A Noninvasive Method for Estimating Nitrogen Balance in Free-Ranging Primates

Towards an integrated understanding of dietary phenotypes

Diet and nutrition comprise a complex, multi-faceted interface between animal biology and food environments. With accumulating information on the many facets of this association arises a need for systems-based approaches that integrate dietary components and their links with ecology, feeding, post-ingestive processes and the functional and ecological consequences of these interactions. We briefly show how a modelling approach, nutritional geometry, has used the experimental control afforded in laboratory studies to begin to unravel these links. Laboratory studies, however, have limited ability to establish whether and how the feeding and physiological mechanisms interface with realistic ecological environments. We next provide an overview of observational field studies of free-ranging primates that have examined this, producing largely correlative data suggesting that similar feeding mechanisms operate in the wild as in the laboratory. Significant challenges remain, however, in establishing causal links between feeding, resource variation and physiological processes in the wild. We end with a more detailed account of two studies of temperate primates that have capitalized on the discrete variation provided by seasonal environments to strengthen causal inference in field studies and link patterns of intake to dynamics of nutrient processing. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

A skew in poo: Biases in primate fecal isotope analysis and recommendations for standardized sample preparation

Feces are a treasure trove in the study of animal behavior and ecology. Stable carbon and nitrogen isotope analysis allows to assess the dietary niches of elusive primate species and primate breastfeeding behavior. However, some fecal isotope data may unwillingly be biased toward the isotope ratios of undigested plant matter, requiring more consistent sample preparation protocols. We assess the impact of this potential data skew in 114 fecal samples of wild bonobos (Pan paniscus) by measuring the isotope differences (Δ¹³ C, Δ¹⁵ N) between bulk fecal samples containing larger particles (>1 mm) and filtered samples containing only small particles (<1 mm). We assess the influence of fecal carbon and nitrogen content (ΔC:N) and sample donor age (subadult, adult) on the resulting Δ¹³ C, Δ¹⁵ N values (n = 228). Additionally, we measure the isotope ratios in three systematically sieved fecal samples of chimpanzees (Pan troglodytes verus), with particle sizes ranging from 20 μm to 8 mm (n = 30). We found differences in fecal carbon and nitrogen content, with the smaller fecal fraction containing more nitrogen on average. While the Δ¹³ C values were small and not affected by age or ΔC:N, the Δ¹⁵ N values were significantly influenced by fecal ΔC:N, possibly resulting from the differing proportions of undigested plant macroparticles. Significant relationships between carbon stable isotope ratios (δ¹³ C) values and %C in large fecal fractions of both age groups corroborated this assessment. Δ¹⁵ N values were significantly larger in adults than subadults, which should be of concern in isotope studies comparing adult females with infants to assess breastfeeding. We found a random variation of up to 3.0‰ in δ¹³ C and 2.0‰ in nitrogen stable isotope ratios within the chimpanzee fecal samples separated by particle sizes. We show that particle size influences isotope ratios and propose a simple, cost-effective filtration method for primate feces to exclude larger undigested food particles from the analysis, which can easily be adopted by labs worldwide.

Minimally invasive biomarkers in human and non-human primate evolutionary biology: Tools for understanding variation and adaptation

BACKGROUND

The use of minimally invasive biomarkers (MIBs - physiological biomarkers obtained from minimally invasive sample types) has expanded rapidly in science and medicine over the past several decades. The MIB approach is a methodological strength in the field of human and non-human primate evolutionary biology (HEB). Among humans and our closest relatives, MIBs provide unique opportunities to document phenotypic variation and to operationalize evolutionary hypotheses.

AIMS

This paper overviews the use of MIBs in HEB. Our objectives are to (1) highlight key research topics which successfully implement MIBs, (2) identify promising yet under-investigated areas of MIB application, and (3) discuss current challenges in MIB research, with suggestions for advancing the field.

