Inter-individual variation in weaning among rhesus macaques (Macaca mulatta): Serum stable isotope indicators of suckling duration and lactation

All together now: Assessing variation in maternal and nonmaternal handling of wild Colobus vellerosus infants

Primatologists have a long-standing interest in the study of maternal care and nonmaternal handling (NH) of infants stemming from recognition that early social relationships can have enduring consequences. Though maternal care and NH often include expression of similar behaviors, they are regularly studied in isolation from each other with nonoverlapping terminology, thereby overlooking possible interplay between them and obfuscating potential developmental ramifications that ensue from trade-offs made between maternal (MH) and NH during infancy. To that end, identifying how MH and NH patterns interact and contribute to the total handling (TH) infants receive is a critical first step. We present durational handling data collected from 25 wild Colobus vellerosus infants from 2016 to 2017 and assess the relationships between TH, MH, and NH. Patterns of social affiliation are shaped in part by surrounding context, and therefore, we also assess whether NH and TH differ in their responsivity to various infant and social group characteristics. Ninety-four percent of observed handling was MH, while just 5.5% was NH. Young infants who received more MH (excluding nursing) also received more NH; there was no relationship between the two in older infants. Infants in larger groups participated in more handling of all types. Additionally, NH time was associated with infant sex and group stability. Non-nursing TH time was associated with group stability and infant cohort size. Though NH variation likely confers social-networking advantage, in this population NH is not a major contributor to TH and would not effectively replace reduced MH. The positive association between MH and NH during early infancy suggests that colobus mothers may play a mediating role in shaping infant socialization. This is a first step in elucidating how different forms of handling relate to one another in wild primates and in identifying the impact of handling on infant socialization.

Maternal care in wild toque macaques (Macaca sinica) involves prolonged lactation and interbirth intervals as adaptations to reduce maternal depletion and infant mortality in harsh environments

Weaning age in primates has been challenging to measure and new methods, involving molecular biomarkers in feces, tissue, or teeth have contributed to a solution. Here, we used a direct approach by briefly anesthetizing 442 female toque macaques (Macaca sinica) of Sri Lanka (over a 17-year period) and manually testing their mammary tissue for the presence or absence of milk. Milk tests were related to known offspring ages and maternal care behaviors and indicated that older infants suckled milk well past the weaning age of 7 months that is often reported for food-provisioned primates. Mothers strongly rejected their infants' nursing attempts in two phases, the first at 7 months as an honest signal "giving notice" promoting a shift to greater independence from milk to solid food, and when "shutting down" at final weaning after 12-18 months. The shift to supplementary lactation coincided also with the cessation of mothers carrying their infants and a resumption of cycling. All infants up to 7.2 months suckled milk, 91% of them did up to 18 months, this continued for 42% of infants beyond 18 months, and normally none received milk after 22 months. Lactation extended into 2.2% of cycling and 10.7% of pregnant females (up to 50% of gestation). The interbirth interval was prolonged by factors predicted to draw on female metabolic energy reserves and included the duration of lactation, growth among primiparas, and dietary limitations. The last also increased menarche. Females offset the metabolic costs of lactation with increased foraging and catabolism, but infants died when lactation costs seemingly compromised maternal condition. The prolonged lactation and slowed reproduction are considered adaptations to promote infant survival and growth in an environment where the natural food supply limits population growth and competition for food and water impacts the mortality of the youngest the most.

Postnatal growth and spatial conformity of the cranium, brain, eyeballs and masseter muscles in the macaque (Macaca mulatta)

Spatial growth constraints in the head region can lead to coordinated patterns of morphological variation that pleiotropically modify genetically defined phenotypes as the tissues compete for space. Here we test for such architectural modifications during rhesus macaque (Macaca mulatta) postnatal ontogeny. We captured cranium and brain shape from 153 MRI datasets spanning 13 to 1090 postnatal days and tested for patterns of covariation with measurements of relative brain, eyeball, and masseter muscle size as well as callosal tract length. We find that the shape of the infant (<365 days) macaque cranium was most closely aligned to masseter muscle and brain size measured relative to face size. Infant brain and juvenile (365-1090 days) cranium shape were more closely linked with brain size relative to basicranium and face size. Meanwhile, the juvenile macaque brain shape was dominated by the size of the brain relative to that of the basicranium. Associations with relative eyeball size and commissural tract lengths were weaker. Our results are consistent with a spatial-packing regime operating during postnatal macaque ontogeny, in which relative growth of the masseter, face and basicranium have a greater influence than brain growth on the overall shape of the cranium and brain.

