Causes of Intraspecific Aggression in Alouatta palliata mexicana: Evidence from Injuries, Demography, and Habitat

Mantled howler monkey males assess their rivals through formant spacing of long-distance calls

Formant frequency spacing of long-distance vocalizations is allometrically related to body size and could represent an honest signal of fighting potential. There is, however, only limited evidence that primates use formant spacing to assess the competitive potential of rivals during interactions with extragroup males, a risky context. We hypothesized that if formant spacing of long-distance calls is inversely related to the fighting potential of male mantled howler monkeys (Alouatta palliata), then males should: (1) be more likely and (2) faster to display vocal responses to calling rivals; (3) be more likely and (4) faster to approach calling rivals; and have higher fecal (5) glucocorticoid and (6) testosterone metabolite concentrations in response to rivals calling at intermediate and high formant spacing than to those with low formant spacing. We studied the behavioral responses of 11 adult males to playback experiments of long-distance calls from unknown individuals with low (i.e., emulating large individuals), intermediate, and high (i.e., small individuals) formant spacing (n = 36 experiments). We assayed fecal glucocorticoid and testosterone metabolite concentrations (n = 174). Playbacks always elicited vocal responses, but males responded quicker to intermediate than to low formant spacing playbacks. Low formant spacing calls were less likely to elicit approaches whereas high formant spacing calls resulted in quicker approaches. Males showed stronger hormonal responses to low than to both intermediate and high formant spacing calls. It is possible that males do not escalate conflicts with rivals with low formant spacing calls if these are perceived as large, and against whom winning probabilities should decrease and confrontation costs increase; but are willing to escalate conflicts with rivals of high formant spacing. Formant spacing may therefore be an important signal for rival assessment in this species.

How it ends: A review of behavioral and psychological phenomena, physiological processes and neural circuits in the termination of aggression in other animals and anger in people

More is known about aggression initiation and persistence in other animals, and anger in people, than about their cessation. This review summarizes knowledge of relevant factors in aggression, mostly in vertebrates, and anger termination in people. The latency, probability and intensity of offensive aggression in mice is controlled by activity in a neuronal subpopulation in ventromedial hypothalamus [VMH]. This activity instantiates an aggressive state termed angriffsbereitschaft ["attack-readiness"]. Fighting in many species is broken into bouts with interbout breaks due to fatigue and/or signals from dorsal raphe to VMH. Eventually, losers decide durations and outcomes of fighting by transitioning to submission or flight. Factors reducing angriffsbereitschaft and triggering these defeat behaviors could include metabolic costs, e.g., lactate accumulation and glucose depletion detected by the hypothalamus, central fatigue perhaps sensed by the Salience Network [insula and anterior cingulate gyrus] and pain of injuries, the latter insufficiently blunted by opioid and non-opioid stress analgesia and transduced by anterior VMH neurons. Winners' angriffsbereitschaft continue for awhile, as indicated by post-victory attacks and, perhaps, triumph displays of some species, including humans. In longer term situations, sensory and/or response habituation of aggression may explain the "Dear enemy" tolerance of competitive neighbors. Prolonged satiation of predatory behavior could involve habenula-regulated reduction of dopaminergic reward in nucleus accumbens. Termination of human anger involves at least three processes, metaphorically termed decay, quenching and catharsis. Hypothesized neural mechanisms include anger diminution by negative feedback from accumbens to anterior cingulate and/or activity in the Salience Network that controls anger's "accumulation/offset" phase.

Behavioral responses of mantled howler monkeys to neighbor long-distance vocalizations

OBJECTIVES

Long-distance vocalizations are used by primates in a variety of contexts and may have different functions. The long-distance vocalizations of howler monkeys (Alouatta spp.) underlie the spatial regulation of neighboring groups and could be associated with the defense of food resources. Here, we test the hypothesis that the behavioral responses of mantled howler monkeys (A. palliata) to neighbor long-distance vocalizations are influenced by the potential for range defensibility while accounting for location within the home range and food availability.