DISCUSSION AND CONCLUSIONS

A range of MIBs are used to investigate focal topics in HEB, including energetics and life history variation/evolution, developmental plasticity, and social status and dominance relationships. Nonetheless, we identify gaps in existing MIB research on traits such as physical growth and gut function that are central to the field. Several challenges remain for HEB research using MIBs, including the need for additional biomarkers and methods of assessment, robust validations, and approaches that are standardized across labs and research groups. Importantly, researchers must provide better support for adaptation and fitness effects in hypothesis testing (e.g., by obtaining complementary measures of energy expenditure, demonstrating redundancy of function, and performing lifetime/longitudinal analyses). We point to continued progress in the use of MIBs in HEB to better understand the past, present, and future of humans and our closest primate relatives.

Concentrations of urinary neopterin, but not suPAR, positively correlate with age in rhesus macaques

Identifying biomarkers of age-related changes in immune system functioning that can be measured non-invasively is a significant step in progressing research on immunosenescence and inflammaging in free-ranging and wild animal populations. In the present study, we aimed to investigate the suitability of two urinary compounds, neopterin and suPAR, as biomarkers of age-related changes in immune activation and inflammation in a free-ranging rhesus macaque (Macaca mulatta) population. We also investigated age-associated variation in gene transcription from blood samples to understand the underlying proximate mechanisms that drive age-related changes in urinary neopterin or suPAR. Neopterin was significantly positively correlated with age, and had a moderate within-individual repeatability, indicating it is applicable as a biomarker of age-related changes. The age-related changes in urinary neopterin are not apparently driven by an age-related increase in the primary signaler of neopterin, IFN-y, but may be driven instead by an age-related increase in both CD14+ and CD14− monocytes. suPAR was not correlated with age, and had low repeatability within-individuals, indicating that it is likely better suited to measure acute inflammation rather than chronic age-related increases in inflammation (i.e., “inflammaging”). Neopterin and suPAR had a correlation of 25%, indicating that they likely often signal different processes, which if disentangled could provide a nuanced picture of immune-system function and inflammation when measured in tandem.

Carbon and nitrogen stable isotopic offsets between diet and hair/feces in captive orangutans

Estimating stable isotopic offset values is crucial for dietary reconstructions. Although research into stable isotope ecology of wild nonhuman primates is increasing overall, only a minority of studies involve laboratory experiments. This study is the first to report the carbon and nitrogen stable isotopic offset values in hair and feces of orangutans. During an experiment lasting 1 week, the weight of each consumed food item was recorded for each of six captive Bornean orangutan (Pongo pygmaeus) individuals. The food, hair, and fecal samples were collected for a few days, and their stable carbon and nitrogen isotope ratios were measured using an elemental analyzer/isotope ratio mass spectrometer. Subsamples of feces were treated with ethanol during the preservation process. Monte Carlo analyses showed that the 95% confidence intervals (CIs) of the carbon and nitrogen offset values between hair and diet were +0.9‰ to +3.9‰ and +2.3‰ to +4.5‰, respectively. The 95% CIs of the carbon and nitrogen offset values between feces and diet were -3.7‰ to -0.9‰ and +0.3‰ to +2.7‰, respectively. The effect of ethanol treatment on the stable isotope ratios of feces was unclear and inconclusive. The computed offset values of hair in captive orangutans are similar to those reported in other nonhuman primates, although those of feces showed greater interspecies variations. The offset values estimated in this study contribute to isotopic studies into the feeding ecology of free-ranging orangutans who are critically endangered in most wild settings.

Wild Bornean orangutans experience muscle catabolism during episodes of fruit scarcity

Pronounced temporal and spatial variation in the availability of food resources can produce energetic deficits in organisms. Fruit-dependent Bornean orangutans face extreme variation in fruit availability and experience negative energy and protein balance during episodes of fruit scarcity. We evaluate the possibility that orangutans of different sexes and ages catabolize muscle tissue when the availability of fruit is low. We assess variation in muscle mass by examining the relationship between urinary creatinine and specific gravity and use the residuals as a non-invasive measure of estimated lean body mass (ELBM). Despite orangutans having a suite of adaptations to buffer them from fruit scarcity and associated caloric deficits, ELBM was lower during low fruit periods in all age-sex classes. As predicted, adult male orangutans had higher ELBM than adult females and immatures. Contrary to expectation, flanged and unflanged males did not differ significantly in ELBM. These findings highlight the precarity of orangutan health in the face of rapid environmental change and add to a growing body of evidence that orangutans are characterized by unique metabolic traits shaped by their unpredictable forest environment.

