as many bats at mineral licks than at control sites in a lowland rainforest in eastern Ecuador. Among bats captured at mineral licks, the sex ratio was heavily biased toward females, and a significantly higher portion of females captured at these sites, compared to control sites, were reproductively active (pregnant and lactating). Enrichment ofN15in relation toN14in wing tissue indicated that bats captured at mineral licks were mostly fruit-eating species. Given the high visitation rates of reproductive active females at mineral licks, it is likely that mineral licks are important for fruit-eating female bats as a mineral source during late pregnancy and lactation. By sustaining high population densities of fruit-eating bats that disperse seeds, mineral licks may have an indirect influence on local plant species richness.

Plants using the C3 and C4 photosynthetic pathways differ in carbon-isotope composition, and this difference offers a means to estimate the relative abundance of these two functional groups in the palaeorecord. We report here results of a study aiming to evaluate pollen δ13C(δ13Cp) of Poaceae (the grass family) as a proxy indicator for palaeoecological studies. On average δ13Cp differs by ∼ 13- between modern C3 (-22.6 to - 26.8) and C4 (-9.2 to -17.7) grass species. δ13Cp is 1.2-3.7 more negative for modern grass pollen treated with the same protocol as for fossil samples than for untreated modern samples. δ13Cp ranges from -20.1 to -25.4 for grass pollen in the middle-Holocene sediments from West Olaf Lake, located near the modern tallgrass prairie forest ecotone in western Minnesota. We applied a two end-member mixing model to estimate fluctuations in C3 and C4 grass abundance around this lake. Both C3 and C4 grasses expanded relative to Ambrosia and Artemisia with the decline of aridity from 8000 to 4000 BP. C3 grasses were generally more abundant than C4 grasses throughout the middle Holocene, suggesting the presence of mixed-grass prairie around West Olaf Lake. The grass δ13Cp-based estimates of C4 plant abundance were lower than charcoal δ13C-based estimates, probably reflecting different source areas of pollen and charcoal. Grass δ13Cp also revealed greater submillennial-scale variability in C3 and C4 abundance than charcoal δ13C. These results suggest that grass δ13Cp can provide palaeoenvironmental information not available from other proxy indicators.

Stable isotopes can be used to evaluate trophic relationships, nutrient state, and temporal and spatial variation in diet, food webs, and behaviour both within and between species. Here we describe the development and application of models to predict habitat use of a common insectivorous bat (Eptesicus fuscus) based upon delta(13)C and delta(15)N signatures of skin tissue. We used a 42-specimen sample collected from three well-characterized ecogeographic regions, disparate both in photosynthetic mechanism and fertilizer use, to generate the models. Significant univariate differences between these three sites in terms of delta(13)C (F-2,(39) = 112.92, P < 0.0001) and delta(15)N (F-2,F-39 = 97.06, P < 0.0001), and multivariate significance of both variables (Wilk's lambda = 0.032, F-4,(76) = 87.02, P < 0.0001), made it possible to develop three predictive models using Fisher's linear discriminant functions: 1) a model predicting if bats forage in C-3 or mixed C-3/C-4 sites, 2) a model predicting if bats forage in agricultural areas, and 3) a combined model using both variables to predict specific habitat use. We present the results of model application to an independent dataset of 329 bats sampled from 10 states that included a broad range of delta(13)C (-26.53 parts per thousand <= delta(13)C <= -17.20 parts per thousand) and delta(15)N (6.36%. <= delta(15)N <= 15.60 parts per thousand) signatures. We validated the use of skin tissue samples (from wing membranes) in the model by comparing the sites used for model development across five tissue types, selecting skin samples for model development due to consistently low variance within this tissue type. Our results indicate nonspecific habitat-use by big brown bats.

The introduction of Nile perch, Lates niloticus, to Lake Victoria, East Africa, interacted with eutrophication to cause a reorganization of the lake's food web and the extirpation of many endemic fishes. The Lake Kyoga satellite system lies downstream from Lake Victoria. It encompasses species-rich lakes where Nile perch are absent or very rare, and low diversity lakes where L. niloticus is abundant. In 1999 we surveyed seven lakes in the Kyoga system using experimental monofilament gill nets (1/4-1 inches variable mesh). At Boston University we assessed delta N-15 signatures of epaxial muscle from subsamples of the catch (n = 361). These signatures are often highly correlated with the near-term mean realized trophic position of an individual organism. A neural network analysis of fish length, species name, trophic level, and lake of origin fish explained 94% of the sample variance in delta N-15. We analysed statistical patterns in these signatures at a number of spatial scales. The relationship between trophic level and delta N-15 varied greatly among lakes. Higher diversity perch-free lakes had greater variance in delta N-15 values and fish lengths than lower diversity Nile perch lakes, suggesting an important relationship between species diversity and functional diversity. Against expectations, lake size was negatively correlated with delta N-15. Patterns in stable isotope signatures indicated that Nile perch lakes have shorter food chains than perch-free lakes. The results throw up two management problems for the Kyoga system. Impacted lakes need to be studied to understand and ameliorate the community-level effects of Nile perch introduction, whereas the species-rich nonperch lakes, which harbour a large proportion of the remaining diversity of regionally endemic taxa, are in need of conservation planning.

During copulation, male Isophya kraussi transfer a large nuptial gift to females. In this study, we hypothesized that the energy content of spermatophores should meet the energy requirements of both body maintenance and egg production of females. We measured the field metabolic rate of male and female I. kraussi using the doubly labeled water method and the energy content of spermatophores and male bodies with microbomb calorimetry. The energy content of male nuptial gifts averaged 0.66+/-0.09 kJ, approximately 20% of the total body energy content of male I. kraussi (3.24+/-0.26 kJ). Field metabolic rates averaged 0.41+/-0.17 kJ d(-1) (n = 8) for males and 0.30+/-0.15 kJ d(-1) (n = 5) for female I. kraussi. Thus, the energy content of spermatophores exceeded the daily energy requirements of existence in male I. kraussi. A single nuptial gift provides for all energy requirements of females for 1 or 2 d, depending on their activity, egg production, and ambient temperature. Because the shortest known remating interval of female katydids varies between 1 and 3 d, female I. kraussi could theoretically exist exclusively on spermatophores to meet their nutritional requirements.