Fecal glucocorticoids as indicators of metabolic stress in female Sykes' monkeys (Cercopithecus mitis albogularis)

Because of their mediating role in the stress response and potential effects on fitness, glucocorticoid (GC) hormones are increasingly used to assess the physiological costs of environmental and behavioral variation among wild vertebrates. Identifying the proximate causes of GC variation, however, is complicated by simultaneous exposure to multiple potentially stressful stimuli. Here, we use data from a partially provisioned social group of Sykes' monkeys to evaluate the effects of potential psychological and metabolic stressors on temporal and individual variation in fecal GC (fGC) excretion among 11 adult females. Despite high rates of agonism over provisioned foods fGCs declined during periods of high provisioning frequency when fruit availability was dominated by neem (Azadirachta indica), an item requiring great feeding effort. Provisioned foods did not prevent fGC increases when availability of the most preferred main fruit item, tamarind (Tamarindus indica), declined drastically. Although rank-related differences in access to provisioned foods and rates of agonism did not lead to an overall effect of rank on fGCs, low-ranking females excreted more fGCs than high-ranking females during a period of high provisioning intensity and low fruit availability. The emergence of this rank effect was associated with elevated feeding effort in all females, a greater access to provisioned items by high-ranking females, and a higher proportion of time spent moving in low-ranking females. Our findings suggest that metabolic stressors were the primary determinants of both temporal and individual variation in fGCs, indicating potential fitness benefits for high-ranking females when food availability is limited.     

Seasonal Energetic Stress in a Tropical Forest Primate: Proximate Causes and Evolutionary Implications

Animals facing seasonal variation in food availability experience selective pressures that favor behavioral adjustments such as migration, changes in activity, or shifts in diet. Eclectic omnivores such as many primates can process low-quality fallback food when preferred food is unavailable. Such dietary flexibility, however, may be insufficient to eliminate constraints on reproduction even for species that live in relatively permissive environments, such as moist tropical forests. Focusing on a forest-dwelling primate with a flexible diet (Cercopithecus mitis) we investigated whether females experience seasonal energetic stress and how it may relate to reproductive seasonality. We used fecal glucocorticoids (fGCs) as an indicator of energetic stress, controlling for the potentially confounding effects of social interactions and reproductive state. We modeled within-female fGC variation with General Linear Mixed Models, evaluating changes in feeding behavior and food availability as main effects. Regardless of reproductive state, fGCs increased when females shifted their diet towards fallback foods (mature leaves and other non-preferred items) and when they spent more time feeding, while fGCs decreased with feeding time on preferred items (insects, fruits, young leaves) and with the availability of young leaves. Changes in fruit availability had no general effects on fGCs, likely because fruits were sought out regardless of availability. As predicted, females in the energetically demanding stages of late pregnancy and early lactation showed greater increases in fGCs between periods of low versus high availability of fruits and young leaves than females in other reproductive states. Potential social stressors had no measurable effects on fGCs. Preliminary evidence suggests that seasonal energetic stress may affect the timing of infant independence from mothers and contribute to unusually long inter-birth intervals compared to closely related species of similar body size. Our findings highlight how the study of stress responses can provide insights into the proximate control of reproductive strategies.     

Effects of reproductive and social variables on fecal glucocorticoid levels in a sample of adult male ring-tailed lemurs (Lemur catta) at the Beza Mahafaly Reserve, Madagascar

Glucocorticoids, a group of adrenal hormones, are secreted in response to stress. In male primates, variables such as breeding seasonality, dominance hierarchy stability, and aggressive and affiliative interactions can affect glucocorticoid levels. In this study, we examined interindividual differences in mean fecal glucocorticoid (fGC) levels among males in three groups of wild ring-tailed lemurs to better understand the physiological costs of group living for males in a female-dominant species that exhibits strict reproductive seasonality. Fecal and behavioral data samples were collected during one mating and two postmating seasons (2001 and 2003). The mean fGC levels were examined in relation to reproductive season, male rank, number of resident males, intermale and female-male agonism, and affiliative behavior with females. The mean fGC levels were not significantly elevated during mating season compared to the postmating period. During the mating season, male dominance hierarchies broke down and rank effects could not be tested; however, there was no relationship between male rank and fGC levels in the postmating periods. In 2001, males that resided in the group with the fewest males exhibited lower fGC levels during the postmating period. They also affiliated more with females than did males in the other groups. During the mating season of 2003, males engaged in more affiliative behaviors with females compared to the postmating season, but female-male agonism did not differ by season. However, rates of intermale agonism were significantly higher during mating compared to postmating periods, but such heightened agonism did not translate to a higher stress response. Thus, neither male-male competition for mates nor heightened agonism between males during the breeding season affected male fGC levels. Fewer males residing in a group, however, did have some effect on male-female affiliation and male fGC levels outside of the mating period. Males that live in a group with only a few (two or three) males may experience less physiological stress than those that live in groups with more males.     