METHODS

We studied two groups for 13 months and a total of 888 h at La Flor de Catemaco (Mexico). Group 1 had a 92-ha home range and Group 2 had a 24-ha home range. We recorded vocalizations (N = 178 calls) and movements (N = 74 movements) of focal groups following long-distance vocalizations produced by their neighbors.

RESULTS

Movement responses, but not vocal responses, were predicted by range defensibility, location, and food availability. As predicted, the group living in the smaller and more defendable range showed stronger movement responses than the group in the larger home range. These movement responses had a shorter latency and longer duration in more valuable spatial and temporal contexts (i.e., the core area and during periods of low food availability).

CONCLUSIONS

These results suggest that the tradeoff between the costs and benefits of range defense varies according to the interactions between home range size and both the spatial (core areas) and temporal (food availability) abundance of resources. Thus, the responses of mantled howler monkeys to neighbor long-distance vocalizations could be related to home range defensibility.

Adult play and the evolution of tolerant and cooperative societies

Play is generally considered an immature affair. However, adult play is present in several mammal species living in complex social systems. Here, I hypothesize that adult social play is favored by natural selection in those species characterized by high level of social tolerance and/or by the need of others' cooperation to reach a goal (i.e., leverage). The integration and comparison of bio-behavioral data on non-human primates and wild social carnivores allows drawing a comprehensive picture on the importance of adult play in facing unpredictable, novel social situations and in overcoming stressful experiences. The ability to cope with potentially competitive interactions through play can favor the emergence of egalitarian societies. A further interesting and beneficial aspect of adult play is its role in synchronizing group activities and favoring collective decision making by renovating the motivation to cooperate in groupmates. As a last step, some considerations about the presence of adult play in the most egalitarian and cooperative human groups (e.g., hunter-gatherer societies) allows discussing the apparent dichotomy between cultural and biological evolution of certain behavioral traits, including social play in adulthood.

Correlates of hormonal modulation in mantled howler monkey males, Alouatta palliata

OBJECTIVES

The study of hormone modulation may offer important insight into the responses of individuals to environmental challenges. Here we studied C-peptide, thyroid hormone (T3), glucocorticoid (GC), and testosterone (T) metabolites of mantled howler males to assess: 1) correlations among hormones; 2) individual and temporal variation in hormone concentrations; and 3) the influence of ecological, climatic, behavioral, social, and reproductive factors on hormone variation.

METHODS

We studied 10 adult males at La Flor de Catemaco (Mexico) from January 2012 to December 2016. We collected information on food availability; ambient temperature; time budgets; male involvement in mating, agonistic interactions, and interactions with extragroup males. We analyzed C-peptide concentrations in urine samples and T3, GC, and T in fecal samples.

RESULTS

C-peptide was negatively correlated with other hormones, whereas T3, GC, and T were positively related. Hormonal variation was unrelated to individual or yearly differences. Food availability was positively related to C-peptide and T3, and negatively related to GC. Involvement in mating was positively related to T3 and T, whereas the rate agonistic interactions was positively related to GC and T. The rate of interactions with extragroup males was positively related to T. When males mated, the increase in C-peptide and the decrease in GC with increasing food availability were less notable.

CONCLUSIONS

Hormonal variation in mantled howler monkey males is generally stable, but it is influenced by several factors. Our results offer a broad picture of the hormonal modulation of mantled howler monkey males in response to diverse challenges.

