Lethal coalitionary attacks of chimpanzees (Pan troglodytes troglodytes) on gorillas (Gorilla gorilla gorilla) in the wild

Abstract

Intraspecies violence, including lethal interactions, is a relatively common phenomenon in mammals. Contrarily, interspecies violence has mainly been investigated in the context of predation and received most research attention in carnivores. Here, we provide the first information of two lethal coalitionary attacks of chimpanzees (Pan troglodytes troglodytes) on another hominid species, western lowland gorillas (Gorilla gorilla gorilla), that occur sympatrically in the Loango National Park in Gabon. In both events, the chimpanzees significantly outnumbered the gorillas and victims were infant gorillas. We discuss these observations in light of the two most widely accepted theoretical explanations for interspecific lethal violence, predation and competition, and combinations of the two-intraguild predation and interspecific killing. Given these events meet conditions proposed to trigger coalitional killing of neighbours in chimpanzees, we also discuss them in light of chimpanzees’ intraspecific interactions and territorial nature. Our findings may spur further research into the complexity of interspecies interactions. In addition, they may aid in combining field data from extant models with the Pliocene hominid fossil record to better understand behavioural adaptations and interspecific killing in the hominin lineage.

Interspecific interactions between sympatric apes

“…We did not observe gorillas or chimpanzees emitting alarm calls in response to the arrival of the other ape species to a subgroup. No predation attempts between the ape species were observed in this study, but chimpanzees at Loango have been observed killing infant gorillas on two occasions ( Southern et al., 2021 ). However, we did observe bidirectional aggressive threats and contact agonism between apes in the Goualougo Triangle.…”

“…Increased size of interspecific groups could reduce the risk of predation without the increased feeding and mating competition associated with larger groups of conspecifics. Furthermore, recent reports of chimpanzees killing immature gorillas ( Southern et al., 2021 ) prompted us to predict that vulnerable gorillas would avoid close interaction with adult chimpanzees if they posed a risk.…”

“… e Tutin and Fernandez, 1985 , Tutin and Fernandez, 1993 . f Head et al., 2011 , Southern et al., 2021 . g Fay, personal comm.…”

“…Sharing of tree crowns by ape species was observed during foraging in the central African forests of Ndoki ( Suzuki, 1992 ; Yamagiwa et al., 1996a , 1996b ). In contrast, scientists reported two observations of lethal attacks of chimpanzees on immature gorillas in Gabon ( Southern et al., 2021 ). Apes may also alter their habitat use, activity patterns, diet selection, or foraging strategies to avoid interspecific overlap through either spatial or temporal niche partitioning ( Schoener, 1983 ; Mitchell et al., 1990 ; Ziv et al., 1993 ; Eccard and Ylonen, 2003 ).…”

Staring death in the face: chimpanzees' attention towards conspecific skulls and the implications of a face module guiding their behaviour

“…Prey species with similar anatomy such as monkeys are also potentially informative; Boesch & Boesch [ 79 ] report on a juvenile chimpanzee with a colobus monkey skull using a tool to scoop out the brain, and another case involved an adult female using sticks to clean the eye sockets of a colobus skull after she had finished eating the eyes. Moreover, in some parts of Africa, chimpanzees are sympatric with lowland gorillas (for lethal encounters, see [ 80 ]), and gorilla skeletons, exceedingly similar to chimpanzees, are probably encountered. Naturally, being the largest extant land mammal, on dying, elephants leave the largest skulls in their habitats (roughly twice as large as a hippopotamus or a rhinoceros), the only other skulls that bear a superficial resemblance to theirs are those of their closest living relatives, the sirenians (dugongs and manatees) which, being strictly aquatic species, do not share the same habitats.…”