Female bonobos exhibit prolonged receptivity with long-lasting maximal swelling phases, which is suggested to lower male mating competition. However, it remains unclear whether easier access to receptive females can reduce male-male competition, given the exclusive nature of male reproductive competition and the skewed paternity toward the alpha male in bonobos. We tested whether wild male bonobos could discern the female fertile phase within the maximal swelling phase using hormonal and behavioral approaches. We found that as the probability of ovulation increased, male mating efforts, including intensive following, solicitation and intervention of copulation, also increased. When multiple females exhibited maximal swelling, males concentrated their mating efforts on the female with older infants and whose maximal swelling phase had started earlier. Although the most frequent copulation was observed between high-ranking males and females with younger infants whose pregnancy probability was low, this result reflects more selective and fewer matings by the females with older infants. We also showed that the low predictability of ovulation can result from large inter- and intra-individual variations in the length of the maximal swelling phase leading to a low day-specific ovulation probability that distributes evenly throughout the whole maximal swelling phases. Taken together, our results demonstrate males' ability to estimate the probability of ovulation from maximal swelling and confirm noisy ovulatory signals in bonobos. Despite the noisy signal from females, our findings highlight that male bonobos can cope with it by adopting a simple but physically demanding mating strategy - concentrating mating efforts on females with older infants throughout the whole maximal swelling phase. Further investigation into the mechanism and evolution of such noisy signals in our closest relatives will not only clarify the function of the erroneous signals, but also help us to re-evaluate the concept and function of so-called concealed ovulation in humans within the evolutionary continuum of different levels of ovulatory signal conspicuity in primates.