Gorillas reside in sympatry with chimpanzees over the majority of their range. Compiling all known reports of overlap between apes and augmenting these with observations made over twenty years in the Ndoki Forest, we examine the potential predation-related, foraging, and social contexts of interspecific associations between gorillas and chimpanzees. We reveal a greater diversity of interactions than previously recognized, which range from play to lethal aggression. Furthermore, there are indications that interactions between ape species may serve multiple functions. Interactions between gorillas and chimpanzees were most common during foraging activities, but they also overlapped in several other contexts. From a social perspective, we provide evidence of consistent relationships between particular chimpanzee-gorilla dyads. In addition to providing new insights into extant primate community dynamics, the diversity of interactions between apes points to an entirely new field of study in early human origins as early hominins also likely had opportunities to associate.

In this study, we compile previously published observations of interactions between sympatric great apes and augment this with new longitudinal studies of identified apes in the Goualougo Triangle to examine the contexts and potential functions of these interspecific associations. More specifically, we categorized these interspecific associations as cofeeding (foraging on the same food source, such as within the same tree crown), feeding in proximity (feeding within 50m, but not on the same food source), spatial intersection (within 50m, but not foraging or interacting), or social interaction (aggression, play, or sexual encounters between members of different species). In this study, we predict that if interspecific ape groups aid in avoiding predation, then we would expect both ape species to recognize the alarm call of the other species and that smaller chimpanzee parties would frequently associate with gorillas for protection from predators. We also predict that more vulnerable chimpanzee parties, such as those comprised of a single individual or a mother with a dependent offspring might be more likely to associate with gorillas than parties comprised of a greater number of adult chimpanzees, particularly parties with large numbers of adult males. If chimpanzees threaten the survival of vulnerable gorillas, then we would expect to see attempted capture of gorilla infants by chimpanzees or avoidance by gorillas with infants of groups of chimpanzees, particularly groups with adult male chimpanzees. If mutual attraction to food sources is a driving force in associations between species, then we would expect overlap between apes to coincide with the availability of particular foods. It is also possible that one species may capitalize on the other’s knowledge of rare and asynchronously available foods (such as figs). If either ape species is gaining social benefits, then we would expect to detect consistent social contacts and relationships between species. In reviewing all reported interactions between sympatric apes, we consider the detection of overlap events at different sites and the frequency of interspecific associations in various contexts (e.g., foraging, spatial intersection, agonism). It is clear that the present study overcomes two key challenges that are associated with the study of sympatric great ape social interactions: 1) both species are habituated to the presence of researchers and 2) members of the great ape groups are individually identifiable by researchers. With regard to the biological importance of interspecific interactions, they directly link to fitness via reduction of predation risk and enhanced foraging potential which is essential to understanding the present and past primate communities.

Given this evidence, consistent observations of great ape overlap likely represents active choice over avoidance or simply chance encounters. Although not yet detailed in the literature, there are potential social benefits of mixed-species associations and social relationships between great ape species. In addition to establishing tolerant relationships and gaining services (such as grooming or alloparental care), the potential for information transmission should also not be overlooked as both chimpanzees and gorillas are capable of social learning. Interspecific associations and social interactions may provide pathways for the spread of behaviors and innovations between ape societies. However, a potential trade-off of social contact between closely related species is the spread of infectious diseases. This study will contribute to the long-term monitoring of social contacts between sympatric apes to provide quantifiable metrics on potential cross-species pathways of disease transmission which is a conservation concern for these endangered species.

Interactions between sympatric great apes have most commonly been characterized as strategies to avoid competition or as direct competition over food sources (). Based on the nest site choice of sympatric apes in east Africa, it was concluded that chimpanzees avoided nesting in trees bearing fruits eaten by mountain gorillas, Gorilla beringei (). Scientists observed contest competition between chimpanzees and mountain gorillas in the Bwindi forests of Uganda (). The counterparts of mountain gorillas in western Africa, lowland gorillas (Gorilla gorilla) are more frugivorous than eastern gorillas (Gorilla beringei) and thus one might expect them to show a higher degree of overlap with chimpanzees than their eastern counterparts who incorporate less fruit in their diet. There is some evidence that foraging western lowland gorillas and chimpanzees converge during times of fruit abundance and diverge during fruit scarcity (). Sharing of tree crowns by ape species was observed during foraging in the central African forests of Ndoki (). In contrast, scientists reported two observations of lethal attacks of chimpanzees on immature gorillas in Gabon (). 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 ().

