Research in social mammals has revealed the complexity of female counter-strategies to reproductive competition and sexual conflict. For example, comparative research has shown that the length of female sexual receptivity varies with infanticide risk, but whether individuals can strategically adjust their period of receptivity from cycle to cycle remains unknown. This study addresses this gap by exploring whether wild female chacma baboons (Papio ursinus) modulate their receptivity in response to the social environment. Given that female chacma baboons (a) compete for paternal care and (b) that infanticide risk and coercion are high, we predicted that: females could (a) shorten their receptive period to reduce intrasexual aggression and (b) male coercion, or (c) increase their conceptive period to access multiple or their preferred male. We quantified 158 receptivity cycles from 47 females recorded over 15 years. Female swelling duration, but not maximal-swelling, had low repeatability between females, but most variation in both phases stemmed from within females. We found evidence that females decrease their oestrous duration in response to an increasing number of in-group males, possibly to decrease their exposure to sexual coercion. We thus present preliminary evidence for an unexplored mechanism under sexual selection: female cycle length manipulation.