Urinary suPAR: a non-invasive biomarker of infection and tissue inflammation for use in studies of large free-ranging mammals

Studies of large free-ranging mammals incorporating physiological measurements typically require the collection of urine or faecal samples, due to ethical and practical concerns over trapping or darting animals. However, there is a dearth of validated biomarkers of immune activation and inflammation that can be measured non-invasively. We here evaluate the utility of urinary measurements of the soluble form of the urokinase plasminogen activator receptor (suPAR), for use as a health marker in studies of wild large mammals. We investigate how urinary suPAR concentrations change in response to viral infection and surgical trauma (inflammation), comparing it to the measurement of a marker of cellular immune activation, urinary neopterin (uNEO), in captive rhesus macaques. We then test the field utility of urinary suPAR, assessing the effects of soil and faecal contamination, sunlight, storage at different temperatures, freeze-thaw cycles, and lyophilization. We find that suPAR concentrations rise markedly in response to both infection and surgery-associated inflammation, unlike uNEO concentrations, which only rise in response to the former. Our field validation demonstrates that urinary suPAR is reasonably robust to many of the issues associated with field collection, sample processing, and storage, as long as samples can be stored in a freezer. Urinary suPAR is thus a promising biomarker applicable for monitoring various aspects of health in wild primates and potentially also other large mammals.

Playing it safe? Solitary vervet monkeys (Chlorocebus pygerythrus) choose high-quality foods more than those in competition

An important goal in foraging ecology is to determine how biotic and abiotic variables impact the foraging decisions of wild animals and how they move throughout their multidimensional landscape. However, the interaction of food quality and feeding competition on foraging decisions is largely unknown. Here we examine the importance of food quality in a patch on the foraging decisions of wild vervet monkeys (Chlorocebus pygerythrus) at Lake Nabugabo, Uganda using a multidestination platform array. The overall nutritional composition of the vervet diet was assessed and found to be low in sodium and lipids, thus we conducted a series of experimental manipulations in which the array was varied in salt and oil content. Although vervets prioritized platforms containing key nutrients (i.e., sodium and lipids) overall, we found that solitary vervets prioritized nutrient-dense platforms more strongly than competing vervets. This finding was opposite to those in a similar experiment that manipulated food site quantity, suggesting that large, salient rewards may be worth competing over but slight differences in nutritional density may be only chosen when there are no potentially negative social consequences (i.e., aggression received). We also found that vervets chose platforms baited with oil-only, and oil combined with salt, but not salt-only, suggesting that energy was an important factor in food choice. Our findings demonstrate that when wild vervets detect differences in feeding patches that reflect nutritional composition, they factor these differences into their navigational and foraging decisions. In addition, our findings suggest that these nutritional differences may be considered alongside social variables, ultimately leading to the complex strategies we observed in this study.

Stable isotope variation in savanna chimpanzees (Pan troglodytes verus) indicate avoidance of energetic challenges through dietary compensation at the limits of the range

OBJECTIVES

Food scarcity is proposed to be a limitation to chimpanzees at the limits of their range; however, such a constraint has never been investigated in this context. We investigated patterns of δ¹³ C and δ¹⁵ N variation along a latitudinal gradient at the northwestern West African chimpanzee (Pan troglodytes verus) range limit with the expectation that isotope ratios of chimpanzees at the range limit will indicate different dietary strategies or higher physiological constraints than chimpanzees further from the edge.

MATERIALS AND METHODS

We measured δ¹³ C and δ¹⁵ N values in hair (n = 81) and plant food (n = 342) samples from five chimpanzee communities located along a latitudinal gradient in Southeastern Senegal.

RESULTS

We found clear grouping patterns in hair δ¹³ C and δ¹⁵ N in the four southern sites compared to the northernmost site. Environmental baseline samples collected from these sites revealed overall higher plant δ¹⁵ N values at the northernmost site, but similar δ¹³ C values across sites. By accounting for environmental baseline, Δ¹³ C and Δ¹⁵ N values were clustered for all five sites relative to total Pan variation, but indicated a ¹³ C-enriched diet at the range limit.