Multiproxy approach to reconstruct fossil primate feeding behavior: Case study for macaque from the Plio-Pleistocene site Guefaït-4.2 (eastern Morocco)

The genus Macaca belongs to Cercopithecidae (Old World monkeys), Cercopithecinae, Papionini. The presence of Macaca in North Africa is well known from the Late Miocene to the Late Pleistocene. However, the diet of fossil Macaca has been poorly described in the literature. In this study, we investigated the feeding habits of Macaca cf. sylvanus (n = 4) from the Plio-Pleistocene site Guefaït-4.2 in eastern Morocco through multiproxy analysis combining analyses of stable carbon and oxygen isotopes from tooth enamel, buccal microtexture, and low-magnification occlusal dental microwear. For both microwear analyses, we compared the macaques with a new reference collection of extant members of Cercopithecoidea. Our occlusal microwear results show for the fossil macaque a pattern similar to the extant Cercocebus atys and Lophocebus albigena, African forest-dwelling species that are characterized by a durophagous diet based mainly on hard fruit and seed intake. Buccal microtexture results also suggest the consumption of some grasses and the exploitation of more open habitats, similar to that observed in Theropithecus gelada. The δ13C of M. cf. sylvanus indicates a C3 based-diet without the presence of C4 plants typical of the savanna grassland in eastern Africa during this period. The high δ18O values of M. cf. sylvanus, compared with the contemporary ungulates recovered from Guefaït-4.2, could be associated with the consumption of a different resource by the primate such as leaves or fresh fruits from the upper part of trees. The complementarity of these methods allows for a dietary reconstruction covering a large part of the individual’s life.

Horizontal transmission of endemic viruses among rhesus macaques (Macaca mulatta): Implications for human cytomegalovirus vaccine/challenge design

INTRODUCTION

Rhesus macaques are natural hosts to multiple viruses including rhesus cytomegalovirus (RhCMV), rhesus rhadinovirus (RRV), and Simian Foamy Virus (SFV). While viral infections are ubiquitous, viral transmissions to uninfected animals are incompletely defined. Management procedures of macaque colonies include cohorts that are Specific Pathogen Free (SPF). Greater understanding of viral transmission would augment SPF protocols. Moreover, vaccine/challenge studies of human viruses would be enhanced by leveraging transmission of macaque viruses to recapitulate expected challenges of human vaccine trials.

MATERIALS AND METHODS

This study characterizes viral transmissions to uninfected animals following inadvertent introduction of RhCMV/RRV/SFV-infected adults to a cohort of uninfected juveniles. Following co-housing with virus-positive adults, juveniles were serially evaluated for viral infection.

RESULTS

Horizontal viral transmission was rapid and absolute, reaching 100% penetrance between 19 and 78 weeks.

CONCLUSIONS

This study provides insights into viral natural histories with implications for colony management and modeling vaccine-mediated immune protection studies.

Elemental composition of primary lamellar bone differs between parous and nulliparous rhesus macaque females

Extracting life history information from mineralized hard tissues of extant and extinct species is an ongoing challenge in evolutionary and conservation studies. Primary lamellar bone is a mineralized tissue with multidien periodicity that begins deposition prenatally and continues until adulthood albeit with concurrent resorption, thus maintaining a record spanning several years of an individual's life. Here, we use field-emission scanning electron microscopy and energy-dispersive X-ray analysis to measure the relative concentrations of calcium, phosphorous, oxygen, magnesium and sodium in the femora of seven rhesus macaque with known medical and life-history information. We find that the concentration of these elements distinguishes parous from nulliparous females; that in females calcium and phosphorus are lower in bone formed during reproductive events; and that significant differences in relative magnesium concentration correlate with breastfeeding in infants.

