Sexual dimorphism in canine length of woolly spider monkeys (Brachyteles arachnoides,E. Geoffroy 1806)

We measured canine teeth from 28 woolly spider monkeys (Brachyteles arachnoides) to assess sexual dimorphism and population differences. The specimens are from the Brazilian states of Bahia, Minas Gerais, Espírito Santo, Rio de Janeiro, and São Paulo. We found strong sexual dimorphism in canine length for individuals belonging to populations south of 22°00 latitude but no sexual dimorphism in canine length from individuals of populations north of 21°00 latitude. Canine length did not vary among females of northern and southern populations. However, southern males had significantly longer canines than northern males. This geographical difference in canine morphology, together with the presence or absence of thumbs and published accounts of differences in genetics and social structure between northern and southern populations, suggests thatBrachyteles arachnoides may be composed of at least two subspecies, which appear to be separated by the rivers Grande and Paraiba do Sul and the Serra da Mantiqueira.     

Capture techniques and morphometrics for the woolly spider monkey,or muriqui (Brachyteles arachnoides,E. Geoffroy 1806)

A total of 12 free-ranging muriquis (Brachyteles arachnoides) were captured with Telazol® at Fazenda Esmeralda, Minas Gerais, Brazil, and at Fazenda Barreiro Rico, São Paulo, Brazil. All animals were measured, marked, weighed, and released. Previously reported data suggested that Brachyteles is a sexually dimorphic species with female-male body weights of 12–15 kg, respectively. We found no statistically significant difference in body weight between females (mean = 8.4 kg, range = 6.9–9.3 kg, n = 4), and males (mean =9.6 kg, range = 9.3–10.2 kg, n = 4). Our results are at variance with previously published body weights in the literature. Larger sample size may reveal a significant sexual difference, particularly in body weight. © 1993 Wiley-Liss, Inc.     

Sexual selection and canine dimorphism in New World monkeys

Social and ecological factors are important in shaping sexual dimorphism in Anthropoidea, but there is also a tendency for body-size dimorphism and canine dimorphism to increase with increased body size (Rensch's rule) (Rensch: Evolution Above the Species Level. London: Methuen, 1959.) Most ecologist interpret Rensch's rule to be a consequence of social and ecological selective factors that covary with body size, but recent claims have been advanced that dimorphism is principally a consequence of selection for increased body size alone. Here we assess the effects of body size, body-size dimorphism, and social structure on canine dimorphism among platyrrhine monkeys. Platyrrhine species examined are classified into four behavioral groups reflecting the intensity of intermale competition for access to females or to limiting resources. As canine dimorphism increases, so does the level of intermale competition. Those species with monogamous and polyandrous social structures have the lowest canine dimorphism, while those with dominance rank hierarchies of males have the most canine dimorphism. Species with fission-fusion social structures and transitory intermale breeding-season competition fall between these extremes. Among platyrrhines there is a significant positive correlation between body size and canine dimorphism However, within levels of competition, no significant correlation was found between the two. Also, with increased body size, body-size dimorphism tends to increase, and this correlation holds in some cases within competition levels. In an analysis of covariance, once the level of intermale competition is controlled for, neither molar size nor molar-size dimorphism accounts for a significant part of the variance in canine dimorphism. A similar analysis using body weight as a measure of size and dimorphism yields a less clear-cut picture: body weight contributes significantly to the model when the effects of the other factors are controlled. Finally, in a model using head and body length as a measure of size and dimorphism, all factors and the interactions between them are significant. We conclude that intermale competition among platyrrhine species is the most important factor explaining variations in canine dimorphism. The significant effects of size and size dimorphism in some models may be evidence that natural (as opposed to sexual) selection also plays a role in the evolution of increased canine dimorphism.     