The potential benefits of interspecific associations among monkeys have been relatively well documented ( Table 1 ). However, comparable assessments of sympatric great apes are lacking despite the fact that gorillas (Gorilla spp.) and chimpanzees (Pan troglodytes) reside in sympatry over the majority of their range ( Table 2 and Figure 2 ). It is challenging to observe interspecific interactions of these species as there are few sites where both species have been habituated to the presence of researchers. In the few studies that have considered gorilla and chimpanzee sympatry, potential anti-predation advantages have generally been considered irrelevant because it was assumed that the large body sizes of great apes were sufficient to deter predation. However, documented leopard attacks provide evidence to the contrary (). 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 () prompted us to predict that vulnerable gorillas would avoid close interaction with adult chimpanzees if they posed a risk.

Both western lowland gorillas (Gorilla gorilla) and mountain gorillas (Gorilla beringei) reside in sympatry with chimpanzees (Pan troglodytes) throughout most of their ranges. Inset shows members of the Moto chimpanzee community and Loya gorilla group cofeeding in the crown of a Treculia africana tree in the Goualougo Triangle, Republic of Congo.

Foraging includes both cofeeding and feeding in proximity. Spatial intersection included chimpanzees and gorillas being observed within 50m of each other, but with no apparent interaction. At Loango, two particular interactions between chimpanzees and gorillas involved lethal aggression and so were categorized as agonism. Social interactions among sympatric apes in Goualougo were observed in various contexts.

“Context/Potential Function”: F = foraging efficiency, A = anti-predation, S = social advantages (if listed within parentheses, context-dependent functional advantages were reported). “Benefit” defines which species (or all) were reported to benefit from the mixed species associations. Other species may have been studied in these listed investigations (e.g.,); however, only species discussed with possible functional advantages to mixed species associations are listed here. ∗Note:review the literature on polyspecific callitrichine troops, reporting very low costs to mix-species association of this taxa; Boinksi (1989) reports C. capucinus to be the primary benefactor (anti-predation) of their sustained association with S. oerstedii who may experience a plausible reduction of foraging efficiency;reports that C. mitis appear to be the primary benefactors in some instances while both species experience only minor costs due to mixed-species associations.

Theoretical expectations suggest that predator avoidance and/or enhanced foraging may be potential benefits of mixed-species associations, while costs may include increased feeding competition or parasitism () ( Figure 1 ). Associating with another species facing similar predators could improve the probability of predator detection (), lead to predator confusion (), increase dilution effects (), or enhance the ability to more effectively deter predators (). Furthermore, the benefits of locating food more efficiently through another species’ knowledge of food availability and/or their differential ability to access particular food sources may defray the potential costs of interspecific associations (). There are also potential social benefits of interspecific grouping which may include establishing tolerant relationships (), grooming services (), alloparental care (), and reproductive advantages of increasing group size without increasing breeding competition (). Furthermore, it should be noted that these potential functions need not be mutually exclusive () and that one primate species may benefit more than another () or at another’s cost ().

The contexts of interspecific associations are predation-related, foraging, and social. The potential benefits of interspecific associations are listed, as well as the deterrents of chimpanzee and gorilla association. It should be noted that these are not mutually exclusive and that interspecific interactions occur in several contexts and may serve multiple functions.

Primate communities may be comprised of ten or more species, with local compositions and interactions determined by abiotic and biotic factors operating at variable spatial scales (). The organization of these communities has typically been attributed to bottom-up forces, wherein populations are regulated by resource limitations (). Interspecific associations can generally be defined as two or more species in such close proximity that they can be regarded as members of the same social unit, during which they may engage in both direct and indirect interactions characterized by territoriality, aggressive behavior, agonism, or competition over shared resources (). From a traditional perspective, sympatric species that occupy similar ecological niches would be expected to intensely compete and eventually exclude each other from communities (). However, recent research across an array of species has revealed complex combinations of both resource competition and facilitation (). It has been shown that a species can actually enhance the survival, growth, or fitness of another species by providing protection from predation, accumulating nutrients consumed by the other species, or even by mitigating disturbance ().