Maternal lactational investment is higher for sons in chimpanzees

Abstract

Maternal lactational investment can affect female reproductive rates and offspring survival in mammals and can be biased towards infants of one sex. We compared estimates of lactation effort among mothers, assessed as their potential milk contribution to age-specific infant diets (mother-infant differences in fecal stable nitrogen isotopes, δ15N), to the timing of weaning (infant age at last nursing bout) and to maternal inter-birth interval lengths for male and female infant chimpanzees (Pan troglodytes) at Ngogo, Uganda. Infant males had greater proportions of milk in their age-specific diets, indicated by higher mother-infant differences in δ15N (Generalized Estimating Equation, GEE: p < 0.01). This may mean that mothers of sons showed greater lactation effort than mothers of daughters. Infant males stopped nursing at older ages than infant females (Kaplan–Meier product limit estimate, Breslow estimator: p < 0.05). Mothers of sons showed longer interbirth intervals than mothers of daughters (GEE: p < 0.01). All three measures indicated maternal lactational investment was higher for sons. Male infants may cost mothers more to ensure infant survival than female infants because males are more vulnerable and/or because maternal genetic returns on investment are greater for sons than daughters, as male philopatry means that chimpanzee mothers can have more influence on the reproductive success of sons. Chimpanzee females may trade off growth-related benefits of high lactational investment in male offspring against reduced reproductive rates.

Significance statement

Maternal investment via lactation affects the reproductive success of female mammals and their offspring and can be biased towards infants of one sex. We investigated lactational variation among wild chimpanzees in relation to infant sex using three proxies for maternal lactational investment: fecal stable nitrogen isotopes, a physiological biomarker that may provide an estimate of lactation effort; observations of nursing, which we used to establish weaning ages; and the lengths of intervals between births of surviving infants. Chimpanzee mothers biased lactational investment toward sons on all three indicators and showed reduced fecundity due to longer inter-birth intervals for mothers of sons than for mothers of daughters. These results would be expected if greater maternal investment toward sons leads to better condition and higher reproductive success for sons later in life, thus to greater inclusive fitness for mothers.

The poly(I:C)-induced maternal immune activation model; a systematic review and meta-analysis of cytokine levels in the offspring

The maternal polyinosinic:polycytidylic acid (poly(I:C)) animal model is frequently used to study how maternal immune activation may impact neuro development in the offspring. Here, we present the first systematic review and meta-analysis on the effects of maternal poly(I:C) injection on immune mediators in the offspring and provide an openly accessible systematic map of the data including methodological characteristics.

Pubmed and EMBASE were searched for relevant publications, yielding 45 unique papers that met inclusion criteria. We extracted data on immune outcomes and methodological characteristics, and assessed the risk of bias. The descriptive summary showed that most studies reported an absence of effect, with an equal number of studies reporting an increase or decrease in the immune mediator being studied.

Meta-analysis showed increased IL-6 concentrations in the offspring of poly(I:C) exposed mothers. This effect appeared larger prenatally than post-weaning. Furthermore, poly(I:C) administration during mid-gestation was associated with higher IL-6 concentrations in the offspring. Maternal poly(I:C) induced changes in IL-1β, Il-10 and TNF-α concentrations were small and could not be associated with age of offspring, gestational period or sampling location. Finally, quality of reporting of potential measures to minimize bias was low, which stresses the importance of adherence to publication guidelines.

Since neurodevelopmental disorders in humans tend to be associated with lifelong changes in cytokine concentrations, the absence of these effects as identified in this systematic review may suggest that combining the model with other etiological factors in future studies may provide further insight in the mechanisms through which maternal immune activation affects neurodevelopment.

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Long-term effects of maternal poly(I:C) on immune mediators in the offspring appear limited.

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Prenatal measurements and mid gestation poly(I:C) injection are associated with increases in IL-6 concentrations.

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Variety in methodological conduct hampers identification of key elements that affect cytokine concentrations.

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The quality of reporting of potential measures to minimize bias is poor.

Faecal proteomics as a novel method to study mammalian behaviour and physiology

Mammalian faeces can be collected noninvasively during field research and provide valuable information on the ecology and evolution of the source individuals. Undigested food remains, genome/metagenome, steroid hormones, and stable isotopes obtained from faecal samples provide evidence on diet, host/symbiont genetics, and physiological status of the individuals. However, proteins in mammalian faeces have hardly been studied, which hinders the molecular investigations into the behaviour and physiology of the source individuals. Here, we apply mass spectrometry-based proteomics to faecal samples (n = 10), collected from infant, juvenile, and adult captive Japanese macaques (Macaca fuscata), to describe the proteomes of the source individual, of the food it consumed, and its intestinal microbes. The results show that faecal proteomics is a useful method to: (i) investigate dietary changes along with breastfeeding and weaning, (ii) reveal the taxonomic and histological origin of the food items consumed, and (iii) estimate physiological status inside intestinal tracts. These types of insights are difficult or impossible to obtain through other molecular approaches. Most mammalian species are facing extinction risk and there is an urgent need to obtain knowledge on their ecology and evolution for better conservation strategy. The faecal proteomics framework we present here is easily applicable to wild settings and other mammalian species, and provides direct evidence of their behaviour and physiology.

