Costs and benefits of allomaternal care to mothers and others in wild Phayre's leaf monkeys

OBJECTIVES

Allomaternal care (AMC) is suggested to be energetically beneficial to mothers and costly to allomothers. However, among primates, AMC is a heterogeneous phenomenon and its implications are less clear especially in female dispersal species. Here, we investigated infant care in a female dispersal species, Phayre's leaf monkeys (Trachypithecus phayrei crepusculus), to evaluate whether mothers were constrained by infant care and benefitted energetically from AMC, whether AMC was energetically costly for allomothers and how maternal experience was associated with AMC.

MATERIALS AND METHODS

Data were collected via instantaneous focal animal sampling between 2004 and 2005 for juvenile and adult females (N = 18) from two groups at the Phu Khieo Wildlife Sanctuary, Thailand (440 h). We used generalized linear mixed models to determine how infant care during the first month after birth affected the time mothers and allomothers spent feeding, socializing, resting, and locomoting and how AMC varied.

RESULTS

In the first month, infants spent 26% of their time with an allomother. We found no differences in mothers' overall activity before versus after birth, although mothers fed significantly more and rested less when without their infant. Allomothers fed and rested less when with an infant. AMC varied between 0.0% and 20.5%, with immature females being most active.

DISCUSSION

Mothers appear to benefit energetically from AMC such that their overall activity after birth remained unchanged. Costs and benefits for allomothers seem to be variable. Some very active immature females may be benefitting from learning-to-mother. The overall low cost of AMC may facilitate a reciprocal social network among unrelated females.

Independent fitness consequences of group size variation in Verreaux's sifakas

The costs and benefits of group living are also reflected in intraspecific variation in group size. Yet, little is known about general patterns of fitness consequences of this variation. We use demographic records collected over 25 years to determine how survival and reproductive success vary with group size in a Malagasy primate. We show that female reproductive rates of Verreaux's sifakas (Propithecus verreauxi) are not affected by total group size, but that they are supressed by the number of co-resident females, whereas mortality rates are significantly higher in larger groups. Neither annual rainfall nor the adult sex ratio have significant effects on birth and death rates. Hence, these sifakas enjoy the greatest net fitness benefits at small, and not the predicted intermediate group sizes. Thus, independent fitness proxies can vary independently as a function of group size as well as other factors, leading to deviations from optimal intermediate group sizes.

Female-biased birth sex ratio in a female dispersal primate suggests local resource competition

Group living may entail local resource competition (LRC) which can be reduced if the birth sex ratio (BSR) is biased towards members of the dispersing sex who leave the group and no longer compete locally with kin. In primates, the predicted relationship between dispersal and BSR is generally supported although data for female dispersal species are rare and primarily available from captivity. Here, we present BSR data for Phayre's leaf monkeys (Trachypithecus phayrei crepusculus) at the Phu Khieo Wildlife Sanctuary, Thailand (N = 104). In this population, nearly all natal females dispersed, while natal males stayed or formed new groups nearby. The slower reproductive rate in larger groups suggests that food can be a limiting resource. In accordance with LRC, significantly more females than males were born (BSR 0.404 males/all births) thus reducing future competition with kin. This bias was similar in 2-year-olds (no sex-differential mortality). It became stronger in adults, supporting our impression of particularly fierce competition among males. To better evaluate the importance of BSR, more studies should report sex ratios throughout the life span, and more data for female dispersal primates need to be collected, ideally for multiple groups of different sizes and for several years.

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.

Reproductive plasticity in response to the changing cluster size during the breeding period: a case study in a spider mite

Animals living in clusters should adjust their reproductive strategies to adapt to the social environment. Theories predict that the benefits of cluster living would outweigh the costs of competition. Yet, it is largely unknown how animals optimize their reproductive fitness in response to the changing social environment during their breeding period. We used Tetranychus ludeni Zacher, a haplodiploid spider mite, to investigate how the ovipositing females modified their life-history traits in response to the change of cluster size (i.e., aggregation and dispersal) with a consistent population density (1 ♀/cm2). We demonstrate that (1) after females were shifted from a large cluster (16 ♀♀) to small ones (1 ♀, 5 and 10 ♀♀), they laid fewer and larger eggs with a higher female-biased sex ratio; (2) after females were shifted from small clusters to a large one, they laid fewer and smaller eggs, also with a higher female-biased sex ratio, and (3) increasing egg size significantly increased offspring sex ratio (% daughters), but did not increase immature survival. The results suggest that (1) females fertilize more larger eggs laid in a small population but lower the fertilization threshold and fertilize smaller eggs in a larger population, and (2) the reproductive adjustments in terms of egg number and size may contribute more to minimize the mate competition among sons but not to increase the number of inhabitants in the next generation. The current study provides evidence that spider mites can manipulate their reproductive output and adjust offspring sex ratio in response to dynamic social environments.