DISCUSSION

Clustering in Δ¹³ C and Δ¹⁵ N values supports that strategic shifting between preferred and fallback foods is a likely ubiquitous but necessary strategy employed by these chimpanzees to cope with their environment, potentially allowing chimpanzees at their limits to avoid periods of starvation. These results also underline the necessity of accounting for local isotopic baseline differences during inter-site comparison.

Ethics of Primate Fieldwork: Toward an Ethically Engaged Primatology

Field primatologists have ethical responsibilities that extend beyond study subjects to the local human communities living near primate populations and their surrounding ecosystems. In this review, we explore the history of ethical discussions within anthropological primatology and examine the best practices for an ethically engaged primatology that should be followed and role-modeled by primatologists. An increasing number of primates are showing reduced population sizes and are in imminent danger of extinction; thus, we need to carefully consider the ethics of intervening to ensure the survival of remaining populations, the impact of anthropogenic factors (e.g., climate change), and whether long-term field research results in conservation outcomes that consider local human communities. Because best practices change over time as theoretical frameworks and methodological tools advance and scientific goals change, field primatologists must continually reflect on what constitutes ethical practice and consider how research influences the overlapping dimensions of fieldwork: primates, people, and ecosystems.

Energetics of feeding, social behavior, and life history in non-human primates

Energy is a variable of key importance to a wide range of research in primate behavioral ecology, life history, and conservation. However, obtaining detailed data on variation in energetic condition, and its biological consequences, has been a considerable challenge. In the past 20years, tremendous strides have been made towards non-invasive methods for monitoring the physiology of animals in their natural environment. These methods provide detailed, individualized data about energetic condition, as well as energy allocations to growth, reproduction, and somatic health. In doing so, they add much-needed resolution by which to move beyond correlative studies to research programs that can discriminate causes from effects and disaggregate multiple correlated features of the social and physical environment. In this review, I describe the conceptual and methodological approaches for studying primate energetics. I then discuss the core questions about primate feeding ecology, social behavior, and life history that can benefit from physiological studies, highlighting the ways in which recent research has done so. Among these are studies that test, and often refute, common assumptions about how feeding ecology shapes primate biology, and those that reveal proximate associations between energetics and reproductive strategies.

Energetics of feeding, social behavior, and life history in non-human primates

Energy is a variable of key importance to a wide range of research in primate behavioral ecology, life history, and conservation. However, obtaining detailed data on variation in energetic condition, and its biological consequences, has been a considerable challenge. In the past 20years, tremendous strides have been made towards non-invasive methods for monitoring the physiology of animals in their natural environment. These methods provide detailed, individualized data about energetic condition, as well as energy allocations to growth, reproduction, and somatic health. In doing so, they add much-needed resolution by which to move beyond correlative studies to research programs that can discriminate causes from effects and disaggregate multiple correlated features of the social and physical environment. In this review, I describe the conceptual and methodological approaches for studying primate energetics. I then discuss the core questions about primate feeding ecology, social behavior, and life history that can benefit from physiological studies, highlighting the ways in which recent research has done so. Among these are studies that test, and often refute, common assumptions about how feeding ecology shapes primate biology, and those that reveal proximate associations between energetics and reproductive strategies.

Carbon and nitrogen stable isotopic offsets between diet and hair/feces in captive chimpanzees

RATIONALE

Estimation of the stable isotopic offsets between tissue and diet is important for dietary reconstructions. Although stable isotopic studies of chimpanzees (Pan troglodytes) are increasing, the isotopic offsets in chimpanzees have never been studied. In this study, the carbon and nitrogen stable isotopic offset values in hair and feces were measured for 13 captive chimpanzees for the first time.

METHODS

All consumed food items and quantities were recorded for each individual for 1 week. Food samples were typically collected three times, hair was collected 3 weeks after the experimental week, and feces were collected ad libitum during the experimental week. The stable carbon and nitrogen isotope ratios were measured using elemental analyzer-isotope ratio mass spectrometry (EA-IRMS).

