Male infanticides play crucial role in primate monogamy, new research suggests. The paper indicates that the role male infanticide plays among primates is even more important than paternal care or female ranging patterns. According to the National Institutes of Health, such behavior is rare among primates and is often difficult to study in free-ranging primates, with direct observation of the act existing for roughly 6 percent of primate species.

In response to this lack of direct infanticide observations, the researchers relied on a presumed “infanticide risk,” consisting of the ratio between gestation length and lactational length and the duration of the lactation. Using this approach, the researchers presume that female primates have generally longer periods of lactation, with a greater risk of males killing dependent infants.

The authors indicate that the this presumed ratio is greatly affected by phylogeny. Therefore, one problem when comparing this measure among various taxonomic groups of primates is that evolution could have been influenced by various selective pressures among different lineages. In this instance, the researchers could be comparing monogamous New World callitrichids, or marmosets/tamarins, with monogamous Old World gibbons.

Marmosets lack delayed ovulation during lactaction, with females instead showing a luteinizing hormone surge followed by ovulation and conception approximately 14 days after giving birth to twins. These primates continue to lactate long after the conception has occurred.

As opposed to marmosets, gibbons have a very different reproductive physiology. Following the death of an infant gibbon, the mother typically resumes menstrual cycles earlier and has a shorter interbirth interval.