Activity budgets of woolly spider monkeys,or muriquis (Brachyteles arachnoides)

One group of woolly spider monkeys, or muriquis (Brachyteles arachnoides), was observed from June 1983 through July 1984 at Fazenda Montes Claros, Minas Gerais, Brazil. The study subjects spent an average of 49% of their daylight time resting, 29% traveling, and 19% feeding. They shifted their diurnal schedule of activities in response to seasonal changes in temperature and rainfall. The activity budgets of adult males and females were similar. However, a comparison of three adult females in different reproductive conditions revealed that the lactating female spent a greater proportion of time feeding than did both the pregnant and nonreproductive females. Intraspecific differences in group size and diet appear to be important to understanding differences in the activity budgets of Brachyteles. Differences between locomotion patterns of woolly spider monkeys and sympatric howler monkeys may explain activity budget differences based on group size and diet.     

Diet in one group of woolly spider monkeys,or muriquis (Brachyteles arachnoides)

Data on feeding behavior in one group of muriquis (Brachyteles arachnoides) were collected during a 14 month study at Fazenda Montes Claros, Minas Gerais, Brazil. Muriquis devoted an average of 51% of their annual feeding time to leaves, 32% to fruits, and 11% to flowers. The high proportion of leaves in their diet is consistent with predictions from the energetics of body size. However, they devoted a greater proportion of their feeding time to patchy fruit and flower resources than was expected from comparisons with smaller, sympatric howler monkeys. Muriqui diet varied with the availability of preferred food types across sample months. Fruit and flower consumption corresponded to the availability and abundance of these food resources in the forest. Leaves contributed substantially to muriqui diet throughout the year but appeared to be eaten primarily to provide necessary protein and/or necessary bulk. Male and female diets differed only in the greater proportion of feeding time females devoted to flowers.     

Intrasexual competition and canine dimorphism in anthropoid primates.

A number of factors, including sexual selection, body weight, body-weight dimorphism, predation, diet, and phylogenetic inertia have been proposed as influences on the evolution of canine dimorphism in anthropoid primates. Although these factors are not mutually exclusive, opinions vary as to which is the most important. The role of sexual selection has been questioned because mating system, which should reflect its strength, poorly predicts variation in canine dimorphism, particularly among polygynous species. Kay et al. (1988) demonstrate that a more refined estimate of intermale competition explains a large proportion of the variation in canine dimorphism in platyrrhine primates. We expand their analysis, developing a more generalized measure of intermale competition based on the frequency and intensity of male-male agonism. We examine the relative influences of predation (inferred by substrate use), female body weight, body-weight dimorphism, diet, and sexual selection on the evolution of anthropoid canine dimorphism. Intermale competition is very strongly associated with canine dimorphism. Predation also has a marked effect on canine dimorphism, in that savanna-dwelling species consistently show greater canine dimorphism than other species, all other factors being held equal. Body-weight dimorphism is also strongly associated with canine dimorphism, though apparently through a common selective basis, rather than through allometric effects. Body weight seems to play only a minor, indirect role in the evolution of canine dimorphism. Diet plays no role. Likewise, we find little evidence that phylogenetic inertia is a constraint on the evolution of canine dimorphism.     

New World primates,new frontiers: Insights from the woolly spider monkey,or muriqui (Brachyteles arachnoides)

Despite over 25 years of intensive research, much of our knowledge of primate behavior has been limited to a small number of Old World, semiterrestrial species. With the exception of chimpanzees, these species share consistent patterns of behavior, including aggressive competition between males, male dominance over females, male dispersal and female kin groups, that have come to characterize the Order Primates. However, as research has expanded to include a number of newly studied species, such generalizations about primate behavior are no longer appropriate. Data presented on the wooly spider monkey, or muriqui (Brachyteles arachnoides), provide examples of the ways in which traditional views of primate behavior are being reconsidered. In muriquis, and in several other New World monkeys, female kin groups do not occur because females disperse from their natal groups. In addition, aggressive competition between males is virtually absent; and, female muriquis appear to be codominant with males. Constraints on sexual dimorphism may help to explain why muriquis (and other New World primates) differ so fundamentally from the better known semi-terrestrial Old World monkeys.Logo of the 1988 International Primatological Society.     

