Wild chimpanzees deprived a leopard of its kill: Implications for the origin of hominin confrontational scavenging

This study reports the first observed case of wild chimpanzees (Pan troglodytes) obtaining animal prey freshly killed by a sympatric leopard (Panthera pardus) and scavenging it with the leopard still nearby. This observation has important implications for the emergence of confrontational scavenging, which may have played a significant role in human evolution. Many scholars agree that eating meat became important during human evolution, and hominins first obtained meat by scavenging. However, it is debatable whether scavenging behavior was "passive" or "confrontational (power)." The latter is more dangerous, as it requires facing the original predator, and it is thus considered to have been important for the evolution of several human traits, including cooperation and language. Chimpanzees do scavenge meat, although rarely, but no previous evidence of confrontational scavenging has hitherto emerged. Thus, it was assumed that they are averse to confrontation with even leopard-sized predators. However, in the observed case the chimpanzees frequently emitted waa barks, which indicated that they were aware of the leopard's presence but they nevertheless continued to eat the scavenged meat. In addition, we compiled and reviewed 49 cases of chimpanzee encounters with animal carcasses in the Mahale Mountains of Tanzania in 1980-2017. Chimpanzees scavenged meat in 36.7% of these cases, and tended to eat the meat when it was fresh or if the animal species was usually hunted by chimpanzees. However, no evidence indicated that carcasses were avoided when leopard involvement was likely. These results suggest that chimpanzee-sized hominins could potentially confront and deprive leopard-size carnivores of meat.

The social organization of Homo ergaster: Inferences from anti-predator responses in extant primates

Patterns of primate socioecology have been used to suggest that the first truly savanna-dwelling hominin, Homo ergaster, lived in sizeable groups. Here, we revisit these estimates and infer additional features of the social organization of these early hominins based on anti-predator responses observed across the primate taxon. We first show that the effect of habitat on primate group size, composition, and sexual dimorphism is negligible after controlling for substrate use and phylogeny: terrestrial species live in larger groups with more and bigger males than arboreal taxa. We next hypothesize that groups can only survive in open habitats if males are able to engage in joint counter-attacks against the large carnivorans typical of such environments. To test this, we analyze reports on primate counter-attacks against known predators and find these are indeed disproportionately frequent in terrestrial taxa living in open habitats, sometimes even involving the use of tentative weapons. If we subsequently only examine the taxa that are particularly adept at this (chimpanzees and baboons), we find an effect of habitat type on group size: groups on the savanna are larger than those in the forest. We thus infer that H. ergaster lived in very large groups with many males that jointly defended the group against carnivorans, and argue that these counter-attacks will readily have turned into confrontational scavenging and cooperative hunting, allowing Homo to move into the niche of social carnivore. These two features (life in very large multi-male groups and a switch to persistent carnivory) shaped the evolution of our lineage to such an extent that the social organization of H. ergaster may already have contained many key elements characterizing modern day foragers: male bonding, incipient male-female friendships with food sharing, a tendency toward endogamy, and the presence of large communities that eventually turned into the ethno-linguistic units we can still recognize today.