It seems that despite the wealth of information we know about primate behavior, especially chimpanzee behavior, we cannot fully grasp nor explain infanticide. In my opinion, primatology has not yet full investigated this unique and complicated behavior… well at least not until now. Before I get into the thick of new research on infanticide, let’s first define the term. Infanticide is defined as the killing of an infant and it is known to occur in many primate species. Most often in primates, and in other species, it is thought of as a male trait.

Take for example lions, when a male lion becomes head of a pride, he most often systematically kills all lion cubs. It sounds harsh. An unsupported evolutionary hypothesis for this behavior is that the male ‘wants’ to ensure his efforts as a leader influence the survival of his offspring and not the offspring of others… the whole selfish gene concept. The same could be said for male primates when they kill infants in their groups.

Chimpanzee with BabyThere is no explanation for when females engage in infanticide. Some justify female led infanticide as a pathological condition, where there’s something wrong with the female’s psychology. Surely it seems crazy, that the female, who invests so much time and energy and one of her eggs in rearing offspring, to kill one of her own. It gets a bit more complicated as far as jealousy/craziness when females begin to kill another’s infant. The pathological condition is what was hypothesized from Jane Goodall’s observations around the 70’s,

“Passion and Pom, a mother-daughter duo who cooperated in the killing and cannibalization of at least two infant offspring of other females.”

In a new Current Biology paper titled, “Female-led infanticide in wild chimpanzees,” authors Townsend et al., report three instances of female-led infanticidal attacks among the Sonso chimpanzee community in Budongo Forest in Uganda. From what they observed, they speculate these situations of female led infanticide are due to competition for limited resources. From ScienceDaily,

“Alerted to the killings by sounds of chimpanzee screams, the researchers directly observed one infanticide, and found strong circumstantial evidence for two others. Evidence suggested that in two of the cases, the killings were perpetrated by groups of resident females against “stranger” females from outside the resident group. Infants were taken from the mothers, who were injured in at least two of the attacks; in at least one case, adult males in the area exhibited displaying behavior, with one old male unsuccessfully attempting to separate the females.

The authors point out that these new observations indicate that such female-led infanticides are neither the result of isolated, pathological behaviors nor the by-product of male aggression, but instead appear to represent part of the female behavior repertoire in chimpanzees.

What drives the behavior is not yet clear, but may stem from demographic shifts that alter sex ratios and put increased pressure on females competing for foraging areas. In their report, the authors note that the Sonso community had experienced a significant population increase in the ten years prior to the infanticide observations (42 individuals in 1996 to 75 in 2006), and that there had been an influx of at least 13 females with dependent offspring since 2001. The population changes resulted in a highly skewed male:female sex ratio of 1:3, with relatively few males available to increase the home range.”

So there you have it, infanticide in this situation seems to be a numbers game — a means by which members of a chimpanzee community regulate population growth by taking into account the growth in the number of females in relation to growth in the number of males.

I doubt it is really as simple as that but it does provide us a bit more relevance to understand what is going on in the minds of these chimpanzees. We actually have a working model to base infanticide off of. I think we have proven time and time again how complicated chimpanzees are, they rival us in behavioral complexity. As hard as it is to derive why humans kill one another, it is equally challenging to pick that information out of a non-human species we observe in the wild. But this is a step into the right direction.