Habitat,diet, and activity patterns of free-ranging woolly spider monkeys (Brachyteles arachnoides E. Geoffroy 1806)

Results of a 10-month study of the ecology and behavior of free- ranging woolly spider monkeys (Brachyteles arachnoides)in Brazil show that these animals are strongly folivorous. Leaf-eating accounted for more than 50% of the total feeding time in all samples but one and accounted for more than 80% of the total feeding time in three samples. Mature foliage was routinely eaten. Woolly spider monkeys consistently spend more than 50% of each day quietly resting and sleeping. Animals travel little except when actively feeding and show low levels of social interaction. Such an activity profile suggests that woolly spider monkeys may often be living near the limits of their energetic resources. The social organization of the species is unusual for a folivorous primate in that small groups of females and associated immature animals confine their activities to discrete home-range areas, whereas males are itinerant, traveling over the home ranges of various female groups. Animals sharing a common home-range area show no permanent daily pattern of association other than that of mother-dependent offspring. Foraging alone or with few conspecifics should maximize each individual’s returns from foraging by minimizing the day range that must be traveled each day to locate foods while simultaneously lowering interference competition for higher-quality dietary resources.     

Hormonal changes during the mating and conception seasons of wild northern muriquis (Brachyteles arachnoides hypoxanthus)

We investigated hormonal and behavioral changes in wild male and female northern muriquis (Brachyteles arachnoides hypoxanthus) at the Estação Biológica de Caratinga, Minas Gerais, Brazil, during a 6‐mo period that encompassed the onset of the 1998–1999 mating and conception seasons. Individual females resumed mating with the resumption of ovarian cycling, which was not synchronized among them or related to their cortisol levels. Females experienced two to seven cycles prior to conceiving, and the first conception occurred 2 mo after the onset of the group's mating season. There were no differences in female cortisol levels across their premating, mating, and conception conditions. Cortisol levels were significantly higher in females than in males prior to the conception season, consistent with the prediction that energy reserves may be associated with breeding readiness in females, but not males, in this species. The sustained elevation in male cortisol occurred after the peak in their sexual activity, which resulted in the first conception of the year. Male cortisol levels were positively correlated between years that were similar in rainfall, but differed in the timing of sexual and reproductive events. The timing of cortisol elevations in males appears to be generally regulated by environmental cues, but is responsive to fine‐tuning by social and behavioral cues related to the unpredictable timing of reproductive opportunities within their extended mating season. Am. J. Primatol. 61:85–99, 2003. © 2003 Wiley‐Liss, Inc.     

Sexual dimorphism of the canine roots in theMacaca fuscata

In situ radiographic analysis of the maxillary canines ofMacaca fuscata was conducted on 88 specimens in 44 individuals (23 dry skulls and 21 live animals) in order to examine the number of roots. The left canines were then extracted from ten female skulls for measurement, further radiographic examination, and visual morphological observation. The results showed a clear sexual dimorphism in root morphology: all male canines were clearly distinguished as single-rooted from the radiograph, whereas more than 40% of the female canines were double-rooted. Variation was also found among the single-rooted female canines, in that some of these teeth appeared to have a bifurcated canal. This sexual dimorphism in the number of maxillary canine roots and the individual variation found among the females in root and canal morphology are previously unreported for this species. No observations were attempted on mandibular canines, however, because of the incomplete nature of the sample.     

