Sexual selection arises when individuals compete for access to mates, resulting in differences in reproductive success. In mammals, where females invest much more in offspring through gestation and lactation, sexual selection is usually stronger in males, whose fitness is limited by access to receptive females and who compete to monopolize them. Males can maintain exclusive access to a group of females by avoiding other groups, aggressively excluding rival males, or by herding females or defending an area in which they range. In polygynous species, male weapons, ornaments, and displays are often well developed for intrasexual competition, and males may use vocal displays in particular to resolve conflicts without costly fights. These signals may provide information about the caller, such as his body size, age, rank, or physical condition; however, the honesty of these calls can only be maintained by costs or constraints on callers that prevent cheating. This suite of behavioral traits is exemplified by the Asian colobines, which typically live in one-male social groups inhabiting small home ranges. Male territorial behavior and loud calls are common, and have been proposed to function in male-male competition. As female dispersal occurs in most species, these behaviors also have the potential to play a role in female mate choice. One Asian colobine, however, has been described as an exception. Simakobu (Simias concolor) are reported to form adult male-female pairs, giving the appearance of a monogamous mating system, though their pronounced sexual dimorphism makes monogamy seem unlikely. Since information on simakobu is based on very few studies, most of which involved brief observations of unhabituated groups, often at disturbed sites, it is possible that their unusual groups are a result of human disturbance. In fact, early researchers suggested that the social organization of simakobu may be influenced by hunting either by increasing mortality or selecting for cryptic behavior. Thus, a long-term study of habituated groups at an undisturbed site was needed to clarify the status of simakobu. This dissertation investigated male-male competition in one-male groups of simakobu and the role of loud calls as honest signals of male competitive ability at a protected forest in northeastern Siberut, Indonesia, with five main objectives. The first three objectives provided the social, temporal and spatial context of within- and between-group relationships and established the potential for male-male competition and female choice. 1) To establish the social setting for male-male competition, I described the social organization and dispersal patterns for groups, with the secondary goal of explaining the frequent occurrence of small socially monogamous groups often reported in the past. 2) To understand the seasonal patterns of ecology and reproduction, I documented the annual distribution of births and conceptions in relation to climate, phenology, feeding behavior, and female physical condition. 3) I investigated the spatial relationships among groups by establishing the size and exclusivity of home ranges and describing the nature and frequency of intergroup encounters. The final two objectives evaluated the use of loud calls in male-male competition. 4) To assess whether loud calls are honest signals of male energy status, I examined how calling rate and call duration vary in relation to long-term and short-term changes in energy availability. 5) To determine which acoustic features differentiate callers and contexts, I used acoustic analysis to assess which call characteristics may signal male stamina and fighting ability. To address these objectives, I studied simakobu during July-August 2005 and November 2006-December 2008 at Pungut, Siberut Island, Indonesia. Behavioral, demographic and acoustic data were collected from 11 groups (nine one-male and two all-male) of simakobu as well as phenological data from 404 trees and lianas. As part of the first objective, I addressed the apparent paradox that simakobu represent as pair-living Asian colobines with signs of strong sexual selection, including marked sexual dimorphism. I documented the social organization and demographic changes of 10 simakobu groups throughout the two-year study. I hypothesized that the small groups documented in previous reports were the result of human disturbance due to hunting and habitat disturbance. To test this, I compiled data on hunting rates, habitat disturbance and demography (group size, adult sex ratio, number of immatures, immature-to-adult female ratio) for seven additional populations from the literature. I found that simakobu at this site reside in one-male groups that comprise three adult females and immatures and in all-male groups with males of all age classes in addition to juvenile females. Both male and female dispersal occurred, and juveniles seemed to disperse more frequently than adults. The results of the inter-population comparison revealed that the adult male-female pairs reported for this species appear to be a result of hunting pressure reducing group size. For the second objective, I examined the timing of simakobu reproduction in relation to patterns of environmental seasonality. Southeast Asian primates in general show moderate birth seasonality due to either low environmental seasonality or unpredictable fluctuations of mast-fruiting food resources. Simakobu, however, have been reported to be strict seasonal breeders with births occurring in June and July only. It is unclear whether these observations are characteristic of the species