Integrated approaches to howler monkey (Alouatta spp.) medicine in professional care and conservation

Howler monkeys (Alouatta spp.) are threatened by anthropogenic pressures such as habitat fragmentation and deforestation, while conservation efforts are challenging to coordinate as natural geographic distribution ranges are the largest of any New World primate. On a One Health front, howler monkeys represent a great model to investigate the infectious disease dynamics between wild primates and humans as several infectious diseases affecting howlers have a demonstrated zoonotic potential. Howler monkey populations in professional care offer a window to investigate susceptibility to diseases in this species such as yellow fever (YF) and malaria, plus a myriad of endoparasite phyla, as well as vector-borne diseases such as Chagas disease and leishmaniasis. More studies are urgently needed to provide species-specific, medically relevant information as well as clinical descriptions of animals considered medically healthy. Moreover, howler monkeys are a challenging species to breed and maintain in professional care; additionally, reproductive parameters have been published only for a handful of species in this genus. On a One Health approach communication and collaborative health surveillance involving wildlife and zoo experts will ease the identification of factors that contribute to disease emergence facilitating the integration of human, animal, and environmental health. The One Welfare concept is based on the inextricable connection among animal welfare, human welfare, and environmental conservation. Integrating One Health and One Welfare into actions both in-situ and ex-situ will promote the sustainability of the forests and restoration of the ecosystems that those species inhabit, transitioning to a comprehensive One Conservation approach.

Social complexity as a driving force of gut microbiota exchange among conspecific hosts in non-human primates

The emergent concept of the social microbiome implies a view of a highly connected biological world, in which microbial interchange across organisms may be influenced by social and ecological connections occurring at different levels of biological organization. We explore this idea reviewing evidence of whether increasing social complexity in primate societies is associated with both higher diversity and greater similarity in the composition of the gut microbiota. By proposing a series of predictions regarding such relationship, we evaluate the existence of a link between gut microbiota and primate social behavior. Overall, we find that enough empirical evidence already supports these predictions. Nonetheless, we conclude that studies with the necessary, sufficient, explicit, and available evidence are still scarce. Therefore, we reflect on the benefit of founding future analyses on the utility of social complexity as a theoretical framework.

Social drivers of maturation age in female geladas

Female reproductive maturation is a critical life-history milestone, initiating an individual's reproductive career. Studies in social mammals have often focused on how variables related to nutrition influence maturation age in females. However, parallel investigations have identified conspicuous male-mediated effects in which female maturation is sensitive to the presence and relatedness of males. Here, we evaluated whether the more "classic" socioecological variables (i.e., maternal rank, group size) predict maturation age in wild geladas-a primate species with known male-mediated effects on maturation and a grassy diet that is not expected to generate intense female competition. Females delayed maturation in the presence of their fathers and quickly matured when unrelated, dominant males arrived. Controlling for these male effects, however, higher-ranking daughters matured at earlier ages than lower-ranking daughters, suggesting an effect of within-group contest competition. However, contrary to predictions related to within-group scramble competition, females matured earliest in larger groups. We attribute this result to either: 1) a shift to "faster" development in response to the high infant mortality risk posed by larger groups; or 2) accelerated maturation triggered by brief, unobserved male visits. While earlier ages at maturation were indeed associated with earlier ages at first birth, these benefits were occasionally offset by male takeovers, which can delay successful reproduction via spontaneous abortion. In sum, rank-related effects on reproduction can still occur even when socioecological theory would predict otherwise, and males (and the risks they pose) may prompt female maturation even outside of successful takeovers.

A milk-sharing economy allows placental mammals to overcome their metabolic limits

Here, we demonstrate that a naturally evolving behavior (allonursing) has greater effect on reproductive power (mass per unit of time) and output (litter mass at birth) than does artificial selection (domestication). Additionally, we demonstrate the importance of resource optimization afforded by sociality (rather than resource abundance per se) in shaping a species’ life history profile and its ability to overcome its own physiological constraints.

Maternal resource availability and metabolism have a strong limiting effect on reproductive output. Allomaternal care and domestication increase the energy available to the mother and should correlate with an increase in reproductive output. Here, we take a comparative approach to understand how this increase is accomplished (e.g., litter mass, reproductive frequency, etc.) and the strength of the effect among different forms of external energetic supplementation. We find that domestication and all forms of allocare correlate with increased fertility. All forms of provisioning correlate with larger litters without compromising offspring size. The greatest increase we observe in reproductive power is in species that practice allonursing. Our results suggest that the ultimate factor limiting reproductive output in placental mammals is maternal metabolic power rather than resource availability.