We observed a range of social interactions between chimpanzees and gorillas in the Ndoki Forest, ranging from affiliative to aggressive encounters. Interspecific aggression was bidirectional and most frequently consisted of threats. During this study in the Goualougo Triangle, contact aggression was rare and never escalated to the types of lethal attacks documented in Loango, Gabon (). Affiliative interactions included play with individuals of both species engaged in chasing, wrestling, play biting, and play hitting. An interspecific sexual interaction was observed in which a juvenile male gorilla repeatedly dorsally mounted and thrust his groin into contact with the anogenital region of an immature female chimpanzee. We also observed gesturing between species to initiate social interactions. Intriguingly, chimpanzees exhibited chest-beating which is a behavior characteristic of gorillas. Although most interactions involved a chimpanzee-gorilla pair, we also observed polyadic interactions.

In accordance with conventional definitions, we refer to repeated patterns of both associations and interactions between individually identifiable chimpanzees and gorillas as social relationships ( Figure 4 ) (). Although on average chimpanzees had more total social ties (degree, chimpanzee: 30.67 ± 8.58, n = 36; gorilla: 27.6 ± 1.34, n = 5) and displayed higher levels of social integration (eigenvector centrality, chimpanzee: 0.48 ± 0.32, n = 36; gorilla: 0.14 ± 0.03, n = 5) than gorillas when feeding on figs, the range of network measures for all the individual gorillas in this context was within the range of chimpanzees. The silverback male had the lowest eigenvector centrality score among the gorillas in this context, whereas the younger male gorillas had the highest measures of connectedness and social integration. This was confirmed by average dyadic simple ratio index associations of gorillas from 2017 to 2020 which showed that the younger males (Kao = 0.29 ± 0.17, Modiaye = 0.26 ± 0.16, Eteko = 0.22 ± 0.16) had stronger collective ties to chimpanzees and engaged in more social relationships with chimpanzees than the silverback (0.21 ± 0.12) or female (0.24 ± 0.16) and infant (0.21 ± 0.10). In accordance with differences in network ties between chimpanzee-gorilla dyads, we observed that some individuals repeatedly sought particular social partners over time.

The graphs show gorillas (green-colored nodes) and chimpanzees (rose-colored nodes) encountered in parties together while cofeeding on Ficus from 2014 to 2016, cofeeding on Ficus from 2017 to 2020, in other social contexts from 2017 to 2020, and aggregated across all contexts from 2014 to 2020. Node size is scaled to layer-specific eigenvector centrality (larger nodes indicate greater network importance) and shaded darker with age. Edge widths are scaled to the strengths of gorilla-chimpanzee dyadic relationships (thicker lines indicate stronger relationships). Horizontal lines across these network layers connect nodes that represent the same individual across all contexts. Overall, the integrated structure of these networks across time and contexts highlights the consistency in relationships between chimpanzees and gorillas.

The beneficial sociality hypothesis predicts that species are attracted to mixed-species associations by the social benefits of interactions with and/or forming a relationship with an individual(s) of another species. Network statistics were used to assess the strength of social associations (calculated as simple ratio indexes) between individually identifiable gorillas and chimpanzees. We confirmed that the observed distribution of gorilla and chimpanzee association rates significantly differed from those that would be expected by randomly permuted network datasets (2014-2020 observed mean ± s.d. = 0.16 ± 0.17, random = 0.15 ± 0.15; p < 0.001, 1000 permutations ()). Furthermore, we found that particular dyads were associated on multiple occasions and across different contexts (cofeeding, resting, traveling, socializing) over a period of at least six years ( Figure 3 ). We confirmed the consistency of these associations using a subset of the multidimensional network that contained those individuals active in all layers (mean edge overlap = 71.07%; Spearman correlation analysis; see STAR Methods ). All layers were highly correlated (Ficus 2014 to 2016 – Ficus 2017 to 2020: r= 0.93; Ficus 2014 to 2016 – social 2017 to 2020: r= 0.82; Ficus 2017 to 2020 – social 2017 to 2020: r= 0.82).