or resulted from a sampling bias. To address this question, I documented the annual distribution of 11 births and conceptions in eight groups of simakobu over two consecutive years, and assessed annual variation in ecology and reproduction via rainfall, temperature, food availability, feeding time, and female physical condition. I hypothesized that, like other Southeast Asian species, simakobu would not exhibit strict birth seasonality, and if they were capital breeders, conceptions would occur immediately after food peaks when female physical condition should be best. I found that simakobu foods were abundant throughout the year, but fruit feeding showed seasonal peaks in June and September. Breeding was not strictly seasonal, as births occurred in seven months, with a peak in the wettest month of the year (October) at the end of the period of highest fruit availability. Females seemed to conceive following a peak in unripe fruit, and when physical condition was best, suggesting a capital breeding strategy. To address the third objective, I established the spatial distribution of groups as well as the nature of intergroup relationships. I hypothesized that males would attempt to maintain exclusive access to females via aggressive intergroup encounters and loud calls. I compiled ranging and intergroup encounter data from all-day follows of four groups over five consecutive months. Using GIS methods, I mapped and described the size and overlap of their home range areas. I compared behavior, ranging patterns and space use of one-male groups and all-male groups and evaluate the hypothesis that males attempt to monopolize females by defending the areas in which they range. I found that simakobu home ranges were very small, 6-7 ha on average, and are used almost exclusively (ca. 10% overlap). Most encounters involved aggressive behavior from adult males and occurred in the peripheral areas of OMG home ranges, and in core areas for AMGs only. Overall, the exclusive use and aggressive defense of areas by males in OMGs suggested that males are indirectly defending females through the defense of the areas in which they range. For the fourth objective, I investigated whether loud calls are honest advertisements of male energy status, testing the hypothesis that loud calls are energetically costly handicap signals. I combined behavioral and ecological data with acoustic analysis to determine the influence of energy status on calling effort, measured as calling rates and call duration. Using data from all-day follows of three adult males and loud call recordings from an additional three, I examined the relationship between short-term (time of day, temperature, amount of rain, and travel distance) and long-term (fruit availability, physical condition) measures of energy availability. I found that the duration of simakobu loud calls was negatively affected by short-term changes in energy status, with shorter calls produced during wet periods and at higher post-dawn temperatures, supporting the hypothesis that calls may be energetically costly.. A peak of early morning calls of long duration suggested this might be the best time for males to advertise their quality. Contrary to predictions, daily calling rate increased with decreasing fruit availability and did not appear to reflect male energy status. I hypothesize that calling rate may instead be linked to strategies involving feeding and/or mate competition and signal the motivation, rather than fighting ability, of the caller. To address the fifth objective, I examined the structure and usage of loud calls to determine which acoustic features vary within and between individuals. Using four acoustic parameters, call duration, inter-unit interval (IUI), fundamental frequency (F0) and peak frequency (pf), I tested hypotheses about call variation related to the size, arousal, and exhaustion of the callers. I analyzed 170 recordings made from 10 adult males and described acoustic differences among contexts, individuals, and callers of different ages, as well as changes that occur during a single call bout. I found that individuals, were well discriminated by the acoustic features of their loud calls, including F0, IUI and duration. Contexts were not well discriminated, spontaneous calls had higher pf's, suggesting a higher level of arousal. The loud calls of older males had shorter IUI and lower F0. These differences likely reflect the caller's size, stamina and fighting ability, and could thus be a way for males and females to assess competitors and mates from long distances. Overall, results from this research revealed that, rather than being an exceptional primate, simakobu appear to fit the Asian colobine pattern with respect to group size and composition, dispersal, reproduction, ranging, and male competition. Simakobu show morphological and behavioral signs of strong male-male competition. Adult males attempt to monopolize females and appear to aggressively defend at least part of the area in which they range. They produce loud calls several times daily, and despite very small home range areas, meet other groups only once or twice per week. Results suggest that males use loud calls to signal their presence and fighting ability to defend females and/or the resources within their home ranges. Loud calls may function as honest signals of male condition, age, and strength, which are likely used by males and females alike to assess rivals and mates. Future research is needed to fully investigate the role of loud calls as part of a strategy by males to defend females, food resources, and/or infants.