Cord Blood-Based Approach to Assess Candidate Vaccine Adjuvants Designed for Neonates and Infants

Neonates and infants are particularly susceptible to infections, for which outcomes tend to be severe. Vaccination is a key strategy for preventing infectious diseases, but the protective immunity achieved through vaccination typically is weaker in infants than in healthy adults. One possible explanation for the poor acquisition of vaccine-induced immunity in infants is that their innate immune response, represented by toll-like receptors, is immature. The current system for developing pediatric vaccines relies on the confirmation of their safety and effectiveness in studies involving the use of mature animals or adult humans. However, creating vaccines for neonates and infants requires an understanding of their uniquely immature innate immunity. Here we review current knowledge regarding the innate immune system of neonates and infants and challenges in developing vaccine adjuvants for those children through analyses of cord blood.

Isotopic and elemental corroborates for wild bearded capuchin (Sapajus libidinosus) omnivorous dietary adaptation at Fazenda Boa Vista, Brazil

RATIONALE

This study analyzes variability in the diets of wild bearded capuchin monkeys, Sapajus libidinosus, by analyzing stable carbon (δ¹³ C) and nitrogen (δ¹⁵ N) isotope ratios and elemental concentrations (%C and %N) of fecal samples and food items. Developing isotopic and elemental correlates for diets of habituated subjects is a necessary step towards applying similar methods to interpret diets of unhabituated or cryptic subjects.

METHODS

Fecal samples from wild capuchins and their foods were collected at Fazenda Boa Vista, Brazil. Fecal samples from laboratory-housed Sapajus spp. and their foods were analyzed to establish diet-feces offsets for δ¹³ C, δ¹⁵ N, %C, and %N. Samples were dried, powdered, and measured for isotopic and elemental values. A Bayesian mixing model commutes isotopic and elemental data from wild capuchins into likely proportions of different food categories.

RESULTS

The captive study shows small diet-feces spaces for Sapajus spp. of -0.8 ± 0.7‰ for δ¹³ C, -0.2 ± 0.4‰ for δ¹⁵ N, -6.1 ± 1.7% for %C, and -1.0 ± 0.6% for %N. The wild study shows omnivorous diets based on C₃ , C₄ , and CAM plants, and fauna. Subject diets are highly varied within and between days. Fecal data show age-related differences in diet and crop-raiding. There is no consistent isotopic or elemental difference between mothers and infants.

CONCLUSIONS

Fecal stable isotope and elemental evidence employed in a Bayesian mixing model reflects the highly varied diets of capuchin monkeys in an isotopically heterogeneous environment. The isotopic and elemental variability reported here will aid similar diet reconstructions among unhabituated subjects in the future, but precludes tracking weaning isotopically among capuchins in this environment.

Higher early life mortality with lower infant body mass in a free-ranging primate

Traits that reflect the amount of energy allocated to offspring by mothers, such as infant body mass, are predicted to have long-lasting effects on offspring fitness. In very long-lived species, such as anthropoid primates, where long-lasting and obligate parental care is required for successful recruitment of offspring, there are few studies on the fitness implications of low body mass among infants. Using body mass data collected from 253 free-ranging rhesus macaque Macaca mulatta infants on Cayo Santiago, Puerto Rico, we examined if lower infant body mass predicts lower chance of survival through to reproductive maturation (4th year of life). We also used data on inter-birth intervals and suckling behaviours to determine whether the duration of maternal care was adjusted to infant body mass. Rhesus macaque infants experienced on average 5% reduced hazard of death for an increase in body mass of 0.1 SD (~100 g) above the mean within their age-sex class. The positive association between body mass and early life survival was most pronounced in the 1st year of life. Infant body mass tended to be lower if mothers were young or old, but the link between infant body mass and early life survival remained after controlling for maternal age. This finding suggests that maternal effects on early life survival such as maternal age may act through their influence on infant body mass. Mothers of heavier infants were less likely to be delayed in subsequent reproduction, but the estimated association slightly overlapped with zero. The timing of the last week of suckling did not differ by infant body mass. Using infant body mass data that has been rarely available from free-ranging primates, our study provides comparative evidence to strengthen the existing body of literature on the fitness implications of variation in infant body mass.