Contribution of adult sex ratio to trauma and reproductive output in large breeding groups of rhesus macaques (Macaca mulatta)

Maintaining stable breeding groups of rhesus macaques (Macaca mulatta) can be challenging due to the complex social dynamics and despotic nature of the species. Trauma from aggression is a common problem in rhesus colonies and can cause social disruption, strain veterinary and animal management resources, and potentially affect reproduction. Previous research has shown that increasing the number of non-natal adult males in a breeding group can improve group stability, reduce trauma, and increase reproduction. Here, we used mixed-effects regression models to examine the effects of sex ratio and other factors on trauma and reproduction at the Yerkes National Primate Research Center using a historical dataset made up of four large rhesus groups over an eleven-year period (2003–2013). As expected, sex ratio was a significant predictor for both trauma and reproduction. However, group age since formation was a stronger predictor of trauma frequency and the amount of space available was a slightly better predictor of reproduction than sex ratio or trauma. These results indicate that improving sex ratios can be a viable management strategy to reduce trauma and improve reproduction, particularly when it is difficult to manipulate the group compositions and/or their housing situations. Reducing trauma is a primary goal for rhesus breeding colonies, as it directly impacts the monkeys' health and psychological well-being. Such improvements are necessary for the ethical treatment and care of the animals themselves, but also to reduce financial burdens and maintain a healthy colony for research purposes.

The Infertility Trap: The Fertility Costs of Group-Living in Mammalian Social Evolution

Mammal social groups vary considerably in size from single individuals to very large herds. In some taxa, these groups are extremely stable, with at least some individuals being members of the same group throughout their lives; in other taxa, groups are unstable, with membership changing by the day. We argue that this variability in grouping patterns reflects a tradeoff between group size as a solution to environmental demands and the costs created by stress-induced infertility (creating an infertility trap). These costs are so steep that, all else equal, they will limit group size in mammals to ∼15 individuals. A species will only be able to live in larger groups if it evolves strategies that mitigate these costs. We suggest that mammals have opted for one of two solutions. One option (fission-fusion herding) is low cost but high risk; the other (bonded social groups) is risk-averse, but costly in terms of cognitive requirements.

Determinants of Harem Size in a Polygynous Primate: Reproductive Success and Social Benefits

We used long-term data on the variation in harem size in Yunnan snub-nosed monkeys to research the effects of harem size on reproductive success and the ratio of grooming received to given (RGRG). The results suggest that harem holders derive reproductive benefits commensurate with harem size, whereas the females' reproductive success is unaffected by harem size. Males of larger harems groomed less and had higher RGRG than males of smaller harems. In the case of females, grooming given increased, and RGRG decreased with an increase in harem size. The males' reproductive success seems to be a driver of harem size maximization. From the females' perspective, dwindling social benefits appear to set the upper limit for harem enlargement. We also showed that males of monogamous units ('single-female harems') invested more into grooming their female, presumably to prevent unit disintegration and loss of mating privileges.

The Goldilocks effect: female geladas in mid-sized groups have higher fitness

The cost-benefit ratio of group living is thought to vary with group size: individuals in 'optimally sized' groups should have higher fitness than individuals in groups that are either too large or too small. However, the relationship between group size and individual fitness has been difficult to establish for long-lived species where the number of groups studied is typically quite low. Here, we present evidence for optimal group size that maximizes female fitness in a population of geladas (Theropithecus gelada). Drawing on 14 years of demographic data, we found that females in small groups experienced the highest death rates, while females in mid-sized groups exhibited the highest reproductive performance. This group size effect on female reproductive performance was largely explained by variation in infant mortality (and, in particular, by infanticide from immigrant males) but not by variation in reproductive rates. Taken together, females in mid-sized groups are projected to attain optimal fitness due to conspecific infanticide and, potentially, predation. Our findings provide insight into how and why group size shapes fitness in long-lived species.

Developmental transitions in body color in chacma baboon infants: Implications to estimate age and developmental pace

OBJECTIVES

In many primates, one of the most noticeable morphological developmental traits is the transition from natal fur and skin color to adult coloration. Studying the chronology and average age at such color transitions can be an easy and noninvasive method to (a) estimate the age of infants whose dates of birth were not observed, and (b) detect interindividual differences in the pace of development for infants with known birth dates.

MATERIALS AND METHODS

Using a combination of photographs and field observations from 73 infant chacma baboons (Papio ursinus) of known ages, we (a) scored the skin color of six different body parts from pink to gray, as well as the color of the fur from black to gray; (b) validated our method of age estimation using photographic and field observations on an independent subset of 22 infants with known date of birth; and (c) investigated ecological, social, and individual determinants of age-related variation in skin and fur color.

RESULTS

Our results show that transitions in skin color can be used to age infant chacma baboons less than 7 months old with accuracy (median number of days between actual and estimated age = 10, range = 0-86). We also reveal that food availability during the mother's pregnancy, but not during lactation, affects infant color-for-age and therefore acts as a predictor of developmental pace.

DISCUSSION

This study highlights the potential of monitoring within- and between-infant variation in color to estimate age when age is unknown, and developmental pace when age is known.