Persistence of maternal effects in baboons: Mother's dominance rank at son's conception predicts stress hormone levels in subadult males

Dominance status and reproductive experience are maternal characteristics that affect offspring traits in diverse taxa, including some cercopithecine primates. Maternal effects of this sort are widespread and are sources of variability in offspring fitness. We tested the hypothesis that maternal dominance rank and reproductive experience as well as a male's own age and dominance rank predicted chronic fecal glucocorticoid (fGC) concentrations in 17 subadult wild male baboons, Papio cynocephalus (median age 6.5 years), in the Amboseli basin, Kenya. Among these variables, maternal dominance rank at a subadult male's conception was the sole significant predictor of the male's fGC and accounted for 42% of fGC variance; sons of lower ranking mothers had higher fGC than did those of high-ranking mothers. This result is striking because subadult male baboons are approximately 4-6 years past the period of infant dependence on their mothers, and are larger than and dominant to all adult females. In addition, many males of this age have survived their mothers' death. Consequently, the influence of maternal dominance rank persisted well beyond the stage at which direct maternal influence on sons is likely. Persistence of these major maternal influences from the perinatal period may signal organizational effects of mothers on sons' HPA axis. Although short-term, acute, elevations in GC are part of adaptive responses to challenges such as predators and other emergencies, chronically elevated GC are often associated with stress-related pathologies and, thereby, adverse effects on fitness components.     

Within-group female-female agonistic interactions in Taiwanese macaques (Macaca cyclopis)

Feeding-related agonism among wild female Taiwanese macaques was investigated in two study groups at ecologically diverse sites (Fu-shan and Ken-ting) to determine whether contest-feeding competition was present in these groups. Females that contest for food within a primate group are hypothesized to form dominance hierarchies and tend to be philopatric. In this study we tested 1) whether Taiwanese macaque females show higher agonism in a feeding context, 2) whether they exhibit stronger agonism over higher-quality foods, and 3) whether higher agonism rates occur in smaller food patches. Female Taiwanese macaques at both study sites showed similar agonism rates in a feeding context (0.30 events/hr). They exhibited higher agonism rates inside food patches than outside food patches in the spring. Higher agonism rates occurred during seasons of higher fruit availability, and a lower agonism rate occurred in winter when the macaques switched to feeding on fallback foods. Females in the Fu-shan group exhibited higher proportions of aggressive interactions over higher-quality foods, such as animal matter and the reproductive parts of plants. In the Ken-ting group, 95.8% of feeding-related agonistic interactions among females occurred over fruits. Agonistic interactions that occurred in small food patches tended to result in the agonism recipient leaving the food patch. We conclude that female Taiwanese macaques show contest feeding competition in certain contexts. The patterns we observed have also been documented in other primate species in which females are philopatric and form linear dominance hierarchies.     

The Association of Intergroup Encounters,Dominance Status,and Fecal Androgen and Glucocorticoid Profiles in Wild Male White‐Faced Capuchins (Cebus capucinus)

Androgens play a role in male reproductive competition, frequently via aggression, while glucocorticoids are associated with the stress response. However, the relationships of these hormones with different sources of competition (intra‐ vs. intergroup) and dominance status are highly variable. Here, we consider the fecal androgen (fA) and glucocorticoid (fGC) profiles of alpha and subordinate male Cebus capucinus in the context of intergroup competition during a rare period of low intragroup competition (i.e. all females were either pregnant or lactating). Intergroup encounters (IGEs) are a long‐term reproductive strategy in male white‐faced capuchins, enabling them to assess the composition of neighboring groups. IGEs pose a threat to resident males as these can result in injury or death, loss of dominance rank, group eviction, and group takeovers that are frequently associated with infanticide. From February to July 2007, fecal samples were collected from eight males in three groups of white‐faced capuchins in the Santa Rosa Sector of the Área de Conservación Guanacaste, Costa Rica. IGE rate was positively associated with both fA and fGC levels, indicating that IGEs are perceived as reproductive challenges by resident males, and may be associated with elevated metabolic costs. Alpha males sire the majority of group offspring and, accordingly, the threat of IGEs to both future (via rank loss or eviction) and current (via infanticide) reproductive success is greater than for subordinate males. Consistent with this observation, alpha males had higher fA and fGC levels than subordinate males. Given that all females were either pregnant or lactating and pronounced overt intragroup competition was absent, we interpret the difference in hormone profiles of alpha and subordinate males as being primarily associated with variation in the perceived threats of IGEs according to dominance status. Future studies should focus on the interaction of intra‐ and intergroup competition by examining hormone levels in the presence of periovulatory females. Am. J. Primatol. 75:107‐115, 2013. © 2012 Wiley Periodicals, Inc.     

