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26One of the basic principles of sexual selection is that male reproductive success should be 27 skewed towards strong males in species with anisogamous sex. Studies on primate multi-male 28 groups, however, suggest that other factors than male fighting ability might also affect male 29 reproductive success. The proximate mechanisms leading to paternity in multi-male primate 30 groups still remain largely unknown since in most primate studies mating rather than 31 reproductive success is measured. Furthermore, little research focuses on a female's fertile 32 phase. The aim of this study was to investigate the relative importance of male monopolisation 33 and female direct mate choice for paternity determination. We also investigated the extent to 34 which paternity was decided post-copulatory, i.e. within the female reproductive tract. We used a

Ovarian cycles in catarrhine primates are uniquely characterized by prolonged periods of sexual activity in which the timings of ovulation and copulation do not necessarily correspond. According to current hypotheses of primate social evolution, extended sexuality in multi-male groups might represent part of a female strategy to confuse paternity in order to reduce the risk of infanticide by males. We test this hypothesis by examining mating behaviour in relation to timing of ovulation and paternity outcome in a multi-male group of free-living Hanuman langurs. Using faecal progestogen measurements, we ¢rst document that female langurs have extended receptive periods in which the timing of ovulation is highly variable. Next, we demonstrate the capacity for paternity confusion by showing that ovulation is concealed from males and that copulations progressively decline throughout the receptive phase. Finally, we demonstrate multiple paternity, and show that despite a high degree of monopolization of receptive females by the dominant male, non-dominant males father a substantial proportion of o¡spring. We believe that this is the ¢rst direct evidence that extended periods of sexual activity in catarrhine primates may have evolved as a female strategy to confuse paternity.