No abstract

We tested the competing hypotheses that (1) nitrogen discrimination in mammals and birds increases with dietary nitrogen concentration or decreasing C:N ratios and, therefore, discrimination will increase with trophic level as carnivores ingest more protein than herbivores and omnivores or (2) nitrogen discrimination increases as dietary protein quality decreases and, therefore, discrimination will decrease with trophic level as carnivores ingest higher quality protein than do herbivores. Discrimination factors were summarized for five major diet groupings and 21 different species of birds and mammals. Discrimination did not differ between mammals and birds and decreased as protein quality (expressed as biological value) increased with trophic level (i.e., herbivores to carnivores). Relationships between discrimination factors and dietary nitrogen concentration or C:N ratios were either the opposite of what was hypothesized or non-significant. Dietary protein quality accounted for 72% of the variation in discrimination factors across diet groupings. We concluded that protein quality established the baseline for discrimination between dietary groupings, while other variables, such as dietary protein intake relative to animal requirements, created within-group variation. We caution about the care needed in developing studies to understand variation in discrimination and subsequently applying those discrimination factors to estimate assimilated diets of wild animals.

Nitrogen isotope analysis is a common technique for investigating dietary behaviour in modern and archaeological populations. One of its primary uses is to provide trophic level information. This application is possible because of a ∼3‰ enrichment in 15 N along each step in the food chain, resulting in carnivores having higher δ 15 N values than herbivores, which in turn have higher δ 15 N values than plants. Much variation has also been observed within a trophic level, although the reasons for this are poorly understood. Here we present the results of a controlled feeding study designed to test the effects of gut anatomy and dietary protein levels on hair δ 15 N values within a trophic level. The data reveal that mammalian herbivores eating identical diets can have hair δ 15 N values that differ by as much as 3.6‰. This is particularly striking as it suggests that interspecific physiological differences can lead to larger shifts in δ 15 N values than a shift in trophic level. We also found that diet-hair fractionation was 2.3‰ greater when herbivores were fed high-protein (19%) diets than when they were fed low-protein (9%) diets. The primary nitrogen losses in mammalian herbivores are 15 N-depleted urine and 15 N-enriched faeces. We reason that an increase in the ratio of urinary to faecal nitrogen efflux leads to greater diet-hair fractionation on the high-protein diet.

Temporal stable isotope records derived from animal tissues are increasingly studied to determine dietary and climatic histories. Despite this, the turnover times governing rates of isotope equilibration in specific tissues following a dietary isotope change are poorly known. The dietary isotope changes recorded in the hair and blood bicarbonate of two adult horses in this study are found to be successfully described by a model having three exponential isotope pools. For horse tail hair, the carbon isotope response observed following a dietary change from a C3 to a C4 grass was consistent with a pool having a very fast turnover rate ( t1/2 approximately 0.5 days) that made up approximately 41% of the isotope signal, a pool with an intermediate turnover rate ( t1/2 approximately 4 days) that comprised approximately 15% of the isotope signal, and a pool with very slow turnover rate ( t1/2 approximately 140 days) that made up approximately 44% of the total isotope signal. The carbon isotope signature of horse blood bicarbonate, in contrast, had a different isotopic composition, with approximately 67% of the isotope signal coming from a fast turnover pool ( t1/2 0.2 days), approximately 17% from a pool with an intermediate turnover rate ( t1/2 approximately 3 days) and approximately 16% from a pool with a slow turnover rate ( t1/2 approximately 50 days). The constituent isotope pools probably correspond to one exogenous and two endogenous sources. The exogenous source equates to our fast turnover pool, and the pools with intermediate and slow turnover rates are thought to derive from the turnover of metabolically active tissues and relatively inactive tissues within the body, respectively. It seems that a greater proportion of the amino acids available for hair synthesis come from endogenous sources compared to the compounds undergoing cellular catabolism in the body. Consequently, the isotope composition of blood bicarbonate appears to be much more responsive to dietary isotope changes, whereas the amino acids in the blood exhibit considerable isotopic inertia.

The carbon-isotope composition of hair and feces offers a glimpse into the diets of mammalian herbivores. It is particularly useful for determining the relative consumption of browse and graze in tropical environments, as these foods have strongly divergent carbon-isotope compositions. Fecal δ13C values reflect the last few days consumption, whereas hair provides longer term dietary information. Previous studies have shown, however, that some fractionation occurs between dietary δ13C values and those of hair and feces. Accurate dietary reconstruction requires an understanding of these fractionations, but few controlled-feeding studies have been undertaken to investigate these fractionations in any mammalian taxa, fewer still in large mammalian herbivores. Here, we present data from the first study of carbon-isotope fractionation between diet, hair, and feces in multiple herbivore taxa. All taxa were fed pure alfalfa (Medicago sativa) diets for a minimum period of 6 months, at which point recently grown hair was shaved and analyzed for carbon isotopes. The mean observed diet–hair fractionation was +3.2‰, with a range of +2.7 to +3.5‰. We also examined diet–feces fractionation for herbivores on alfalfa and bermudagrass (Cynodon dactylon) feeds. The mean diet–feces fractionation for both diets was –0.8‰, with less fractionation for alfalfa (–0.6‰) than bermudagrass (–1.0‰). Fecal carbon turnover also varies greatly between taxa. When diets were switched, horse (Equus caballus) feces reflected the new diet within 60 h, but alpaca (Lama pacos) feces did not equilibrate with the new diet for nearly 200 h. Thus, fecal carbon isotopes provide far greater dietary resolution for hindgut-fermenting horses than foregut-fermenting alpacas.