Female dispersal patterns influenced by male tenure duration and group size in western lowland gorillas

Abstract

Evaluating the factors influencing the patterns of female dispersal in mammals is critical to understanding its importance for male and female reproductive strategies and the evolution of social systems. In western lowland gorillas, females emigrate from their natal group (natal dispersal) but also disperse between groups multiple times in their life (secondary dispersal). This strategy is uncommon in primates and may reflect females seeking to reduce feeding competition or female choice for males that can provide protection against infanticide. In this study, we test how inbreeding avoidance, group size, and male competitive ability could influence female dispersal in western gorillas, using 25 natal and 285 secondary dispersal events collected over 20 years at Mbeli Bai. We found that while all females dispersed out of their natal group, presumably to avoid inbreeding, females also left their group before having their first offspring when the dominant male was not their father, pointing towards unconditional dispersal by nulliparous females. Regarding secondary dispersal, we found that females were more likely to stay with males that were at the beginning of their tenure than transfer and more likely to disperse away from (presumably old) males that were near the end of their tenure or closer to their death. Females were also more likely to leave larger groups for smaller ones suggesting a potential effect of within-group feeding competition or avoidance of outsider males or predators. This study highlights the intersexual conflict found in gorillas: while female choice for high-quality males may influence the formation of their relatively rare social system, smaller group size may be more important for females than previously thought, which runs counter to the males’ interest of having a high number of females.

Significance

Dispersal is a very important life strategy for most group-living mammals. Secondary dispersal by females is a rare behavior but is believed to be a counterstrategy against sexual coercion by males, reduces feeding competition, assists in predator avoidance, and facilitates mate choice. We studied the factors influencing secondary dispersal in western lowland gorillas. We found that the duration of male tenure and the size of the group influences female transfer decisions. This study shows that female reproductive strategies such as secondary dispersal may be very powerful to counter the risk of infanticide through female choice for better protector males but also to reduce the impact of feeding competition or to avoid predators or outsider males.

Influence of habitat conditions on group size, social organization, and birth pattern of golden langur (Trachypithecus geei)

We studied endangered golden langurs in fragmented and altered habitats to understand the consequences of habitat conditions on group size, social organization, and birth seasonality. We selected 12 groups inhabiting forest edge and forest core of Chakrashila Wildlife Sanctuary (henceforth Chakrashila WLS) and adjoining the Abhaya rubber plantation. Each group was monitored every month from May 2013 to September 2016 and recorded the age-sex of individuals in the group. The births were recorded with the individual identity of females in five focal groups. The overall group size of golden langur was 11.3 ± 3.5_SD, and ranged between 5 and 18. The mean group size in forest core, forest edge, and rubber plantation differed significantly. We recorded a total of 46 births in 12 groups across the three different habitats. The number of infants correlates positively with adult females and group size across all the 12 groups for all the years. The number of births that occurred in all the months varied significantly across the months. Births occurred in all the months but peaked between May and September (82.6%). The mean number of births positively correlated with mean monthly rainfall. Mean inter-birth interval was 24.5 ± 1.6_SD months that did not vary between the females. It therefore appears that group size is sensitive to forest type, and births are positively related to social and environmental factors. The behavioral parameters may influence life-history traits if continuous habitat alteration persists.

Fertility as a constraint on group size in African great Apes

Gorillas and chimpanzees live in social groups of very different size and structure. Here I test the hypothesis that this difference might reflect the way fertility maps onto group demography as it does in other Catarrhines. For both genera, birth rates and the number of surviving offspring per female are quadratic (or ∩-shaped) functions of the number of adult females in the group, and this is independent of environmental effects. The rate at which fertility declines ultimately imposes a constraint on the size of social groups that can be maintained in both taxa. The differences in group size between the two genera seem to reflect a contrast in the way females buffer themselves against this cost. Gorillas do this by using males as bodyguards, whereas chimpanzees exploit fission–fusion sociality to do so. The latter allows chimpanzees to live in much larger groups without paying a fertility cost (albeit at a cognitive cost).

Trade-off between fertility and predation risk drives a geometric sequence in the pattern of group sizes in baboons

Group-living offers both benefits (protection against predators, access to resources) and costs (increased ecological competition, the impact of group size on fertility). Here, we use cluster analysis to detect natural patternings in a comprehensive sample of baboon groups, and identify a geometric sequence with peaks at approximately 20, 40, 80 and 160. We suggest (i) that these form a set of demographic oscillators that set habitat-specific limits to group size and (ii) that the oscillator arises from a trade-off between female fertility and predation risk.