Utility of Automated Feeding Data to Detect Social Instability in a Captive Breeding Colony of Rhesus Macaques (Macaca mulatta): A Case Study of Intrafamily Aggression

Some captive breeding colonies of rhesus macaques live in large outdoor multimale, multifemale social groups. These groups are composed of several matrilineal families, governed by a clear female dominance hierarchy. Aggression within the same or between different matrilineal families due to social instability can result in trauma and mortality. Therefore, a primary management goal is to detect emerging social unrest before the onset of significant fighting and wounding. Accordingly, groups are monitored routinely for changes in dominance and alliance relations as well as for increases in trauma frequency and severity. Decreased food intake is a normal physiologic response to acute stress; therefore, inappetence in key animals or groups of monkeys might be used as an indicator of increased social stress and emerging instability. An incident of intrafamily aggression occurred recently in a breeding group at our facility and resulted in considerable fighting. Because this compound was equipped with an automated feeding system that tracks the caloric intake of individual animals, we retrospectively analyzed feeding data to determine whether significant reduction in caloric consumption occurred prior to the onset of aggression, compared with baseline values. Neither the entire group nor individual families showed any significant differences in total caloric intake between baseline and previous 24 h values; however, the affected family exhibited a 20% reduction in total caloric during the 24 h prior to the aggression. Most notably, the deposed subfamily showed a marked 58% reduction in caloric intake during the prior 24 h, whereas remaining subfamilies showed no significant changes in intake. High-ranking animals of the group, including the α female, β female, and α male, similarly exhibited marked decreases in caloric intake during that period. These findings indicate that automated feeders can assist management staff with monitoring social stability in breeding colonies of rhesus macaque.

Living near the limits: Effects of interannual variation in food availability on diet and reproduction in a temperate primate, the Taihangshan macaque (Macaca mulatta tcheliensis)

Nutrient intake of animals is influenced by an interplay of external and internal factors, such as food availability and reproductive state, respectively. We used the nutritional geometry framework to analyze individual-based data on energy and nutrient intake in relation to reproductive state in a population of rhesus macaques (Macaca mulatta tcheliensis), which live in a harsh high-latitude habitat, the Taihangshan mountains of China, and exhibit strong reproductive seasonality. We combined data over a 3-year period on food availability, diets, reproductive output, and components of maternal investment to understand how Taihangshan macaques respond to variation in food availability and nutrition in reproduction. Our results show there was high interannual variation in availability of an important staple source of fat and carbohydrates (nonprotein energy), seeds of oak (Quercus spp). Despite this variability in seed availability skewing the dietary macronutrient ratios considerably (from 12.96% to 30.12% dietary energy from protein), total metabolizable energy intake was maintained across years during pregnancy. Lactating females had higher mean daily energy intakes than pregnant females. As in pregnant females, energy intake was maintained constant across years, but only when seed availability enabled the contribution of available protein to energy intake to be maintained between 15.32% (2013) and 17.97% (2015). In 2014, when seeds were scarce, lactating females had a shortfall in energy intake compared with 2013 and 2015. This corresponded with a reduction in the number of females giving birth (11 out of 23), but there was no interannual difference in survival rates. Compared to 2013 and 2015, in 2014 females had greater weight loss (drew on body reserves), moved less, and spent more time nursing their offspring. We discuss implications of these results for range limitation in Taihangshan macaques.

Age at reproductive debut: Developmental predictors and consequences for lactation, infant mass, and subsequent reproduction in rhesus macaques (Macaca mulatta)

OBJECTIVES

The age at which females initiate their reproductive career is a critical life-history parameter with potential consequences on their residual reproductive value and lifetime fitness. The age at reproductive debut may be intimately tied to the somatic capacity of the mother to rear her young, but relatively little is known about the influence of age of first birth on milk synthesis within a broader framework of reproductive scheduling, infant outcomes, and other life-history tradeoffs.

MATERIAL AND METHODS

Our study investigated the predictors of age at first reproduction among 108 captive rhesus macaque (Macaca mulatta) females, and associations with their milk synthesis at peak lactation, infant mass, and ability to subsequently conceive and reproduce.

