Another, more recent, hypothesis is that concealed ovulation is an adaptation in response to a promiscuous mating system, similar to that of our closest evolutionary relatives, bonobos and chimpanzees. The theory is that concealed ovulation evolved in women to lessen paternity certainty, which would both lessen the chances of infanticide (as a father is less likely to kill offspring that might be his), and potentially increase the number of men motivated to assist her in caring for her offspring (partible paternity). This is supported by the fact that all other mammals with concealed ovulation, such as dolphins and gray langurs, are promiscuous, and that the only other ape species that have multi-male communities, as humans do, are promiscuous. It is argued that evidence such as the Coolidge effect, showing that a man does not seem to be naturally geared towards sexual mate-guarding behavior (that is, preventing other males from having access to his sexual partner), supports the conclusion that sexual monogamy (though perhaps not social monogamy and/or pair bonding) was rare in early modern humans.