Vocal and movement responses of mantled howler monkeys (Alouatta palliata) to natural loud calls from neighbors

Loud calls (i.e., long-range acoustic signals) regulate resource competition among neighboring groups of conspecifics in several nonhuman primate species. Ultimate explanations for primate loud calls include mate, offspring, and food defense. Additionally, loud calls may provide valuable information pertaining to the identity and health status of callers, their competitive abilities, and their spatial location. The loud calls of howler monkeys (Alouatta) have been thoroughly studied and seem to play an important function in the defense of valuable resources in a variety of socioecological contexts. Here, we examined whether the behavioral responses of mantled howler monkeys (Alouatta palliata) to natural loud calls from neighbors are linked to three factors: food availability, familiarity, and distance between groups. We studied three groups of mantled howler monkeys at La Flor de Catemaco (Los Tuxtlas, Mexico) for 15 months (1817 observation hours), during which we recorded 236 neighbor loud calls. Food availability per se did not influence the behavior of groups receiving loud calls, although males produced longer vocal responses toward unfamiliar neighbors when food availability decreased. Groups vocalized quicker and both vocalized and moved for longer after loud calls from unfamiliar neighbors. Additionally, groups vocalized and moved for longer at shorter distances from unfamiliar neighbors compared with familiar neighbors. Finally, groups usually moved away from calling neighbors that were closer. These results indicate that the behavioral responses of mantled howler monkeys to loud calls from neighbor groups are associated with the integration of information pertaining to caller identity as well as to their ecological and spatial context.

Demographic population structure of black howler monkeys in fragmented and continuous forest in Chiapas, Mexico: Implications for conservation

For wild primates, demography studies are increasingly recognized as necessary for assessing the viability of vulnerable populations experiencing rapid environmental change. In particular, anthropogenic changes such as habitat loss and fragmentation can cause ecological and behavioral changes in small, isolated populations, which may, over time, alter population density and demographic structure (age/sex classes and group composition) in fragment populations relative to continuous forest populations. We compared our study population of Endangered black howler monkeys (Alouatta pigra) in 34 forest fragments around Palenque National Park (PNP), Mexico (62 groups, 407 individuals), to the adjacent population in PNP, protected primary forest (21 groups, 134 individuals), and to previous research on black howlers in fragments in our study area (18 groups, 115 individuals). We used χ² and Mann-Whitney U tests to address the questions: (a) what is the current black howler demographic population structure in unprotected forest fragments around PNP? (b) How does it compare to PNP's stable, continuous population? (c) How has it changed over time? Compared to the PNP population, the fragment populations showed higher density, a significantly lower proportion of multimale groups, and significantly fewer adult males per group. The population's age/sex structure in the fragmented landscape has been stable over the last 17 years, but differed in a higher proportion of multifemale groups, higher density, and higher patch occupancy in the present. In the context of conservation, some of our results may be positive as they indicate possible population growth over time. However, long-term scarcity of adult males in fragments and associated effects on population demographic structure might be cause for concern, in that it may affect gene flow and genetic diversity. The scarcity of adult males might stem from males experiencing increased mortality while dispersing in the fragmented landscape, whereas females might be becoming more philopatric in fragments.

A demographic history of a population of howler monkeys (Alouatta palliata) living in a fragmented landscape in Mexico

Long-term field studies are critical for our understanding of animal life history and the processes driving changes in demography. Here, we present long-term demographic data for the northernmost population of mantled howler monkeys (Alouatta palliata) residing in a highly anthropogenically fragmented landscape in Los Tuxtlas, Mexico. We carried out 454 monthly group visits to 10 groups of mantled howler monkeys between 2000 and 2011. The population remained relatively stable over the 11-year study period, with an overall increase in the total number of individuals. Birth rates and inter-birth intervals were comparable to those of howler monkeys at non-fragmented sites, suggesting that living in a fragmented landscape did not affect the reproductive output of our study population. Moreover, despite the landscape, dispersal events were commonplace, including many secondary dispersals (individuals emigrating from groups that they had previously immigrated into). Finally, we found a marked effect of seasonality on the dynamics of our study population. In particular, the period of lowest temperatures and resource scarcity between November and March was associated with higher mortality and reproductive inhibition, while the period of resource abundance between April and May was associated with the majority of conceptions and weaning of offspring. This, in turn, could be influencing dispersal patterns in our study area, as male howler monkeys seem to time some of their immigrations into new groups to coincide with the start of the period of higher fertility, while females preferentially joined new groups several months before the onset of this period. These data have important implications for the conservation and management of howler monkeys in fragmented landscapes, as well as for our understanding of the effect of seasonality over howler monkey dispersal, reproduction and survival.