RESULTS

The majority of females reproduced in their fourth year (typical breeders); far fewer initiated their reproductive career one year earlier or one year later (respectively early and late breeders). Early breeders (3-year-old) benefited from highly favorable early life development (better juvenile growth, high dominance rank) to accelerate reproduction, but were impaired in milk synthesis due to lower somatic resources and their own continued growth. Comparatively, late breeders suffered from poor developmental conditions, only partially compensated by their delayed reproduction, and evinced compromised milk synthesis. Typical breeders not only produced higher available milk energy but also had best reproductive performance during the breeding and birth seasons following primiparity.

DISCUSSION

Here, we refine and extend our understanding of how life-history tradeoffs manifest in the magnitude, sources, and consequences of variation in age of reproductive debut. These findings provide insight into primate reproductive flexibility in the context of constraints and opportunities.

Alloparenting is associated with reduced maternal lactation effort and faster weaning in wild chimpanzees

Alloparenting, when individuals other than the mother assist with infant care, can vary between and within populations and has potential fitness costs and benefits for individuals involved. We investigated the effects of alloparenting on the speed with which infants were weaned, a potential component of maternal fitness because of how it can affect inter-birth intervals, in wild chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Uganda. We also provide, to our knowledge, the first description of alloparenting in this population and present a novel measure of the contribution of milk to infant diets through faecal stable nitrogen isotopes (δ15N). Using 42 mother-infant pairs, we tested associations of two alloparenting dimensions, natal attraction (interest in infants) and infant handling (holding, carrying), to the proportion of time mothers spent feeding and to maternal lactation effort (mean nursing rates and mother-infant δ15N differences). Neither natal attraction nor infant handling was significantly associated with feeding time. Infant handling was inversely associated with both measures of lactation effort, although natal attraction showed no association. Alloparenting may benefit mothers by enabling females to invest in their next offspring sooner through accelerated weaning. Our findings emphasize the significance of alloparenting as a flexible component of female reproductive strategies in some species.

A novel fecal stable isotope approach to determine the timing of age-related feeding transitions in wild infant chimpanzees

OBJECTIVES

Determining nutritional development in wild primates is difficult through observations because confirming dietary intake is challenging. Physiological measures are needed to determine the relative contributions of maternal milk and other foods at different ages, and time of weaning. We used fecal stable carbon and nitrogen isotopes (δ¹³ C, δ¹⁵ N) and fecal nitrogen concentrations (%N) from wild chimpanzees at Ngogo, Uganda, to derive physiological dietary indicators during the transition from total reliance on maternal milk to adult foods after weaning.

MATERIALS AND METHODS

We analyzed 560 fecal samples collected non-invasively from 48 infants, their mothers, and 6 juvenile siblings. Most infant and juvenile samples (90%) were matched to samples collected from mothers on the same day. Isotopic assessments were compared with observations of nursing and feeding.

RESULTS

Infants ≤1 year old showed average δ¹⁵ N, δ¹³ C and %N ratios that were 2.0‰, 0.8‰ and 1.3% greater than their mothers, respectively, interpreted as trophic level effects. Although data collected on newborns were few, results suggest that solid foods were consumed within 2-5 months after birth. Trophic level differences decreased steadily after 1 year, which indicates a decreasing relative contribution of milk to the diet. Isotopic results indicated infants were weaned by 4.5 years old-more than a year earlier than observations of nipple contacts ceased, which revealed the occurrence of "comfort nursing." Juvenile isotopic signatures indicate no nursing overlap between siblings.

DISCUSSION

Our results resemble the stable isotope differences of human babies. This study contributes to a model of chimpanzee nutritional development required to understand early life history patterns in hominins.

Advances in primate stable isotope ecology-Achievements and future prospects

Stable isotope biogeochemistry has been used to investigate foraging ecology in non-human primates for nearly 30 years. Whereas early studies focused on diet, more recently, isotopic analysis has been used to address a diversity of ecological questions ranging from niche partitioning to nutritional status to variability in life history traits. With this increasing array of applications, stable isotope analysis stands to make major contributions to our understanding of primate behavior and biology. Most notably, isotopic data provide novel insights into primate feeding behaviors that may not otherwise be detectable. This special issue brings together some of the recent advances in this relatively new field. In this introduction to the special issue, we review the state of isotopic applications in primatology and its origins and describe some developing methodological issues, including techniques for analyzing different tissue types, statistical approaches, and isotopic baselines. We then discuss the future directions we envision for the field of primate isotope ecology. Am. J. Primatol. 78:995-1003, 2016. © 2015 Wiley Periodicals, Inc.