Chimpanzees use weight to pick the best tool, and monkeys beg more when they're paid attention to, as reported in two independent research reports published July 18 in the open access journal PLoS ONE. In the chimp study, researchers found that the chimpanzees used weight to choose the best hammer to crack open nuts. Nut cracking is one of the most sophisticated instances of tool use in chimpanzees, and learning how to do it has been shown to be very difficult for some chimps. In work led by Cornelia Schrauf of the University of Vienna, the researchers showed that the chimps were able to choose the best tool to crack nuts based solely on the weight of the tool. Schrauf notes, "Experience clearly affected the subjects' attentiveness to the relevant tool properties. Whereas the most skilled chimpanzee showed a preference for the most efficient hammers from the early beginning of the experiment, the unskilled individuals became selective over time."

In another study, old world monkeys called Mangabeys were shown to modulate（调节） their begging behavior based on whether the experimenter was paying attention to them. The monkeys were trained to make "requesting gestures," and the researchers, led by Audrey Maille of the University of Rennes 1 in France, found that the monkeys gestured more and faster when the experimenter's body and head were facing the monkey than when they were oriented away. The monkeys did not modulate their behavior simply based on the direction of the experimenter's gaze, though.

Maille explains, "Our study deals with…whether functional similarities may be found between human language and nonhuman primates communication. By investigating the flexibility of gestures production, we showed that old world monkeys, and not only great apes, may use communicative signals intentionally."