The East African hominin Paranthropus boisei was characterized by a suite of craniodental features that have been widely interpreted as adaptations to a diet that consisted of hard objects that required powerful peak masticatory loads. These morphological adaptations represent the culmination of an evolutionary trend that began in earlier taxa such as Australopithecus afarensis, and presumably facilitated utilization of open habitats in the Plio-Pleistocene. Here, we use stable isotopes to show that P. boisei had a diet that was dominated by C 4 biomass such as grasses or sedges. Its diet included more C 4 biomass than any other hominin studied to date, including its congener Paranthropus robustus from South Africa. These results, coupled with recent evidence from dental microwear, may indicate that the remarkable craniodental morphology of this taxon represents an adaptation for processing large quantities of low-quality vegetation rather than hard objects.C4 photosynthesis | C3 photosynthesis T he East African hominin Paranthropus boisei possessed large and low-cusped postcanine dentition, large and thick mandibular corpora, and powerful muscles of mastication, which are generally believed to be adaptations for a diet of nuts, seeds, and hard fruit (1-3). This notion emerged from interpretations of P. boisei's morphology, but gained indirect support from dental microwear studies of its congener, Paranthropus robustus; these concluded that wear on the molars of South African Paranthropus was consistent with its having ingested and chewed small, hard food items, if not as primary resources, then at least as fallback foods (4-6). Although some have suggested that the craniodental morphology of P. boisei is consistent with the consumption of tough rather than hard foods (7,8), this idea has been largely eschewed by most workers. Thus, when a recent study using dental microwear texture analysis revealed no evidence for the consumption of hard foods by P. boisei (9), it challenged decades of received wisdom, and underscored the need for independent lines of paleodietary evidence.Stable carbon isotope analysis has proven a powerful tool for testing hypotheses about the diets of extinct herbivorous mammals (10, 11). It is based on the idea that carbon isotope compositions vary predictably between plant foods [e.g., plants using the C 3 photosynthetic pathway (most dicotyledonous plants including trees, shrubs, forbs, herbs) and those using the C 4 pathway (predominantly tropical grasses and sedges, which are monocotyledonous plants)], and further that dietary carbon remains locked in tooth enamel even after millions of years (10). Carbon isotope studies of P. robustus from South Africa indicated that it consumed some plants using C 4 photosynthesis such as tropical grasses or sedges, but were also consistent with most of its dietary carbon (approximately 70%) having been derived from the C 3 food items favored by extant chimpanzees (Pan troglodytes) such as tree fruits (12, 13). In contrast, stable isotopes measuremen...

Recent advances in mass spectrometry now allow relatively routine measurements of sulphur isotopes (δ 34 S) in small samples (>10 mg) of tissue from archaeological human, plant, and faunal samples. δ 34 S values of human and faunal bone collagen can indicate residence or migration and can provide palaeodietary information. Here we present a review of applications of sulphur isotopes to archaeological materials, and we also present preliminary results from one of the few controlled feeding experiments undertaken for sulphur isotopes. This study indicates that there is relatively little fractionation (−1‰) between diet and body protein (keratin) δ 34 S values for modern horses on a protein adequate C 3 plant diet. In contrast, horses fed a possible low protein C 4 feed have a diet to hair fractionation of +4‰ that could be the result of the input of endogenous sulphur from the recycling of body proteins.

Hypotheses to explain diversity among African ungulates focus largely on niche separation along a browser/grazer continuum. However, a number of studies advocate that the browser/grazer distinction insufficiently describes the full extent of dietary variation that occurs within and between taxa. Disparate classification schemes exist because of a lack of uniform and reliable data for many taxa, and failure to incorporate spatio-temporal variations into broader assessments of diet.In this study, we tested predictions for diet and dietary niche separation of African savanna ungulates using stable carbon isotope evidence from faeces for proportions of C 3 (browse) to C 4 (grass) intake among 19 species from the Kruger National Park, South Africa. Dietary predictions from the literature are confirmed in the case of browsers (black rhinoceros Diceros bicornis, giraffe Giraffa camelopardalis, bushbuck Tragelaphus scriptus, kudu Tragelaphus strepsiceros), mixed-feeders (impala Aepyceros melampus, nyala Tragelaphus angasii), and most grazers (white rhinoceros Ceratotherium simum, Burchell's zebra Equus burchellii, warthog Phacochoerus africanus, hippopotamus Hippopotamus amphibius, blue wildebeest Connochaetes taurinus, tsessebe Damaliscus lunatus, waterbuck Kobus ellipsiprymnus). In contrast, several species showed results differing from most expectations derived from the available literature, including eland Taurotragus oryx, steenbok Raphicerus campestris, grey duiker Sylvicapra grimmia, buffalo Syncerus caffer, roan antelope Hippotragus equinus and sable antelope Hippotragus niger. Many of these discrepancies can be accounted for by seasonal and/or regional dietary differences. Cluster analysis based on a data matrix that incorporates the extent of spatio-temporal dietary variation among Kruger Park ungulates reveals several distinct categories of feeding preferences that extend beyond a two-edged browser/grazer dichotomy, such as mixed-feeders with a preference for either forage class, and spatial/seasonal shifts between uniform and mixed-feeding styles among variable browsers (e.g. grey duiker) and variable grazers (e.g. buffalo). These results highlight the need for approaches that are sensitive to spatio-temporal variations and the continuity of diet.