Nocturnal prosimians, as basal taxa in primate phylogeny, are models for studying early primate evolution. In most theoretical studies, nocturnal prosimians are grouped as solitary and with a simple polygynous social and mating system. Recent work dispels the previously held view that they are solitary foragers, but their socioecology remains poorly studied. This thesis reports a suite of unexpectedly complex ranging, social, and mating behaviors, and habitat use among the slender loris Loris tardigradus. Thirty-three individuals were radio tracked during 2002-2003 in Kalakad-Mundanthurai Tiger Reserve (KMTR) in India. Three types of males were described based on their range size and access to mates: roamers, settled paired, and settled unpaired. Roamers had the largest ranges and no females associated with them; settled males had smaller established territories. Within the latter, settled unpaired males were never seen with females; settled paired males had associated females (based on range overlap and co-sleeping). The morphological and behavioral factors that influence males' access to females were investigated, and an index of male body condition developed to analyze its effects on ranging and social behavior. Body condition is a strong predictor of males' social status as roaming or settled. Also, paired males had relatively larger testes than other males, suggesting a role for both sperm competition and mate-guarding. Both types of settled males and females are found in better habitats than the roaming males. However, there are no inter- or intea-sexual differences in microhabitat use, suggesting that the roamers are using similar areas within poorer habitats. These results support the conclusion that lorises are likely to have a complex mating system that combines pair-living and elements of polygyny with sperm competition, instead of the simple mating system that was previously described. Such a combination of pair-living and high sperm competition has not been reported previously in primates. These findings have strong implications for understanding the complexity of prosimian social systems and the relationship between habitats and social organization. These results will also help in developing management plans for the slender loris both in KMTR and elsewhere in its range.