To assess whether one species was potentially benefiting from the knowledge of the other species, we examined the temporal pattern of interspecific associations at cofeeding events when we followed the gorilla group two days prior to, during, and for two days after cofeeding events with chimpanzees. A total of 10 cofeeding events met this criterion, with five of the trees representing species with synchronized fruiting and the other five being asynchronous Ficus resources (figs). At four of the synchronous resources, the gorilla group was observed feeding at the trees on days prior to the cofeeding event and continued to feed on these trees for two days after, suggesting that their presence at the trees was independent of chimpanzees. In contrast, gorillas were not observed visiting any of the five Ficus locations on the two days prior to the cofeeding event and visitation to figs was rare on the days after cofeeding. During ape follows, we also observed the gorilla group immediately change their travel direction to head toward chimpanzee vocalizations originating from the canopy of Ficus with ripe figs. Chimpanzees did not exhibit any similar behaviors that would indicate they were attracted to food sources by gorillas.

The enhanced foraging hypothesis predicts that interspecific associations between apes would coincide with particular foods. Supporting this assertion, these gorillas and chimpanzees are most often associated in contexts where both species were feeding either on the same food source (cofeeding) or on different foods but in close proximity. Cofeeding at the same food source (e.g., same tree) represented 34% of interspecific associations, with another 18% of observations involving apes foraging in close spatial proximity but on different foods. At least 20 different plant species were targeted by apes during cofeeding events in this study ( Table 4 ), which greatly expands our knowledge of the diversity of resources shared during interspecific associations. Despite the extreme rarity of Ficus spp. in this region, figs were consumed during 64% of the observed cofeeding events.

Cofeeding events involved at least one member of both species feeding at the same food source (e.g., in the same tree crown). Lifeform describes plant morphology of the species cofed upon by sympatric apes (seefor the lifeforms of Ficus spp.). Size classes of trees are small (10-30 cm diameter at breast height, DBH), medium (30-80 cm DBH), and large (>80 cm DBH).

While gathering in large nesting groups could serve to reduce the risk of predation at night, previous research has shown that gorillas and chimpanzees have distinct habitat preferences for nesting which could preclude association in this context (). In our transect surveys of ape nests, we observed that only 2.4% of all fresh chimpanzee nest sites/groups (5 of 206 total sites) were within 50m of a gorilla nest site. A slightly higher percentage of fresh gorilla nest sites (6.7%, 7 of 105 sites) were found within 50m of chimpanzee nest groups.

Counter to predictions of reducing vulnerability by increasing numbers, we found that the initial chimpanzee parties in association with gorillas were larger than parties consisting only of chimpanzees ( Table 3 ). The anti-predation hypothesis predicts that lone individuals would more frequently associate with gorillas than other types of chimpanzee parties, but we found that interspecific associations occurred in all types of chimpanzee parties. Interspecific relationships were strongest between gorillas and adult female chimpanzees with dependents ( Figure 3 ), followed by gorilla associations with mature male chimpanzees, and then mature chimpanzee females without infants.

Chimpanzees (rose-colored nodes) and gorillas (green-colored nodes) maintained a selective set of relationships with particular individuals. Also, gorillas associated twice as much with chimpanzee females that had dependent offspring (0.37 ± 0.14) compared to either chimpanzee males (0.18 ± 0.09) or females without dependents (0.12 ± 0.07). Female apes are represented by circles and males as squares. All nodes are shaded darker with age and scaled in size to represent eigenvector centrality. Strong relationships are represented by thicker and darker lines.

Chimpanzee parties consisted of all individuals travelling, feeding, resting, or socializing within 50m of one another (adopted fromand). Interspecific associations can generally be defined as two or more species in such close proximity that they can be regarded as members of the same group. We have applied this definition to identify interspecific associations.