Costs of and Investment in Mate-Guarding in Wild Long-Tailed Macaques (Macaca fascicularis): Influences of Female Characteristics and Male–Female Social Bonds

Male primates living in multimale groups tend to direct mate and mate-guarding choices toward females of high reproductive value, i.e., high-ranking, parous females, or females with which they share strong bonds. Little is known, however, about the constraints that may limit male mate-guarding choices (the costs of this behavior) and the influence of the females’ quality on male investment in mate-guarding. We aimed to study the effects of female rank, parity status, and male–female social bond strength on the costs of and investment in mate-guarding by males. We carried out our study during two reproductive seasons on three groups of wild long-tailed macaques in Indonesia. We combined behavioral observations on male locomotion and activity with noninvasive measurements of fecal glucocorticoids (fGC). Males spent less time feeding when mate-guarding nulliparous females than when mate-guarding parous females and tended to have higher fGC levels when mate-guarding low-ranking nulliparous females than when mate-guarding high-ranking nulliparous ones. Evolution should thus favor male choice for high-ranking parous females because such a decision brings benefits at proximate (reduced costs of mate-guarding) and ultimate (higher reproductive value) levels. Further, male investment in mate-guarding was flexible and contingent on female reproductive and social value. Males were more vigilant and more aggressive toward other males when mate-guarding females to which they were strongly bonded and/or high-ranking ones than when mate-guarding other females. Our findings bring a new dimension to the study of mate choice by showing that males not only mate preferentially with high-quality females but may also aim to secure paternity with these females through optimized monopolization.     

Male Response to Female Ovulation in White-Faced Capuchins (Cebus capucinus): Variation in Fecal Testosterone,Dihydrotestosterone, and Glucocorticoids

Reproductive success is linked to dominance in male primates, reflecting the benefits of male competition. However, not all males compete successfully, suggesting that the costs of obtaining and maintaining high dominance status are significant. Here we examine the fecal metabolites of testosterone (fT) and dihydrotestosterone (fDHT) as bioactive androgens reflecting male reproductive effort, as well as fecal glucocorticoid (fGC) excretion as an index of stress in male white-faced capuchins (Cebus capucinus). We investigated the influence of female fertility (periovulatory vs. nonovulatory) on the hormonal responses of alpha and subordinate males. Over a 17-mo field season, we collected and analyzed weekly fecal samples (N = 992) from all 14 adult (> 10 yr) and subadult (≥ 6–10 yr) males residing in three study groups in the Santa Rosa Sector of the Área de Conservación Guanacaste, Costa Rica. Fecal samples (N > 2250) were also collected from group females (N = 28) to identify the fertile period using progesterone and estradiol assays. Alpha males had significantly higher fT, fDHT, and fGC levels than subordinate males independent of female reproductive state; further, adult subordinates had significantly higher fT, but not fDHT or fGC, than subadult subordinates. Male fT, fDHT, and fGC levels were significantly higher in the presence of fertile females, regardless of male dominance status and age. These findings indicate that the higher reproductive effort of alpha males comes with some costs (increased fGCs), and the presence of periovulatory females is associated with specific endocrine responses reflecting male reproductive effort and stress in white-faced capuchins.     

Social dominance and female reproductive behaviour in rhesus monkeys (Macaca mulatta)

The relationship between dominance rank and female sexual behaviour was examined in rheusus monkeys (Macaca mulatta) living in a social group. High-ranking females engaged in copulatory series as frequently as lower-ranking females. Furthermore, lower-ranking females copulated with as many available males as did high-ranking females. Social rank did appear to influence the pattern of sexual activity exhibited, in that copulatory series were more often initiated by the higher-ranking animal of the mating pair. Copulatory series involving high-ranking females were characterized by more mounts by males and were longer in duration. Higher-ranking animals interfered more often with copulatory series involving other animals, but such interference was not effective in preventing completion of the series. These data indicate that any reproductive advantage conferred to high-ranking females is not the result of sexual competition in social-living rhesus monkeys.     