RESULTS

As the results of Monte Carlo analysis, the estimated carbon and nitrogen offsets between the hair and diet were +3.0 to +3.9‰ and +2.8 to +3.7‰, respectively, for the 95% confidence intervals (CIs). The 95% CIs of the carbon and nitrogen offset values between the feces and diet were -1.6 to 0.0‰ and +1.2 to +2.7‰, respectively.

CONCLUSIONS

These offset values are generally consistent with those of the other primate species reported in previous studies. However, potential variations in the offset values due to dietary and physiological factors should be studied in detail in the future. Copyright © 2016 John Wiley & Sons, Ltd.

Functions of innate and acquired immune system are reduced in domestic pigeons (Columba livia domestica) given a low protein diet

Racing pigeons are exposed to and act as carriers of diseases. Dietary protein requirement for their maintenance has not been determined experimentally despite their being domesticated for over 7000 years. A maintenance nitrogen (protein) requirement (MNR) for pigeons was determined in a balance study using diets containing 6, 10 and 14% crude protein (CP). Then, the effects of feeding the diets were investigated to determine whether they were adequate to sustain innate and acquired immune functions. Nitrogen intake from the 6% CP diet was sufficient to maintain nitrogen balance and body weight in pigeons. However, the immune functions of phagocytosis, oxidative burst and lymphocyte proliferation in pigeons fed this diet were reduced compared with those fed 10 and 14% CP diets. Pigeons given the 6 and 10% CP diets had lower antibody titres following inoculation against Newcastle disease (ND) than those on the 14% CP diet. A confounding factor found on autopsy was the presence of intestinal parasites in some of the pigeons given the 6 and 10% CP diets; however, none of the pigeons used to measure MNR or acquired immunity to ND were infested with parasites. In conclusion, neither the 6 nor 10% CP diets adequately sustained acquired immune function of pigeons.

Stable Isotope Analyses and the Evolution of Human Diets

Stable isotope analysis of carbon and nitrogen has revolutionized anthropology's approach and understanding of the evolution of human diet. A baseline comparison across extant nonhuman primates reveals that they all depend on C3 plants in forests, forest patches, and woodlands except during rare seasonal intake, in marginal regions, or where maize fields exist. Even large-bodied hominoids that could theoretically rely on hard-to-digest C4 plants do not do so. Some Plio-Pleistocene hominins, however, apparently relied heavily on C4 and/or CAM plants, which suggests that they relied extensively on cecal-colon microbial fermentation. Neanderthals seem less carnivorous than is often assumed when we compare their δ15Nbone collagen values with those of recent human populations, including recent human foragers who also fall at or near the top of their local trophic system. Finally, the introduction of maize into North America is shown to have been more sporadic and temporally variable than previously assumed.

Isotopic nitrogen in fecal fiber as an indicator of winter diet in caribou and muskoxen

RATIONALE

The ratios of stable nitrogen isotopes (δ(15)N values) in excreta have been used to examine aspects of trophic and nutritional ecology across taxa. Nitrogen fractions in feces of herbivores include endogenous (e.g., sloughed intestinal cells, unresorbed digestive secretions, and microbial debris) and dietary sources. For animals such as large herbivores, that have diets and feces with high concentrations of indigestible fiber, endogenous (15)N may constrain the use of fecal δ(15)N values to estimate dietary δ(15)N values and reconstruct diets.

METHODS

We compared two techniques (detergent and detergent-free) to isolate fractions of plant fibers in the forages of caribou (Rangifer tarandus) and muskoxen (Ovibos moschatus); estimated the discrimination factors between the δ(15)N values of fecal fiber residues and of the diets of captive animals; and used the more effective isotopic tracer of dietary δ(15)N values to examine the relationships between the δ(15)N values of fecal residues and diet composition in several populations of wild caribou and muskoxen throughout North America in winter.

RESULTS

The detergent-based approach contaminated the fractions of plant fibers in forages and feces with (14)N, whereas the detergent-free method was a good proxy to estimate δ(15)N values of plant fibers (r(2) = 0.92) and provided a better estimate of the fecal-fiber to diet discrimination factor for both species (caribou = 3.6‰; muskoxen = 2.8‰). In wild populations, the δ(15)N values of fecal fibers reflected diet composition in muskoxen (adjusted R(2) = 0.43) but not caribou (adjusted R(2) = 0.06).

