Grooming in the mantled howler monkey,Alouatta palliata Gray

Dominant individuals groom subordinates in the mantled howler monkey. This relationship is previously unreported for the species and is a characteristic that howler monkeys share with many birds and mammals.     

Visual detection and fruit selection by the mantled howler monkey (Alouatta palliata)

Howler monkeys (platyrrhini) have evolved routine trichromatic color vision independently from catarrhines, which presents an opportunity to test hypotheses concerning the adaptive value of distinguishing reddish from greenish hues. A longstanding hypothesis posits that trichromacy aids in the efficient detection of reddish-ripe fruits, which could be an advantage for the detection of the nutritional content of the fruit, such as sugars. In the present study, we assessed fruit visual conspicuity and selection based on color and sucrose content by wild mantled howler monkeys (Alouatta palliata) on Agaltepec Island, Mexico. We used colorimetry to classify dietary fruits as cryptic (greenish) or conspicuous (reddish) against their background leaves. Species-specific color models indicate that trichromatic howler monkeys should be more efficient in discriminating the conspicuous ripe fruits from leaves compared to detecting cryptic ripe fruits from leaves. We found howler monkeys consume more cryptic fruits compared to conspicuous fruits, and that they consume more unripe fruits than ripe fruits. The consumption (acceptance) of fruit was independent of sucrose content, and thus this disaccharide may not play an essential role in mantled howler food selection. Our findings suggest that routine trichromatic color vision may aid in the detection and discrimination of conspicuously colored fruits, but that the final decision whether to accept or reject a fruit probably involves the use of other senses in addition to vision.     

Blood groups of the mantled howler monkey (Alouatta palliata).

Fifty-two howler monkeys were tested for their human-type A-B-O blood groups. All were group B, as shown by the presence of B and H in their saliva, and anti-A in serum. The B-like agglutinogen of their red cells is common to all New World monkey species tested, and is of different origin and significance than their true A-B-O blood group. Differences among the B-like agglutinogens of the red cells of howler monkeys, marmosets, rabbits and humans group B were demonstrated, and limited tests have also been performed to study the biochemical basis of the anti-B reactions.     

Non-visual senses in fruit selection by the mantled howler monkey (Alouatta palliata)

Primates - There is extensive knowledge about the visual system and the implications of the evolution of trichromatic color vision in howler monkeys (genus Alouatta) related to food selection;...     

Craniofacial sexual dimorphism in Alouatta palliata, the mantled howling monkey

Ontogenetic scaling has been hailed as an explanation of the differences in craniofacial morphology between adult males and females of a number of non human primate species. This inference has implications for the evolutionary processes underlying patterns of sexual variation, as several heterochronic processes (rate and time hypo- and hypermorphosis) predict ontogenetic scaling. Primary among species for which ontogenetic scaling of craniofacial dimensions has been claimed is Alouatta palliata , the mantled howling monkey. This study uses a variety of analytical tools to explore the efficacy of ontogenetic scaling as an explanatory paradigm for this classic example. Multivariate analysis captures shape far better than does bivariate analysis. However, multivariate analysis does not support the traditional inference of ontogenetic scaling. Explanations for contradictory results are considered.     

Sex differences in leadership during group movement in mantled howler monkeys (Alouatta palliata)

Benefits of group life depend in large part on whether animals remain cohesive, which often requires collective decisions about where and when to move. During a group movement, the leader may be considered as the individual occupying the vanguard position of the group progression, when its movement evokes following by other group members. In nondespotic societies, individuals with greater incentives to move frequently are leaders. During 15 months of observations (1,712 contact hours), we investigated two mantled howler monkey (Alouatta palliata) groups at La Flor de Catemaco (Los Tuxtlas, Mexico) to examine whether sex and female reproductive state influenced leadership likelihood in two contexts: movements toward feeding trees; movements associated with loud calls, a group-defense behavior used by males of this genus. Females led and occupied forward positions during group movements toward feeding trees more often than adult males. Adult females led these movements more frequently when they were gestating than when they were lactating or cycling. There were no differences between sexes in the leadership of group movements associated with loud calls. Leadership by gestating females is perhaps the result of their higher nutritional/energetic needs when compared with cycling females, and of their greater mobility when compared with lactating females carrying dependent offspring. Female leadership during movements toward feeding trees may be a mechanism to optimize access to food resources in mantled howler monkeys.     