Determinants of dominance hierarchy in a captive group of pigtailed monkeys (Macaca nemestrina)

Eighteen pigtailed monkeys, all strangers to each other, were placed together in a laboratory compound. Two infants were eliminated from the study shortly after group formation. To determine dominance hierarchy aggressive-submissive interactions were observed among the remaining 16 monkeys during 4 periods covering 5 months of group development.Results suggested that 3 factors were basic determinants of dominance hierarchy: body weight for males, estrus for females, and maturity for both sexes.Aggressive-submissive interactions were far more frequent during Period I, the first hour of group formation, but decreased with stabilization of hierarchical order in subsequent periods.Aggressive-submissive interactions were not evenly distributed among all possible pairs in the group, but tended to involve mostly the high-ranking animals. Also, high-ranking animals concentrated their aggression towards more submissive monkeys of their own rank. Because low ranking animals were involved in fewer aggressive-submissive interactions, their rank determination was difficult.This research was supported by grant No. FR-00166 from the National Institutes of Health, U. S. A.     

Microhabitat Variation and Its Effects on Dietary Composition and Intragroup Feeding Interactions Between Adult Female <Emphasis Type="Italic">Lemur catta</Emphasis> During the Dry Season at Beza Mahafaly Special Reserve,Southwestern Madagascar

We compared diets and female feeding agonism in 2 groups of ring-tailed lemurs living in markedly diverse microhabitats in and near the Beza Mahafaly Reserve, Madagascar during mid-to-late gestation periods and height of the dry season to test predictions concerning differences in resource availability, preferred foods, female rank, and degree and frequency of feeding agonism in relation to usurpability and monopolization of food resources. Quadrat sampling in disturbed forest habitat revealed a greater number of plant species than in the gallery forest home range area, but females in both groups consumed nearly equal numbers of actual food plant species. Higher-ranking females in disturbed forest consumed human food scraps from the researchers camp significantly more often than low-ranking females did, while there was no rank effect for consumption of any food type between females in the gallery forest group. Higher rates of female feeding agonism in both groups occurred around usurpable fruit and leaves, as well as over monopolizable human food scraps in the disturbed forest group. There is no association between degree of agonism and food type in either group, and rate of feeding agonism is similar for both groups. The most highly contested food items came in large packages and were high in nutrients: beneficial foods for gestating females in the height of dry season in southwestern Madagascar.     

Competition among female long-tailed macaques,Macaca fascicularis

In species with permanent groups, the groups tend to move toward a size where competition for food regulates female fitness. Hence, one would expect females to use behavioural means to gain a reproductive advantage over other females in the group. Their competitive ability is likely to reflect dominance rank and age. Adult female Sumatran long-tailed macaques, Macaca fascicularis, in four different groups were studied. Females of a higher dominance rank had a total food intake equal to or higher than that of the lower-ranking females, acquired it at a lower energy cost and probably ate food of higher quality. The older and lower-ranking females avoided competition by more often moving away from the centre of the group. Mortality fell more heavily on females who were less frequently present in the main party. High-ranking females tended to produce more offspring surviving to 1-year, with top-ranking females tending to out-reproduce all others. It is concluded that safety monopolization was the predominant mode of competition among the females caused by the monopolization of clumped food within the main party by high-ranking females.     

Dominance and feeding competition in captive rhesus monkeys

The feeding behavior of 16 adult female rhesus monkeys living in three captive social groups was observed. Estimates of relative food intake, feeding rate, and location of feeding in relation to food sources were compared between females of different dominance ranks. Higher-ranking females had greater access to feeding sites and were supplanted or threatened less frequently while feeding than subordinates. However, no consistent differences in estimates of total intake were found between females of high and females of low rank. The effects of dominance on feeding behavior were most pronounced in the group receiving the least food relative to estimates of overall group nutritional requirements. Higher-ranking females, both over the long term and during the study period, tended to produce more surviving offspring. The effects of dominance on reproductive performance appeared to be less related to food intake than to competitive and aggressive interactions, potentially resulting in higher levels of stress for subordinates.     