CONCLUSIONS

Contamination from detergent residues prohibited the use of detergent extraction in isolating forage (15)N from endogenous (15)N in the feces of herbivores. Although δ(15)N values in fecal fibers can be used to track dietary δ(15)N values in wild herbivores, discrimination between fecal extracts and diet may vary with the contribution of endogenous nitrogen (N), and, therefore, residual endogenous (15)N in feces may limit dietary reconstructions from fecal δ(15)N values for some large herbivores.

Stable isotope analysis in primatology: a critical review

Stable isotope analysis has become an important tool in ecology over the last 25 years. A wealth of ecological information is stored in animal tissues in the relative abundances of the stable isotopes of several elements, particularly carbon and nitrogen, because these isotopes navigate through ecological processes in predictable ways. Stable carbon and nitrogen isotopes have been measured in most primate taxonomic groups and have yielded information about dietary content, dietary variability, and habitat use. Stable isotopes have recently proven useful for addressing more fine-grained questions about niche dynamics and anthropogenic effects on feeding ecology. Here, we discuss stable carbon and nitrogen isotope systematics and critically review the published stable carbon and nitrogen isotope data for modern primates with a focus on the problems and prospects for future stable isotope applications in primatology.

Technical note: variation in muscle mass in wild chimpanzees: application of a modified urinary creatinine method

Individual body size and composition are important variables for a variety of questions about the behavioral ecology and life histories of non-human primates. Standard methodologies for obtaining body mass involve either capture, which poses risks to the subject, or provisioning, which can disrupt the processes being studied. There are no methods currently available to assess body composition from living animals in the wild. Because of its derivation in muscle, the amount of creatinine that an individual excretes in 24 hours is a reliable and frequently used indicator of relative muscle mass in humans and laboratory animals. Although it is not feasible to collect 24-hour urine samples from wild primates, we apply here a simple method to approximate muscle mass variation from collections of spot urine samples. Specific gravity (SG), an alternative method for assessing urinary water content, is both highly correlated to creatinine and free of mass-dependent effects. Individuals with greater muscle mass should excrete more creatinine for a given SG. We examine this relationship in a dataset of 12,598 urine samples from wild chimpanzees in the Kibale National Park, Uganda. As expected from known differences in body composition, the slope of the relationship between SG and creatinine is significantly greater in adult males than adult females and in adults versus immature individuals. Growth curves generated through this method closely approximate published weight curves for wild chimpanzees. Consistent with the role of testosterone in muscle anabolism, urinary testosterone predicted relative creatinine excretion among adult male chimpanzees.

Regional, seasonal and interspecific variation in 15N and 13C in sympatric mouse lemurs

Madagascar provides some of the rare examples where two or more primate species of the same genus and with seemingly identical niche requirements occur in sympatry. If congeneric primate species co-occur in other parts of the world, they differ in size in a way that is consistent with Hutchinson's rule for coexisting species, or they occupy different ecological niches. In some areas of Madagascar, mouse lemurs do not follow these "rules" and thus seem to violate one of the principles of community ecology. In order to understand the mechanisms that allow coexistence of sympatric congeneric species without obvious niche differentiation, we studied food composition of two identical sized omnivorous mouse lemur species, Microcebus griseorufus and M. murinus with the help of stable isotope analyses (δ(15)N and δ(13)C). The two species are closely related sister species. During the rich season, when food seems abundant, the two species do not differ in their nitrogen isotope composition, indicating that the two species occupy the same trophic level. But they differ in their δ(13)C values, indicating that M. griseorufus feeds more on C(4) and CAM (Crassulacean-acid-metabolism) plants than M. murinus. During the lean season, M. murinus has lower δ(15)N values, indicating that the two species feed at different trophic levels during times of food shortage. Hybrids between the two species showed intermediate food composition. The results reflect subtle differences in foraging or metabolic adaptations that are difficult to quantify by traditional observations but that represent possibilities to allow coexistence of species.