Influence of the number of calves and lactating adult females in a herd on the adrenocortical activity of free-ranging Asian elephants

Abstract ContextPhysiological stress has the potential to influence animal population persistence. The endangered Asian elephant (Elephas maximus) is involved in intense conflict with humans in many parts of its range, which likely leads to stress for individuals and groups, with consequences for population survival. Thus, it is important to understand how the elephants’ stress levels are influenced by socio-ecological factors when not directly exposed to human-induced threats, and to use this understanding to improve conservation and management strategies. AimsThe present study was designed to provide baseline information on the link between socio-ecological factors and stress levels of undisturbed populations of elephants. The main aim was to determine the influence of a number of factors – herd size, season, number of calves and adult females present in a herd and their lactational status and body condition – on the adrenocortical activity of free-ranging adult female Asian elephants living in protected forests (without any direct exposure to human-induced threats), by measuring their faecal glucocorticoid metabolite (fGCM) levels. MethodsA total of 145 fresh faecal samples were collected from 123 identified adult female elephants inhabiting Bandipur and Nagarahole National Parks of southern India, between the years 2013 and 2015. fGCM levels were measured by employing a group-specific standardised 11-oxoetiocholanolone enzyme immunoassay (EIA). A generalised linear mixed-effects model (GLMM) was used to assess the influence of socio-ecological factors on fGCM levels of adult female elephants. Key resultsWhen fGCM levels were analysed with a GLMM, the following patterns were observed: fGCM levels were negatively correlated with the number of adult females (herd size) and positively correlated with the number of calves in a herd and active lactational status of an adult female. fGCM levels of adult female elephants were higher during the dry season (February to May) than wet season (August to December) and negatively correlated with body condition scores. ConclusionsAdrenocortical activity of female elephants is significantly influenced by the number of calves and adult females present in the herd, seasonality and lactational status. ImplicationsIt is important to consider the influence of multiple ecological and social correlates when assessing and interpreting the adrenocortical activity of Asian elephants. Our findings highlight the importance of maintaining the social structure of elephants in the wild to avoid detrimental effects on their physiological health. Insights from such assessments could be used to evaluate the stress in elephants that are involved in direct conflicts with humans to take appropriate management decisions for mitigating conflicts.

The Anatomy of Friendship

Friendship is the single most important factor influencing our health, well-being, and happiness. Creating and maintaining friendships is, however, extremely costly, in terms of both the time that has to be invested and the cognitive mechanisms that underpin them. Nonetheless, personal social networks exhibit many constancies, notably in their size and their hierarchical structuring. Understanding the processes that give rise to these patterns and their evolutionary origins requires a multidisciplinary approach that combines social and neuropsychology as well as evolutionary biology.

Births during 7 years after the translocation of a pair of black-and-gold howler monkeys (Alouatta caraya) to a forest fragment in southeast Brazil

Fecundity in female primates is influenced by the nutritional condition. If when translocated howler monkeys exhibit the same breeding patterns as non-translocated members of the same genus, it is an indication that the translocated monkeys have become well adapted to their release site and that they are likely in good nutritional condition. The objective of this study was therefore to investigate this pattern by recording copulations (over 5 years) and births (over 7 years) after the translocation of a pair of black-and-gold howler monkeys (Alouatta caraya) and to evaluate their gestation period, seasonality of births, and intervals between births. The pair was released in November 2009 on the campus of the University of São Paulo in Ribeirão Preto, São Paulo State, Brazil. Data on copulations were collected from January 2010 to March 2011 and from January 2012 to December 2014. Births were collected from January 2010 to December 2016. During the 5-year observation period, 25 copulations were recorded. Seven births were recorded over a period of 7 years, which included reproduction of the offspring of the translocated pair. Births occurred in the dry season between April and August. The interval between births was approximately 1 year. Our data provide insight into the reproduction of howler monkeys that have been translocated to a new habitat. Translocation can provide a valuable approach for rescuing or restoring Alouatta, whose populations have been detrimentally impacted by long-term habitat fragmentation.

Effects of group size and rank on mother-infant relationships and reproductive success in rhesus macaques (Macaca mulatta)

In the present study, we compared mother-infant relationships in 57 mother-infant dyads residing in two wild, semi-provisioned (22 mother-infant dyads in 2014, 35 dyads in 2015) groups of rhesus macaques (Macaca mulatta) in the Nanwan Nature Reserve for Rhesus Macaques, Hainan, China. We also compared reproductive success between these two groups. The ecology and provisioning regime for each group was similar. These groups differed however, in size. Group 1 contained ∼35 individuals and Group 2 contained ∼120 individuals. Data were collected over a 2-year period (2014-2015). We found that during the birth season, mother-infant relationships in the larger group were characterized by less time in contact and more time separated than in the smaller group. Mothers in the smaller group initiated more contact and proximity with their infants. During the mating season when infants were approximately 6 months old, mother-infant relationships in the smaller group were more rejecting. We also found that birth rates were significantly higher but infant survivorship (to 1-year-old) was significantly lower in the smaller group. Moreover, higher-ranking mothers in the larger group were characterized by a higher reproductive output than females of lower rank. In the smaller group, female rank did not affect reproductive output. There was, however, no clear relationship between infant survivorship and maternal rank. We hypothesize that differences in reproductive success and changes in mother-infant relationships between the birth and mating seasons reflect differences in the costs and benefits of group size: females in the larger group faced (i) higher intragroup feeding competition leading to a reduction in birth rate but also (ii) lower predation risk, resulting in higher infant survival rate compared to females in the smaller group. The results of this study point to the tradeoffs that primate females face in living in smaller and larger social groups.