Behavioural dominance and reproductive success in female Japanese monkeys (Macaca fuscata)

Eight years of reproductive data (including 248 births) from a translocated troop of Japanese monkeys (Macaca fuscata) living in a 42-ha enclosure provided three measures of female reproductive success: fecundity, survival of infants to 1 year of age, and age at first parturition. No significant relationship was found between social dominance and these measures. Social dominance was considered with respect to both matrilineal and individual female rank. Additional data on female dominance ranks over four generations of adult females revealed no significant concordance over time. The finding that ranks may not be stable over the lifetime of a female is a significant one because the variation in reproductive success among the females of a group is likely to be further diminished by any instability. For 34 females that were adults for the 8-year period considered, there was no significant correlation between the average rank of a female and either fecundity or survivorship of infants to 1 year of age. These data considered along with the results of other studies of female dominance and reproduction suggest that any effect of female social dominance on reproductive success is probably dependent upon resource availability, with significant benefits accruing to high-ranking individuals only during subsistence periods. It is suggested that dominance competition among female macaques may be a behavioural strategy with a variable payoff.     

The effects of dominance rank and group size on female lifetime reproductive success in wild long-tailed macaques,Macaca fascicularis

Demographic changes were recorded throughout a 12-year period for three social groups ofMacaca fascicularis in a natural population at Ketambe (Sumatra, Indonesia). We examined the prediction that females' lifetime reproductive success depended on dominance rank and group size. Average birth rate was 0.53 (184 infants born during 349 female years). For mature females (aged 8–20 yr) birth rate reflected physical condition, being higher in years with high food availability and lower in the year following the production of a surviving infant. High-ranking females were significantly more likely than low-ranking ones to give birth again when they did have a surviving offspring born the year before (0.50 vs 0.26), especially in years with relatively low food availability (0.37 vs 0.10). Controlled comparisons of groups at different sizes indicate a decline in birth rate with rroup size only once a group has exceeded a certain size. The dominance effect on birth rate tended to be strongest in large groups. Survival of infants was rank-dependent, but the survival of juveniles was not. There was a trend for offspring survival to be lower in large groups than in mid-sized or small groups. However, rank and group size interacted, in that rank effects on offspring survival were strongest in large groups. High-ranking females were less likely to die themselves during their top-reproductive years, and thus on average had longer reproductive careers. We estimated female lifetime reproductive success based on calculated age-specific birth rates and survival rates. The effects of rank and group size (contest and scramble) on birth rate, offspring survival, age of first reproduction for daughters, and length of reproductive career, while not each consistently statistically significant, added up to substantial effects on estimated lifetime reproductive success. The group size effects explain why large groups tend to split permanently. Since females are philopatric in this species, and daughters achieve dominance rank positions similar to their mother, a close correlation is suggested between the lifetime reproductive success of mothers and daughters. For sons, too, maternal dominance affected their reproductive success: high-born males were more likely to become top-dominant (in another group). These data support the idea that natural selection has favored the evolution of a nepotistic rank system in this species, even if the annual benefits of dominance are small.     

Age-related differences in social grooming among adult female Japanese monkeys ( Macaca fuscata)

The present study investigated the influence of dominance rank in combination with kinship on age-related differences in social grooming among adult females in a free-ranging group of Japanese monkeys (Macaca fuscata). Eighty-three adult females were divided into six sub-groups according to age-class (younger: 5–9 years old; middle: 10–14 years old; older: 15–22 years old) and dominance rank (high and low rank). The ratio of the number of unrelated females that each female groomed to the total number of available unrelated females and grooming bouts which she gave to unrelated females decreased with increasing age for both high- and low-ranking females, whereas age did not appear to affect corresponding values for related females. On the other hand, compared with low-ranking females, high-ranking females of all age-classes received grooming more often from a larger number of unrelated females. Moreover, older females of low rank received grooming less often from a smaller number of unrelated females than younger females of low rank. These results indicate that with increasing age females are more likely to concentrate on related females when they have grooming interactions with other females. This tendency seems to be more apparent for low-ranking females. Moreover, the present findings also indicate that older high-ranking females could maintain their social attractiveness as high as younger high-ranking females.     

Hormonal and behavioural correlates of male dominance and reproductive status in captive colonies of the naked mole-rat,Heterocephalus glaber.

Naked mole-rat colonies are societies with a high reproductive skew, breeding being restricted to one dominant female (the ''queen'') and 1-3 males. Other colony members of both sexes are reproductively suppressed. Experimental removal of breeding males allowed us to investigate the relationship between urinary testosterone and cortisol, dominance rank, and male reproductive status. Dominance rank was strongly correlated with body weight, age, and urinary testosterone titres in males. No relationship between urinary cortisol levels and male reproductive status or dominance was found. Breeding males were among the highest-ranking, heaviest and oldest males in their respective colonies, and were succeeded by other high-ranking, large, old colony males. In contrast to females, no evidence of competition over breeding status was observed among males. Male-male agonism was low both before and after removal of breeders and mate guarding was not observed. The lower reproductive skew for males compared with female skew or queen control over male reproduction may explain why males compete less strongly than females over breeding status after removal of same-sexed breeders.     

