A troop of chimpanzees in southeastern Senegal are proving to be a continued source of surprise and amazement for primatologists. Not only do members forge weapons to hunt, making them the only known group to use tools to injure or kill prey, but it turns out that females actually engage in this behavior more than males. This could mean that, unexpectedly, female chimps pioneered tool use for hunting, and that the first weapon-yielding early humans could have also been females. The research has been published in Royal Society Open Science.

Back in 2007, whilst observing a group of savanna chimps in Fongoli, Senegal, Iowa State University anthropologist Jill Pruetz spotted something that had never before been observed: individuals were making sharp spears and using them to hunt vertebrate prey. But that wasn’t the only thing that stood out, as Preutz also noticed that more females were engaging in this behavior than males. At the time, she and her research team did not have enough data to be able to assert that it was indeed more common in females, so they continued to follow the animals for the next seven years.

During this time, the scientists observed troop members snapping off branches, removing the leaves and even using their teeth to trim and sharpen the ends. On average, these tools were around 75 centimeters (30 inches) in length. Weapon in hand, the chimps would then creep up on sleeping bush babies and stab them, either mortally or wounding them enough to make them easy to snatch and kill with hands and teeth.

Throughout the duration of the study, the team observed more than 300 tool-assisted hunts, 175 of which were performed by females. Given the fact that hunting groups were male dominated, with females usually only making up 40% of the members, males were significantly less likely to hunt than females, carrying out only 39% of the hunts. This was surprising since male chimpanzees hunt more than females in general, and within this group they also accounted for the vast majority of all captures.

So why is this behavior more common in females than males? The researchers speculate that it could be because males tend to be more opportunistic, for example grabbing escaped prey that another group member had failed to kill. Given that females are smaller and weaker than males, chasing after prey may also be a pretty inefficient way to hunt, especially since females are often carrying babies and thus not as agile. The researchers also pointed out to Discovery News that females also tend to be more innovative than males, leading them to believe that the first tool-wielding primates were likely females.

The fact that no other chimp groups are known to engage in this behavior is also extremely interesting. The researchers propose this may have something to do with the limited supply of vertebrate prey in the area, which could have encouraged them to become more inventive in order to meet their nutritional needs.

[Via Royal Society Open Science, Iowa State University, New Scientist and Discovery News]