Social structure as a strategy to mitigate the costs of group living: a comparison of gelada and guereza monkeys

In mammals, and especially primates, group size and social complexity are typically correlated. However, we have no general explanation why this is so. I suggest that the answer may lie in one of the costs of group living: mammalian reproductive endocrinology is extremely sensitive to stress, and forms one of the hidden costs of living in groups. Fertility declines with group size widely across the social mammals, including primates, and will ultimately place a constraint on group size. However, some species seem to have been able to mitigate this cost by forming bonded relationships that reduce the impact of experienced aggression, even if rates of aggression remain high. The downside is that they reduce network connectivity and hence risk fragmenting the group by providing fracture lines for group fission. To explore this, I compare network indices and fertility patterns across the same range of group sizes for two species of Old World monkeys, Colobus guereza and Theropithecus gelada: the former relatively unsocial, the latter intensely social with frequent use of grooming-based alliances. Compared to those of the guereza, gelada social networks lose density more slowly, maintain connectedness more effectively and are less likely to fragment as they increase in size. Although fertility declines with group size in both species, in gelada the impact of this effect is deferred to larger group sizes. The differences in fertility and network structure both predict the very different maximum group sizes typical of these two species, as well as the typical sizes at which their groups undergo fission. This finding may explain aspects of wider mammalian sociality.

Risk perception of vervet monkeys Chlorocebus pygerythrus to humans in urban and rural environments

Like other animals, primates respond to predation using behavioural adaptations. Hence, they should optimise their escape strategy under the risk of predation, and flee at a distance, referred to as flight initiation distance (FID), when the fitness-related benefits of staying are balanced against the costs of escape. However, there is an absence of FID studies in primates. In this study, we used vervet monkeys Chlorocebus pygerythrus, a medium-sized African cercopithecoid, as a model species to investigate the influence of environment type (urban and rural), group size (defined as the number of visible neighbours), sex and age on FID when approached by a human. We found significantly shorter FID among urban than rural monkeys; urban individuals delayed their escape compared to rural individuals. We found no relationship between FID and sex and age class, but FID was positively correlated with group size in both settings; urban monkeys live in smaller groups than monkeys in rural areas. As FID and group size are important predictors of predation risk perception in primates, results suggest that, despite probable effects of habituation, vervet monkeys in Uganda adjust their antipredator behaviour when coping with novel environments within human settlements. Our findings are consistent with some previous studies of risk perception in animals, and indicate that FID could be used as an alternative measure for predation risk in primates.

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.

Growing up tough: Comparing the effects of food toughness on juvenile feeding in Sapajus libidinosus and Trachypithecus phayrei crepusculus

Studies of primate feeding ontogeny provide equivocal support for reduced juvenile proficiency. When immatures exhibit decreased feeding competency, these differences are attributed to a spectrum of experience- and strength-related constraints and are often linked to qualitative assessments of food difficulty. However, few have investigated age-related differences in feeding ability relative to mechanical property variation across the diet, both within and among food types. In this study, we combined dietary toughness and feeding behavior data collected in the wild from cross-sectional samples of two primate taxa, Sapajus libidinosus and Trachypithecus phayrei crepusculus, to test the prediction that small-bodied juveniles are less efficient at processing tough foods than adults. We defined feeding efficiency as the time spent to ingest and masticate one food item (item bout length) and quantified the toughness and size of foods processed during those feeding bouts. To make the datasets comparable, we limited the dataset to foods processed by more than one age class and opened without tools. The overall toughness of foods processed by both species overlapped considerably, and juveniles and adults in both taxa processed foods of comparable toughness. Feeding efficiency decreased in response to increasing food toughness in leaf monkeys and in response to food size in both taxa. Age was found to be a significant predictor of bout length in leaf monkeys, but not in bearded capuchins. Juvenile S. libidinosus processed smaller fruits than adults, suggesting they employ behavioral strategies to mitigate the effect of consuming large (and occasionally large and tough) foods. We suggest future intra- and interspecific research of juvenile feeding competency utilize intake rates scaled by food size and geometry, as well as by detailed measures of feeding time (e.g., ingestion vs. mastication), in addition to food mechanical properties to facilitate comparisons across diverse food types and feeding behaviors.