Reproductive rate, not dominance status, affects fecal glucocorticoid levels in breeding female meerkats

Glucocorticoid hormones (GCs) have been studied intensively to understand the associations between physiological stress and reproductive skew in animal societies. However, we have little appreciation of the range of either natural levels within and among individuals, or the associations among dominance status, reproductive rate and GCs levels during breeding. To address these shortcomings, we examined variation in fecal glucocorticoid metabolites (fGC) during breeding periods in free-ranging female meerkats (Suricata suricatta) over 11 years. The vast majority of variation in fGC levels was found within breeding events by the same female (~87%), with the remaining variation arising among breeding events and among females. Concentrations of fGC generally tripled as pregnancy progressed. However, females with a high reproductive rate, defined as those conceiving within a month following parturition (mean = 9 days postpartum), showed significant reductions in fGC in the final 2 weeks before parturition. Despite these reductions, females with a high reproductive rate had higher fGC levels at conception of the following litter than those breeding at a low rate. After controlling for the higher reproductive rate of dominants, we found no association between levels of fGC and either age or dominance status. Our results suggest that one should be cautious about interpreting associations between dominance status, reproductive skew and GCs levels, without knowledge of the natural variation in GCs levels within and among females.     

Ecological strategies of woolly monkeys (Lagothrix lagotricha) at Tinigua National Park,Colombia

Information on the use of space, activity patterns, diet, and social interactions were recorded for a group of woolly monkeys (Lagothrix lagotricha) during 13 months at Tinigua National Park, Macarena, Colombia. In this region, fruit abundance changes throughout the year with a peak during March–April (beginning of the rainy season) and less fruit during September-November (end of rainy season). Woolly monkeys spent most of their time in mature forest where fruit abundance is higher than in opendegraded or flooded forests. Changes in habitat used by monkeys were coupled with changes in fruit supply across vegetation types. On an annual basis, woolly monkeys spent 24% of point samples locomoting, 36% resting, 36% feeding, and 4% on other activities. However, these proportions varied across the year depending on fruit availability. Based on instantaneous samples, the diet consisted mostly of fruits (60%), arthropods (23%), vegetative parts and flowers (17%), and other items (1%). Non-lactating females and juveniles spent more time eating insects than adult males and lactating females; however, significant differences between classes were detected only during the period of fruit scarcity. These differences are probably due to the high extent to which non-lactating females and juveniles were excluded from fruiting trees by males. The high proportion of arthropods in their diet is unusual for primates with large body size and is a possible factor influencing group cohesiveness in woolly monkeys. © 1994 Wiley-Liss, Inc.     

The feeding ecology and activity budget of proboscis monkeys

A group of proboscis monkeys (Nasalis larvatus) consisting of an alpha‐male, six adult females, and several immatures was observed from May 2005–2006. We collected over 1,968 hr of focal data on the adult male and 1,539 hr of focal data on the six females in a forest along the Menanggul River, Sabah, Malaysia. Availability and seasonal changes in plant species consumed by the focal monkeys were determined by vegetation surveys carried out across an area of 2.15 ha along 200–500 m trails in riverine forest. A total of 188 plant species were consumed by the focal monkeys. The activity budget of members of our study group was 76.5% resting, 19.5% feeding, and 3.5% moving. Young leaves (65.9%) and fruits (25.9%) accounted for the majority of feeding time. Over 90% of fruit feeding involved the consumption of unripe fruits and in the majority of case both the fruit flesh and seeds were eaten. Although fruit eating was rare in some months, during other times of the year time fruit feeding exceeded the time devoted to young leaves. We found that monthly fruit availability was positively related to monthly fruit eating and feeding activity, and seasonal fluctuations in dietary diversity were significantly affected by fruit eating. These results suggest that fruit availability and fruit‐eating behaviors are key factors that influence the activity budget of proboscis monkeys. Earlier assumptions that colobine monkeys are obligate folivores do not apply well to proboscis monkeys and certain other colobines. Our findings may help contribute to a better understanding of the dietary adaptations and feeding ecology of Asian colobines. Am. J. Primatol. 71:478–492, 2009. © 2009 Wiley‐Liss, Inc.