The anti-predation hypothesis predicts that apes would attend and respond to the alarm calls of the other species and that smaller groups would seek association with other species to increase vigilance and overall subgroup size. Both species were observed responding to the alarm vocalizations of the other species. Responses included increasing vigilance by orienting toward the origin of the call and visually monitoring the response of other apes in the subgroup. 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 (). However, we did observe bidirectional aggressive threats and contact agonism between apes in the Goualougo Triangle. We also noted behavioral mechanisms which may mitigate such predation attempts, such as adult female gorillas retrieving their young infants who approached and attempted to initiate play with young chimpanzees. However, several play bouts between a subadult male chimpanzee and juvenile male gorilla were observed. Contrary to staying in close proximity to the silverback as protection from predation, juvenile and subadult gorillas regularly traveled more than 300 m from their group to join a chimpanzee party.

In our review of published reports, we found a total of 33 documented interspecific interactions (and two other unpublished interactions) at eight sites, representing studies conducted from 1966 to 2020 ( Table 2 ). While conducting daily follows of chimpanzees and gorillas from 1999 to 2020 in the Goualougo Triangle, we observed an additional 285 interspecific associations between great apes. More specifically, research teams tasked with following chimpanzees in the Moto Community located in the Goualougo Triangle study area observed 206 interspecific associations with gorillas between July 1999 and February 2020. In 2013, a gorilla group within the Moto chimpanzee territory was habituated to humans and teams following the gorilla group observed 62 interspecific associations between February 2013 and February 2020. On 17 occasions, both field teams were present during interspecific associations. Duration of association between chimpanzees and gorillas ranged from 1 min to more than 8 h, with a mean of 67.9 min (median = 32.5, n = 78 cases in which full duration was observed). Importantly, our reported encounters represent minimal frequencies of occurrence as it was not possible to simultaneously follow all chimpanzee parties or conduct ape follows every day throughout the study period.

Discussion

Southern et al., 2021 Southern L.M.

Deschner T.

Pika S. Lethal coalitionary attacks of chimpanzees (Pan troglodytes troglodytes) on gorillas (Gorilla gorilla gorilla) in the wild. In this study, we reveal a greater diversity of interactions than previously documented among sympatric apes, including social relationships between members of different species that persisted over years. Furthermore, we found that interspecific associations between great apes occur in several different contexts and thus may have multiple functions. Supporting the anti-predation hypothesis, both species appropriately responded to the alarm calls of the other species which could increase the detection of potential threats. We have also observed apes approach, investigate, and retreat from recently killed victims of predation that were the other ape species. However, larger chimpanzee parties associated with gorillas more often than smaller parties which are counter to the antipredation hypothesis and could result in increased feeding competition. Some observations could be interpreted either way, such as those of individual gorillas leaving the protection of their group’s silverback to join a chimpanzee party that was more than a hundred meters away. The majority of associations between gorillas and chimpanzees occurred in foraging contexts and we observed cofeeding at many more types of food sources than previously documented. In contrast to predictions of competition between species, nearly all interspecific associations were tolerant or affiliative. Aggression was observed between gorillas and chimpanzees, but did not escalate to killing as reported from Loango, Gabon (). The aim of our study was to broaden perspectives about interspecific interactions that occur within primate communities, particularly those among sympatric great apes, which can be used to inform reconstructions of the contacts and exchanges between early hominin species.

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Oko R.A. Sympatric chimpanzees and gorillas in the Ndoki forest, Congo. Stanford, 2008 Stanford C.B. Apes of the Impenetrable Forest: The Behavioral Ecology of Sympatric Chimpanzees and Gorillas. The majority of associations occurred at a variety of Ficus spp. () which is similar to reports of ape cofeeding on figs at other sites. Figs are asynchronous and found at extremely low densities across the region. Although further research is warranted on the temporal patterning of interspecific associations at particular food resources, we suggest that gorillas may exploit chimpanzee knowledge of the location of ripe figs. Based on our observations in the Goualougo Triangle, large parties of chimpanzees would often broadcast their location while feeding on figs which gorillas responded to by changing their travel orientation toward the food resource. Other loci of association consisted of large trees that produce fruits (e.g., Chrysophyllum lacourtianum, Pancovia laurentii) that are available for weeks compared to the relatively brief period of figs (3-4 days). On some occasions, part of the gorilla group would climb into the tree crown to feed with chimpanzees while others remained on the ground and fed upon fallen fruit. Further research on the availability of preferred foods across habitats is needed to improve our understanding of how competition and niche partitioning vary across ape populations.