Hormonal correlates of life history characteristics in wild female Colobus vellerosus

Documenting primate life history characteristics is important because it provides information about traits that affect the timing and rate of reproduction in these long-lived species. This study describes the hormonal correlates of female reproductive events and quantifies for the first time key life history variables for Colobus vellerosus, using hormonal and observational data. This study also biologically validates that the reproductive events determined in the hormone profiles correspond to observed reproductive events for each female. We collected behavioural data on 18 females in our four study groups during 12 months (May 2012-2013) at the Boabeng-Fiema Monkey Sanctuary, using 10-min continuous focal and ad libitum sampling. We concurrently collected faecal samples (n = 1866) every 2-3 days from these 18 females (prepubescent n = 2, cycling n = 2, lactating n = 12, pregnant, n = 7, and post-reproductive n = 1) and extracted oestrogen (E2) and progesterone (P) metabolites in the field using solid-phase extraction cartridges. We created a hormone profile for each female by analyzing 1586 of our samples for E2 using radio-immuno assays, and P using enzyme-immunoassays at the Wisconsin National Primate Research Center. Mean ovarian cycle length was 24 days ± 1 (n = 2 cycles). Mean gestation length was 23 weeks (range = 21-25 weeks, n = 2 complete pregnancies). For females whose infants survived to nutritional independence, the mean inter-birth interval (IBI) was significantly longer than for females whose infants died prior to reaching nutritional independence (Mann-Whitney U Test; U = 14.5, p = 0.006; IBI surviving infants: 17.75 months, range = 8-20.75 months, n = 11 vs. IBI infant death: 11.89 months, range = 8-18.5 months, n = 9). The values for most life history traits reported in this study are similar to those documented in other similarly sized colobine species. Some values are on the lower end of the range for similarly sized colobines; C. vellerosus shows a cycle of 24 days and gestation length of 5.75 months vs. a range of 24-29 days for cycle length and 5.25-7.5 months for gestation length in other colobines. This may be due to C. vellerosus' smaller body size, or their limited access to higher quality food resources.

Group size, survival and surprisingly short lifespan in socially foraging bats

Background

The relationships between group size, survival, and longevity vary greatly among social species. Depending on demographic and ecological circumstances, there are both positive and negative effects of group size variation on individual survival and longevity. For socially foraging species in particular there may be an optimal group size that predicts maximum individual survival that is directly related to the potential for information transfer, social coordination, and costs of conspecific interference. Our aim was to investigate this central aspect of evolutionary ecology by focusing on a socially foraging bat, Molossus molossus. This species optimizes foraging success by eavesdropping on the echolocation calls of group members to locate ephemeral food patches. We expected to find the highest survival and longest lifespans in small groups as a consequence of a trade-off between benefits of information transfer on ephemeral resources and costs of conspecific interference.

Results

In a mark-recapture study of 14 mixed-sex M. molossus social groups in Gamboa, Panama, spanning several years we found the expected relatively small and intermediate, but stable groups, with a mean size of 9.6 ± 6.7 adults and juveniles. We estimated survival proxies using Cox proportional hazard models and multistate-mark recapture models generated with recapture data as well as automated monitoring of roost entrances in a subset of the groups. Median survival of females was very short with 1.8 years and a maximum estimated longevity of 5.6 years. Contrary to our expectations, we found no relationship between variation in group size and survival, a result similar to few other studies.

Conclusions

Strong selection towards small group size may result from psychoacoustic and cognitive constraints related to acoustic interference in social foraging and the complexity of coordinated flight. The short lifespans were unexpected and may result from life at the energetic edge due to a highly specialized diet. The absence of a relationship between group size and survival may reflect a similar but optimized survival within the selected range of group sizes. We expect the pattern of small group sizes will be consistent in future research on species dependent on social information transfer about ephemeral resources.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-016-0056-1) contains supplementary material, which is available to authorized users.

Understanding stable bi-female grouping in gibbons: feeding competition and reproductive success

INTRODUCTION

Species of the order Primates are highly gregarious with most species living in permanent heterosexual social groups. According to theory in socioecology maximum social group size is limited by rates of intra-group feeding competition and associated increases in travel costs. Unlike other hylobatids, which are predominantly pair living, cao vit gibbons (Nomascus nasutus), and two other species of crested gibbon (Nomascus spp.) living in northern seasonal forest, regularly exhibit larger bi-female groups. To better understand the ability of northern gibbons to live in stable bi-female groups, we examined food distribution, feeding competition and reproductive success over a period of six years in a small cao vit gibbon population at Bangliang, Guangxi, China.

RESULTS

In general, we found weak evidences for within-group contest or scramble competition in our two study groups, which we attribute to high spatial and temporal heterogeneity in the distribution of their important food species. Nevertheless, the larger of the two groups studied increased feeding time and group spread during lean periods, factors that may limit cao vit gibbon group size to a maximum of two breeding females. Relative to tropical pair-living gibbons, there is no evidence that cao vit gibbons travel farther or spend more time feeding, but they did consume more leaves and buds and less fruit and figs. Despite their highly folivorous diet, the average inter-birth interval is comparable to tropical gibbon populations, and the survival rate of infants and juveniles in our study groups is high.