Behavioral Correlates of Dispersal in Female Muriquis (Brachyteles arachnoides)

We documented four adolescent female transfers—two emigrations, two immigrations—during a 12-month study from August 1994 to July 1995 on one group of muriquis (Brachyteles arachnoides) at Estação Biológica de Caratinga in Minas Gerais, Brazil. Intergroup transfers occurred throughout the year independent of season. A total of 613 focal samples of 10-min duration conducted on six adolescent females (5–9 years of age) revealed significant differences in the behavior of migrant versus natal resident females. During the seasons encompassing their respective group transfers, both emigrants and immigrants devoted more of their time to resting than resident females belonging to the same age cohorts did. Time spent feeding on mature fruit was higher for one emigrant and lower for both immigrants compared to the two resident females. Emigrant females had fewer neighbors within a 1-m radius than resident females did, whereas immigrant females were within a 1-m radius of adult females and within a 5-m radius of adult males more often than resident adolescent females were. Adolescent females were displaced on 26 occasions. Displacements occurred mainly during the dry seasons (n = 21) and mainly at food sources (n = 21). Using the number of focal samples conducted on each female as an estimate of observation time, immigrant females were displaced at twice the rate of residents. However, like other behavioral differences detected between resident and migrant adolescent females, differences in individual displacement rates were evident only during the season in which each of the immigrants joined the group. Collectively, our findings imply that female intergroup transfer in this population involves relatively mild, short-term costs.     

Timing of births in sympatric brown howler monkeys (Alouatta fusca clamitans) and northern muriquis (Brachyteles arachnoides hypoxanthus).

We monitored the birth patterns of sympatric brown howler monkeys (Alouatta fusca clamitans) and northern muriquis (Brachyteles arachnoides hypoxanthus) during a 4‐yr period from October 1996 to August 2000 at the Estação Biológica de Caratinga, Minas Gerais, Brazil. Brown howler monkey births (n = 34) occurred throughout the year, and birth frequencies did not differ between rainy and dry season months. The aseasonal birth patterns of the howler monkeys differed significantly from the dry season concentration and dry month peak in muriqui births (n = 23). We found no effects of infant sex or the number of females on interbirth intervals (IBIs) in our 10 howler monkey study troops. IBIs of brown howler monkeys averaged 21.2 ± 2.5 mo (n = 8, median = 21.0 mo), and were significantly shorter following dry season births than rainy season births. Their IBIs and yearling survivorship (74%) were similar to those reported for other species of howler monkeys, but yearling survivorship was much lower than that of muriquis (94%), whose IBIs were more than 12 mo longer than those of the howler monkeys. Our study extends comparative knowledge of birth patterns in Alouatta to a poorly known species, and provides insights into the different ways in which diet and life history may affect the timing of births in large‐bodied platyrrhines under the same seasonal ecological conditions. Am. J. Primatol. 55:87–100, 2001. © 2001 Wiley‐Liss, Inc.     

Population Variation in Patch and Party Size in Muriquis (Brachyteles arachnoides)

We examined the relationship between food patch size and feeding party size with comparative data from two populations of muriquis (Brachyteles arachnoides) in the 37,797-ha forest at the Parque Estadual de Carlos Botelho (PECB), São Paulo, and the 800-ha forest at the Estação Biológica de Caratinga (EBC), Minas Gerais. Precipitation was more abundant and less seasonal at PECB than EBC, and the density of large trees (>25.0 cm) was higher at PECB (206 ha ^–1 ) than at EBC (132 ha ^–1 ). At both sites, the size of feeding parties is positively related to the size of food patches. As predicted, food patches at PECB are significantly larger than those at EBC for both fruit and leaf sources. Contrary to expectations, feeding parties were larger at EBC than PECB. The higher population density of muriquis and sympatric primates at EBC may make large associations more advantageous to these muriquis than to muriquis living at lower population densities in PECB.     