Behavioral and physiological responses to fruit availability of spider monkeys ranging in a small forest fragment

Numerous animal species currently experience habitat loss and fragmentation. This might result in behavioral and dietary adjustments, especially because fruit availability is frequently reduced in fragments. Food scarcity can result in elevated physiological stress levels, and chronic stress often has detrimental effects on individuals. Some animal species exhibit a high degree of fission-fusion dynamics, and theory predicts that these species reduce intragroup feeding competition by modifying their subgroup size according to resource availability. Until now, however, there have been few studies on how species with such fission-fission dynamics adjust their grouping patterns and social behavior in small fragments or on how food availability influences their stress levels. We collected data on fruit availability, feeding behavior, stress hormone levels (measured through fecal glucocorticoid metabolites (FGCM)), subgroup size, and aggression for two groups of brown spider monkeys (Ateles hybridus) in a small forest fragment in Colombia and examined whether fruit availability influences these variables. Contrary to our predictions, spider monkeys ranged in smaller subgroups, had higher FGCM levels and higher aggression rates when fruit availability was high compared to when it was low. The atypical grouping pattern of the study groups seems to be less effective at mitigating contest competition over food resources than more typical fission-fusion patterns. Overall, our findings illustrate that the relationship between resource availability, grouping patterns, aggression rates, and stress levels can be more complex than assumed thus far. Additional studies are needed to investigate the long-term consequences on the health and persistence of spider monkeys in fragmented habitats.

Male reproductive strategies in black and gold howler monkeys (Alouatta caraya)

Behavioral and demographic factors such as group size, social structure, dispersal patterns, and mating systems affect male reproductive success. In the present study, we analyze the relationship between social structure, genetic relatedness of adult males and offspring paternity in one population of Alouatta caraya inhabiting a continuous forest in Northern Argentina. After 14 months of behavioral studies and genotyping 11 microsatellites, we found that dominant or central males achieved greater mating success and fathered all the offspring conceived during our study in two multimale-multifemale groups (both including three adult males). Although skewed toward the dominant males, females copulated with almost all resident males and with extra group males. We found significantly fewer agonistic interactions between adult males in the group with fewer females and where males were more genetically related to each other (average relatedness r = 0.237; 0.015 int/ind/hr vs. r = 0.02; 0.029 int/ind/hr). Paternity was also analyzed in two other neighboring groups which also showed strong skew to one male over a 2-year period. These results reveal that even though female black and gold howlers mate with many males, infants are typically fathered by one dominant male.

Impact of intrasexual selection on sexual dimorphism and testes size in the Mexican howler monkeys Alouatta palliata and A. pigra

One of the goals of physical anthropology and primatology is to understand how primate social systems influence the evolution of sexually selected traits. Howler monkeys provide a good model for studying sexual selection due to differences in social systems between related species. Here, we examine data from the sister howler monkey species Alouatta palliata and A. pigra inhabiting southeastern Mexico and northern Guatemala. We use a resampling approach to analyze differences in sexual dimorphism of body and canine size. In addition, we compare testes size as a way of gauging the intensity of sperm competition in both species. Morphometric data were collected from wild-caught individuals, including body mass and length, and dental data were obtained from casts from wild individuals and from museum specimens. Although A. pigra individuals are larger than their A. palliata counterparts, we find that both species exhibit similar levels of sexual dimorphism for all of the variables considered. Testicular volume results indicate that A. palliata male testes are on average twice as large as those of A. pigra males, suggesting more intense sperm competition in the former species. Our study shows that A. pigra is not highly sexually dimorphic as was once thought, and testes size differences suggest the need for a clearer understanding of howler monkey social systems.