CONCLUSION

Cao vit gibbons do not suffer obvious costs in terms of feeding competition and reproductive success by living in bi-female groups, but within-group feeding competition may determine the upper the limit of cao vit gibbon group size to a maximum of two breeding females. These findings contribute to a growing body of evidence that bi-female grouping can be a stable grouping pattern of gibbons in certain habitats and further emphasize the flexibility of gibbon social organization.

Gastrointestinal Parasites of Ecuadorian Mantled Howler Monkeys (Alouatta palliata aequatorialis) Based on Fecal Analysis

An analysis of gastrointestinal parasites of Ecuadorian mantled howler monkeys, Alouatta palliata aequatorialis, was conducted based on examination of fecal smears, flotations, and sedimentations. At least 1 type of parasite was detected in 97% of the 96 fecal samples screened across 19 howler monkey groups using these techniques. Samples averaged 3.6 parasite species per individual (±1.4 SD). Parasites included species representing genera of 2 apicomplexans: Cyclospora sp. (18% of individual samples) and Isospora sp. (3%); 6 other protozoa: Balantidium sp. (9%), Blastocystis sp. (60%), Chilomastix sp. (4%), Dientamoeba sp. (3%), Entamoeba species (56%), Iodamoeba sp. (5%); 4 nematodes: Enterobius sp. (3%), Capillaria sp. (78%), Strongyloides spp. (88%) which included 2 morphotypes, Trypanoxyuris sp. (12%); and the platyhelminth Controrchis sp. (15%). A statistically significant positive correlation was found between group size and each of 3 different estimators of parasite species richness adjusted for sampling effort (ICE: r(2) = 0.24, P = 0.05; Chao2: r(2) = 0.25, P = 0.05, and Jackknife: r(2) = 0.31, P = 0.03). Two significant associations between co-infecting parasites were identified. Based on the prevalence data, individuals infected with Balantidium sp. were more likely to also be infected with Isospora sp. (χ(2) = 6.02, P = 0.01), while individuals harboring Chilomastix sp. were less likely to have Capillaria sp. present (χ(2) = 4.03, P = 0.04).

Competition for food in a solitarily foraging folivorous primate (Lepilemur leucopus)?

Group-living folivorous primates can experience competition for food, and feeding competition has also been documented for solitarily foraging gummivorous and omnivorous primates. However, little is known about the types and consequences of feeding competition in solitary folivorous foragers. We conducted this study in the spiny forest of Berenty Reserve, southern Madagascar, to characterize the competitive regime of the nocturnal solitarily foraging white-footed sportive lemur (Lepilemur leucopus), a species that lives in dispersed pairs. We analyzed 1,213 hr of behavioral observations recorded simultaneously for the male and female of each of seven social units and recorded seasonal changes in food availability over a complete annual cycle. Lepilemur leucopus exhibited low selectivity in its dietary choice and mainly included the most abundant plant species in its diet. Contrary to our predictions, we did not find evidence for increased rates of contest (i.e., displacement from food trees) or scramble competition (i.e., shared use of food patches) during the lean season, neither within nor between social units. Instead, conflict rates were low throughout the year, and, during these observations, any feeding stress may have been more related to food quality than quantity. The resource defense hypotheses may not explain pair-living in this species as there was no indication that males defend food resources for their female pair-partners. The observed lack of feeding competition may indicate that a cryptic anti-predator strategy is a better predictor of spatial avoidance of pair-partners than conflict over food. While anti-predator benefits of crypsis may explain, at least partly, female-female avoidance, studies on the relationship between territory size/quality and reproductive success are required to understand whether feeding competition reduces the potential for female association in L. leucopus.

Males follow females during fissioning of a group of northern muriquis

Although well documented in matrilocal primate species, group fission is still a poorly known phenomenon among patrilocal primates. In this paper we describe in detail a group fission event in the population of northern muriquis at the Reserva Particular do Patrimônio Natural-Feliciano Miguel Abdala in Caratinga, Minas Gerais, Brazil, using Social Network Analyses (SNA). Data on association patterns were collected during systematic observations from May 2002 to September 2005, and analyzed for dry (from May to October) and rainy seasons (from November to April). The fission process started with subgroup formation in the rainy season 2002-2003, and was completed by the dry season of 2003. By the dry season 2003, the parent group (Jaó) had fissioned to form a second mixed-sex group (Nadir) while a subgroup of males (MU) moved between the parent group and the newly established group. Before the Jaó group fission started (dry season 2002) and during its initial phases (rainy season 2002-2003), females that ultimately composed the daughter group (Nadir) were the most peripheral in the association network. In the rainy season 2002-2003, the median monthly (N=6) operational sex ratio (OSR) of Jaó group was 2.81. However, once Jaó females initiated the fissioning process to establish the Nadir group, the OSR was more favorable to males in the Nadir group than in the Jaó group. Our results suggest that males followed the females to escape an unfavorable OSR in their natal group.