Variation in mating systems and sexual size dimorphism between populations of the Australian python Morelia spilota (Serpentes: Pythonidae)

Although adaptationist hypotheses predict a functional relationship between mating systems and sexual size dimorphism, such predictions are difficult to test because of the high degree of phylogenetic conservatism in both of these traits. Taxa that show intraspecific variation in mating systems hence offer valuable opportunities for more direct tests of evolutionary-ecological hypotheses. Based on a collation of published and unpublished records, we document intraspecific geographic variation in mating systems (presence versus absence of male-male combat) within the widely-distributed Australian python Morelia spilota. Radiotelemetric monitoring of 19 free-ranging pythons in a population in north-eastern New South Wales showed that these animals display a mating system of female defence polygyny. Previous studies on a southern population of the same species found that males engaged in long mate-searching movements, showed no overt agonistic behavior, and formed long-term (>2 months) aggregations around reproductive females. In strong contrast, our adult male carpet pythons (i) moved about relatively little (mean displacement <11 m/day) during the mating season, (ii) remained with females only briefly (<5 days), and (iii) engaged in male-male combat in the vicinity of females. This male-male combat included vigorous biting as well as ritualised wrestling matches, resulting in a high incidence of bite scars in adult males. In keeping with predictions from sexual selection theory, males attain larger body sizes than females in this population, whereas females grow larger than males in the previously-studied southern population where males do not engage in physical combat for mating opportunities.     

Comparison of four simple methods for estimating sexual dimorphism in fossils

Estimating sexual dimorphism in skeletal and dental features of fossil species is difficult when the sex of individuals cannot be reliably determined. Several different methods of estimating dimorphism in this situation have been suggested: extrapolation from coefficients of variation, division of a sample about the mean or median into two subsamples which are then treated as males and females, and finite mixture analysis (specifically for estimating the maximum dimorphism that could be present in a unimodal distribution). The accuracy of none of these methods has been thoroughly investigated and compared in a controlled manner. Such analysis is necessary because the accuracy of all methods is potentially affected by fluctuations in either sample size, sex ratio, or the magnitude of intrasexual variability. Computer modeling experiments show that the mean method is the least sensitive to fluctuations in these parameters and generally provides the best estimates of dimorphism. However, no method can accurately estimate low to moderate levels of dimorphism, particularly if intrasexual variability is high and sex ratios are skewed. © 1994 Wiley-Liss, Inc.     

Ontogenetic changes and sexual dimorphism in the mandible of adult woolly mammoths (Mammuthus primigenius)

Morphometric studies are a fundamental tool in paleontology to answer taxonomic, functional and evolutionary questions. In particular, appropriate functional interpretation often requires consideration of ontogenetic changes in the structures studied. The woolly mammoth (Mammuthus primigenius) is one of the most representative large mammal species of the Eurasian Pleistocene. Available ontogenetic studies on woolly mammoth mandible have focused on the first ontogenetic stages of the mandible development up to 4-5 years and have suggested that the symphysial process is sexually dimorphic. In the present work, we studied ontogenetic changes and sexual dimorphism in 45 mandibles from subadult and adult stages (8-56 African Elephant Years). Our results show positive correlations among almost all the morphometric variables measured, as well as an increase of mandible size with age. This increase does not differ among the variables examined, although the highest values are related with the symphysis height and the opening of the horizontal branches, and the lower ones with the greatest length (dimension), which implies the increase in the relative mandible width and height throughout the individual life. Sexual dimorphism in the mandible is at best slight, and the symphysial process is not diagnostic for sexing purposes. In addition, differences in age were an important confounding factor to assess sexual dimorphism and should be considered in future uses of sexual dimorphism assessment techniques.     

Height and sexual dimorphism of stature among human societies

In this study, which is concerned with the varying degrees of sexual dimorphism of stature between human societies, adult male and female height measurements and male-female height ratios – the measure of sexual dimorphism – from 216 societies are statistically compared with several variables: marriage practices, protein availability, the presence of milking herds, settlement size, and climate. Our results indicate that while greater mean male height is associated with polygynous marriage, marriage practices did not exert an influence on the degree of sexual dimorphism of stature. On the other hand, the results suggest that while sexual dimorphism in height has a strong genetic component, dietary factors can influence the degree of dimorphism.