Effects of habitat fragmentation and disturbance on howler monkeys: a review

We examined the literature on the effects of habitat fragmentation and disturbance on howler monkeys (genus Alouatta) to (1) identify different threats that may affect howlers in fragmented landscapes; (2) review specific predictions developed in fragmentation theory and (3) identify the empirical evidence supporting these predictions. Although howlers are known for their ability to persist in both conserved and disturbed conditions, we found evidence that they are negatively affected by high levels of habitat loss, fragmentation and degradation. Patch size appears to be the main factor constraining populations in fragmented habitats, probably because patch size is positively related to food availability, and negatively related to anthropogenic pressures, physiological stress and parasite loads. Patch isolation is not a strong predictor of either patch occupancy or population size in howlers, a result that may be related to the ability of howlers to move among forest patches. Thus, we propose that it is probable that habitat loss has larger consistent negative effects on howler populations than habitat fragmentation per se. In general, food availability decreases with patch size, not only due to habitat loss, but also because the density of big trees, plant species richness and howlers' home range size are lower in smaller patches, where howlers' population densities are commonly higher. However, it is unclear which vegetation attributes have the biggest influence on howler populations. Similarly, our knowledge is still limited concerning the effects of postfragmentation threats (e.g. hunting and logging) on howlers living in forest patches, and how several endogenous threats (e.g. genetic diversity, physiological stress, and parasitism) affect the distribution, population structure and persistence of howlers. More long-term studies with comparable methods are necessary to quantify some of the patterns discussed in this review, and determine through meta-analyses whether there are significant inter-specific differences in species' responses to habitat loss and fragmentation.

The functions of the "Greeting Ceremony" among male mantled howlers (Alouatta palliata) on Agaltepec Island, Mexico

Nonhuman primates use greeting behaviors as nonaggressive communicatory signals in multiple social contexts. Adult male mantled howlers (Alouatta palliata) perform a ritual greeting that has been associated with bond-strengthening functions. The aim of this study is to explore the greeting patterns of male howlers living on Agaltepec Island, Mexico. Specifically, we analyzed the relationships between greetings and several individual, relational, and contextual variables, such as the expression of affiliation and agonism, dominance rank, age, kinship relationships, spatial organization, activity patterns, and subgrouping patterns. Greetings were more frequent between males with closer dominance ranks. Among those dyads that greeted at least once, dominant males initiated greetings more frequently than less-dominant males. On the other hand, more greetings were observed when one of the participants had recently returned to a subgroup and during locomotion. On the basis of these results, we propose that on Agaltepec greetings are a conflict management mechanism used between males of similar ranks. The fission-fusion social system of this group of howlers allows males with conflicting interests to remain separated, and greetings may reduce tension during fusion events.

Biogeographical and floristic predictors of the presence and abundance of mantled howlers (Alouatta palliata mexicana) in rainforest fragments at Los Tuxtlas, Mexico

This research focuses on identifying the principal habitat characteristics that influence the presence and abundance of mantled howlers in forest fragments. We provide information on the demography of several fragmented Alouatta palliata mexicana subpopulations at Los Tuxtlas, Mexico, and relate this to the biogeographical and floristic characteristics of the forest fragments inhabited. The most important habitat characteristics related to the presence and abundance of howlers in the fragments were fragment size and floristic diversity. On the other hand, some evidence suggests that given the conditions under which howlers in our study area live (i.e., small and degraded fragments with high densities), secondary vegetation may be beneficial for the survival of the howlers. Finally, we discuss the possibility that the very low immature-to-female ratio (IFR) in the groups, and the lack of juveniles found in many of the study groups may be due to high mortality rates in immatures. A reduction in food availability because of the high population densities of these groups may be responsible for this process.