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Pika S. Lethal coalitionary attacks of chimpanzees (Pan troglodytes troglodytes) on gorillas (Gorilla gorilla gorilla) in the wild. Additional study of how relationships between socially acquainted gorillas and chimpanzees change with time and across contexts would also provide clarity on the construction, maintenance, and limits of interspecies tolerance. On several occasions at food sources, we observed young gorillas and chimpanzees seeking out particular partners to engage in bouts of play which may afford unique developmental opportunities to extend their social, physical, and cognitive competencies (). Subadults were observed to engage in interspecific play, but these social dynamics seemed to shift as they matured to adulthood. Additionally, interactions between apes may also take on different severity depending on where they occur in the species’ respective home ranges. Both species have core areas that are surrounded by peripheral zones which are visited less frequently and tend to have more aggressive interactions than core areas (). For example, most of our observations of interspecific interactions occurred within the core area of the chimpanzee community range which is where the gorilla group resides. This is in contrast to observations of lethal killing in the periphery of a chimpanzee community range (). Different groups of sympatric apes may have other types of home range overlap that influence their interactions. Future studies that spatially position interspecies interactions and outcomes within the contexts of the apes’ home ranges will shed light on the dynamics of territoriality and perceptions of risk in certain regions of their ranges. Further, determining the shared strategies of gorillas and chimpanzees holds the promise of revealing a greater depth of their social awareness than previously imagined.

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et al. Contemporaneity of Australopithecus, Paranthropus, and early Homo erectus in South Africa. Our review of interspecific associations between extant great apes indicates that they occur in several different contexts and may serve multiple functions, with broad implications for ape health and culture. It also provides a variety of advantages that are relevant to understanding the behavioral patterns of both present and past primate assemblages. Indeed, multiple hominin species are known to have contemporaneously occupied the same geographic regions, implying that they had opportunities to interact with each other. In eastern Africa, Paranthropus boisei and multiple species of early Homo are found in the Turkana Basin between roughly 2.0 and 1.5 million years ago (Ma) (). Hominin fossils are distributed in an expansive area around Lake Turkana, and there is some evidence that resident hominin species may have partitioned their environment (), so it is difficult to assess how frequently interspecific associations might have occurred. However, in South Africa at ∼2.0 Ma, species of three hominin genera (Australopithecus sediba, P. robustus, and early Homo) occupy a landscape no larger than the Goualougo Triangle research area (). Although sympatry among these taxa cannot be demonstrated conclusively, at a minimum they would have had adjacent ranges whose boundaries might have shifted over time (allowing their fossils to be deposited in the same small area). Thus, these hominins were likely either sympatric or parapatric, implying some opportunity for interaction and transmission of information. Although direct evidence of interspecies interactions in the early hominin fossil record will probably remain elusive, our findings point toward a research program modeling the nature and likelihood of such interactions given what is known about paleoecology, geography, adaptation, and the density of fossil sampling. In short, we can no longer assume that these interactions did not occur.

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et al. Human impact erodes chimpanzee behavioral diversity. We show that extant chimpanzees and gorillas do not so completely partition their habitats to preclude regular interaction and the formation of lasting social relationships. This is interesting given that the chimpanzee, gorilla, and hominin clades diverged from each other during the Miocene when climatic conditions were becoming cooler and drier, which presumably would have placed ancestral ape populations under increasing ecological stress (e.g.,). One might have expected that the divergence of these populations would have been hampered by associative behaviors, especially during the period prior to the evolution of reproductive isolation. It is worth considering whether evolutionary processes might have driven the divergence of these clades without eliminating some level of behavioral interaction, or if the interactions seen among gorillas and chimpanzees today are evolutionarily novel. Unfortunately, anthropogenic disturbances and degradation are removing the environments and resources that draw them together, jeopardizing the future of these apes and with them the opportunity to further understand the relationships between and knowledge shared by our closest living relatives ().