The functions of status in the mantled howler monkey,Alouatta palliata Gray: Intraspecific competition for group membership in a folivorous neotropical primate

In the dominance hierarchies of adult male and female mantled howler monkeys (Alouatta palliata Gray), high-ranking individuals are young adults; intermediate-ranking individuals, middle-aged adults; and low-ranking individuals, old adults. This relationship reverses the trend observed in most group-living animals and is previously unreported for this species. A limiting supply of palatable leaves may create intense intraspecific competition for group membership which, it is hypothesized, has resulted in this rare pattern of hierarchical relations. It is shown that individual-level selection is sufficient to explain the evolution of this apparently “altruistic” status system, though other mechanisms are assessed. The energetic constraints imposed by a folivorous diet appear to restrict the expression of aggression to “ritualized” forms. Two groups of monkeys in two different habitats were studied and the rates of appeasement and aggressive behavior were found to be higher in the more “stressful” forest. The latter group, however, displays significantly more behavioral patterns entailing low or intermediate energy expenditure.     

Rates of fermentative digestion in the howler monkey, Alouatta palliata (primates: ceboidea)

1. Caecal material of wild howler monkeys was analyzed by gas chromatography for evidence of fermentation activity and rates of production and absorption of volatile fatty acids. 2. Results showed a high rate of production of acetic acid and lesser production of propionic, butyric and isobutyric acids. The VFA content of the blood was increase in passage through the caecal vascular system. 3. We estimate that howler monkeys may obtain as much as 31% of their required daily energy from fermentation end products. 4. Energy rich fatty acids may be of particular importance to howlers when they are living on diets high in leaves, which have high cell wall contents and low contents of nonstructural carbohydrates.     

Digestive strategies and food choice in mantled howler monkeys Alouatta palliata mexicana: bases of their dietary flexibility

Mantled howler monkeys (Alouatta palliata) occupy a wide variety of tropical habitats and are the most folivorous of New World primates. However, their diet may include fruits, buds, petioles, and flowers, as well as leaves, suggesting they must cope with variations in the nutrient composition of their food. We studied the physiological basis of the dietary flexibility of these monkeys by comparing food choice, digestive performance and patterns of digesta flow in six adults, fed diets of either leaves or a mixture of fruit and leaves. Although monkeys ate similar amounts of the two diets, they ingested more digestible protein when offered the leaf diet, on which they lost body mass, but they ingested much more soluble sugars when offered fruit and leaves on which they gained mass. Digestibilities of dry matter, fat, energy and fibre did not differ between diets, but those of crude protein, soluble sugars and minerals were higher on the fruit–leaf diet. Mean retention times in the gut of solute (Co-EDTA) and particulate markers (Cr-mordanted cell walls) did not differ between diets, but on both diets the monkeys retained the particulate marker (mean retention time ca 55 h) for longer than they did the solute marker (MRT ca 50 h). A lack of selective retention of solutes and small particles in the gastro-intestinal tract of howler monkeys probably restricts them to mixed diets but their digestive strategy is sufficiently flexible to allow them to feed on a diet of leaves when fruit is unavailable.     

Genetic structure of an isolated population of mantled howler monkeys (Alouatta palliata) on Barro Colorado Island, Panama

Habitat fragmentation may influence genetic variability through reductions in population size and the physical isolation of conspecifics. We explored the effects of these factors on genetic diversity in a population of mantled howler monkeys (Alouatta palliata) on Barro Colorado Island (BCI), Panama. The study population was established in 1914 when an unknown number of resident individuals were isolated from the surrounding mainland by damming of the Río Chagres to create the Panama Canal. Analyses of 10 microsatellite loci indicated that, despite this isolation, the howler monkeys on BCI exhibit levels of genetic diversity (H _S = 0.584 ± 0.063) among the highest reported for any species of Alouatta. These data also revealed that although relatedness among adults in a social group was significantly greater than zero, the BCI population is not highly genetically structured. Tests for genetic bottlenecks based on departures from equilibrium expectations failed to reveal evidence of a recent reduction in population size. In contrast, coalescent modeling indicated that this population has likely experienced a marked decline within the last few 100 years. These findings generate new insights into the genetic structure of A. palliata and suggest that while the formation of BCI may not have substantially reduced genetic variation in these animals, genetic diversity has been influenced by historical changes in population size.     

Craniometric variation in a population of mantled howler monkeys (Alouatta palliata): evidence of size selection in females and growth in dentally mature males

A large body of work on monkey cranial metrics (involving conclusions about interspecific variation, sexual dimorphism, and ontogeny) depends on the assumptions that growth effectively ceases with dental maturity and that intraspecific variation is negligible. We test these assumptions by examining variation in 39 measurements of 166 dentally mature Alouatta palliata skulls from animals found dead on Barro Colorado Island (BCI), Panama. We also investigate whether this population is under size-based selection, since our found-dead sample reflects the natural mortality in this population. The sample was divided into three age stages by occlusal wear (A-C, least to most wear). Female stage A means are significantly smaller than female stage B means for three cranial measures. Female stage B means are significantly smaller than female stage C means for five cranial measures. Male stage A means are significantly smaller than male stage B means for 21 cranial measures. Multivariate analyses confirm this trend of expansion between adult age stages. The dental metric and suture closure data suggest that the cranial expansion in females is due to size-based selection, while the cranial expansion in males is due to significant growth after dental maturity. Sexual dimorphism ratios are highly variable across different samples of A. palliata, indicating that dimorphism varies between populations of this species. These results provide insight into the selective forces operating on the BCI howlers and challenge the validity of the many studies which pool subspecies and assume growth ceases with maturity.     

Testosterone levels of free-ranging resident mantled howler monkey males in relation to the number and density of solitary males: a test of the challenge hypothesis

The challenge hypothesis asserts that testosterone levels and aggression in male adult animals are closely associated with one another in the context of intense reproductive competition, particularly when males challenge one another for the access to females. For mantled howler monkeys (Alouatta palliata), the presence of a solitary male in the vicinity of a social group can be considered a threat for the reproductive success of the resident males, as group takeovers by solitary males are followed by a reestablishment of the access to females among group males and sometimes by infanticide. Therefore, these episodes should be accompanied by an increase in testosterone secretion according to the challenge hypothesis. In order to test our prediction, we relate different group and subpopulation variables to the testosterone levels measured in feces collected from 10 groups living in 6 forest fragments, at Los Tuxtlas, Mexico. Our results suggest that resident A. palliata males regard solitary individuals as potential contesters for their reproductive success and that they respond to interactions with them with an increase in testosterone levels, that is increasing their aggressiveness. Our results also suggest that in the studied groups all males have at least certain access to mates and that the entrance of a new male would affect their reproductive success negatively. Finally, the negative effects of chronically high testosterone levels may be negatively affecting the fitness of our study groups living in high population densities and small fragments.     

Fecundity in the captive howler monkey,Alouatta caraya

Seven female Black howler monkeys, Alouatta caraya, from both wild- and captive-born origins have reproduced at Riverbanks Zoological Park. Three of the five wild-born females arrived as juveniles and grew to maturity; the other two were already mature upon arrival and reproduced shortly therafter. Two females which were born within the park's collection have since reached maturity and also reproduced. The interval between the first and second young of wild-born howlers is longer than between subsequent birth intervals. Subsequent young are also more precocious than their preceding siblings. In at least one case, a socially inferior female did not reproduce until placed within a different troop situation which elevated her social status. Females born in captivity conceived when 42 and 35 months old, the former situation being linked to low social status. Young born to females housed under both seasonal and regulated conditions of photoperiod demonstrated no difference in breeding intervals and seasonality of birth does not appear to be a factor in howler monkey reproduction.     

Parasitic infections of three Mexican howler monkey groups (Alouatta palliata mexicana) living in forest fragments in Mexico

In order to better understand how patterns of parasitism in howler monkeys are affected by forest fragmentation, we carried out a 1 year survey of gastrointestinal parasites in fecal samples from three groups of Mexican howler monkeys inhabiting different forest fragments in the Los Tuxtlas Biosphere Reserve, Mexico. The study groups were chosen because the conditions in which they lived suggested a potentially negative gradient for parasite richness and a positive gradient for levels of parasitism. We report for the first time the presence of Entamoeba coli in Alouatta palliata mexicana and of hookworms (Family Ancylostomidae) in A. palliata. A reduction in home range size and an increase in disturbance was associated with a loss of parasite richness, which in general was high. Parasite prevalence and the proportion of contaminated samples in which each parasite taxon was present was also high in general and there were no differences between groups. A factor related to the generally high levels of parasitism in our study groups could be the high humidity in the study area, because this favors the survival of parasitic free forms and increases the chances of infection. This would also account for the tendency towards higher levels of parasitism observed in the rainy season. Finally we did not find a pattern relating sex and parasitism.