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Chimpanzees make tactical use of high elevation in territorial contexts [1]
['Sylvain R. T. Lemoine', 'Taï Chimpanzee Project', 'Centre Suisse De Recherches Scientifiques', 'Abidjan', 'Côte D Ivoire', 'Max Planck Institute For Evolutionary Anthropology', 'Leipzig', 'Department Of Archaeology', 'University Of Cambridge', 'Cambridge']
Date: 2023-11
Tactical warfare is considered a driver of the evolution of human cognition. One such tactic, considered unique to humans, is collective use of high elevation in territorial conflicts. This enables early detection of rivals and low-risk maneuvers, based on information gathered. Whether other animals use such tactics is unknown. With a unique dataset of 3 years of simultaneous behavioral and ranging data on 2 neighboring groups of western chimpanzees, from the Taï National Park, Côte d’Ivoire, we tested whether chimpanzees make decisions consistent with tactical use of topography to gain an advantage over rivals. We show that chimpanzees are more likely to use high hills when traveling to, rather than away from, the border where conflict typically takes place. Once on border hills, chimpanzees favor activities that facilitate information gathering about rivals. Upon leaving hills, movement decisions conformed with lowest risk engagement, indicating that higher elevation facilitates the detection of rivals presence or absence. Our results support the idea that elevation use facilitated rival information gathering and appropriate tactical maneuvers. Landscape use during territorial maneuvers in natural contexts suggests chimpanzees seek otherwise inaccessible information to adjust their behavior and points to the use of sophisticated cognitive abilities, commensurate with selection for cognition in species where individuals gain benefits from coordinated territorial defense. We advocate territorial contexts as a key paradigm for unpicking complex animal cognition.
Funding: C.C. was supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 679787). L.S. was supported by the Minerva Foundation. S.L., L.S., C.C. and R.M.W. were supported by the Max Planck Society. Research at the Taï Chimpanzee Project has been funded by the Max Planck Society since 1997. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Introduction
“Be before the enemy in occupying the raised and sunny spots (…). Then you will be able to fight with advantage”–Sun Tzu [1]
Intergroup competition in territorial social species incurs costs to individuals, such as loss of feeding and breeding opportunities, increased risk of injuries, and death [2–5]. From an evolutionary perspective, adaptations reducing these costs and increasing competitiveness [6] are expected. These adaptations can take the form of territorial tactics, minimizing the risk imposed by rivals and expressed through adaptive flexible behaviors in the face of intergroup competition. In premodern warfare involving small-scale group conflicts, humans take collective decisions to adopt risk-reduction tactics, such as ambushes and raids into the enemies’ territory [7]. These low-risk tactics are adopted in conditions of numerical superiority [8], maximizing the benefits and minimizing the costs of out-group conflicts. In territorial social species, and in societies that employ flexible fission–fusion dynamics, characterized by subgroups or parties of varying sizes, conflict escalation is determined by the imbalance of power [7]. Tactical decisions to engage in a conflict at low cost, therefore, can be facilitated by early numerical assessment of traveling parties of own and neighboring groups (hereafter called “rivals”) [8].
Taking coordinated decisions to adopt tactics that reduce risks involves demanding cognitive abilities [9] and the ability to adjust own’s behavior based on available knowledge [10]. Thus, warfare is hypothesized to be a driver of brain size and cognition in the hominid lineage [11,12] and potentially among non-ape social species [13]. For example, warfare—a permanent but unpredictable stage of threat and regular conflicts between subgroups of varying sizes—requires tactical and coordinated decisions to outcompete out-groups, while considering the current dynamics of the threat posed by rivals. However, there are few empirical tests of this idea outside of humans [11,12], making evolutionary arguments challenging. As such, it is not known if other species use elaborated territorial tactics, nor if tactical capacities are already evident in hominids, especially in species which engage in intense territorial competition that resembles human small-scale conflicts. While assessing underlying cognition involved in territorial tactics is well recognized to be challenging [14], here we aim at taking a crucial step to assess whether under natural conditions, behavioral patterns are consistent with those expected given tactical decisions that result in minimizing risks.
To this end, we focus our behavioral analysis on the use of topography by 2 neighboring groups of wild western chimpanzees (Pan troglodytes verus) from the Taï National Park (TNP) [15], Côte d’Ivoire. In premodern human warfare, a common low-risk tactic favoring rivals detection and assessment of their power is the use of topography [16–18], particularly elevated areas that facilitate visual and auditory early detection of rivals [19]. Archaeological and historical evidence suggests that high elevation is used in a territorial context by humans to ambush enemies [20], to gain protection, as elevated areas are difficult to attack by enemies without detection [21], and to acquire tactical advantage over enemies through early detection of their numbers and location [22]. Whether collective decision-making [9] to use the topography to facilitate detection of rivals is a unique feature of human warfare or whether it is a shared capacity with other animals, however, has not been investigated. Use of rugged tectonic landscape has been proposed as an adequate niche during the transition between tree and ground dwelling in ancient hominins [23,24], enabling better protection against predators. Given the potential highly competitive context between conspecifics and between hominin species, controlling and exploiting key high points in the landscape could have provided significant territorial advantages. Unraveling how wild chimpanzees combine territorial behavior with landscape use would shed light on the importance of high elevated landscape throughout human evolution.
In fact, while elevation use for acoustic signaling [25] to conspecifics and for predator detection [26] is common in animals, its use in intergroup competition remains unknown, for example, whether or not information gathered at high elevation is used to gain a tactical advantage over out-groups. In territorial species where numerical assessment is key in intergroup conflicts, one would expect behaviors that (i) enhance early detection of out-groups; and (ii) enable advantageous tactical use of this information to minimize risk, such that an advantageous situation promotes engagement with the rival groups, while a disadvantageous situation promotes retreat. In contrast to signaling and anti-predation, hostile rivals pose a higher cognitive challenge, particularly in fission–fusion societies in which groups coordinate their territorial activity and their numbers and movements regularly change.
In chimpanzees (Pan troglodytes), a social species with fission–fusion group dynamics like humans [27,28], and one of humans’ closest living relatives, intergroup competition between rival neighboring communities likely acts as a selective pressure [3,4], favoring in-group cooperation [4,29] during territorial contexts, such as collective border patrols [30,31] and coalitionary killing of rivals [32]. As in premodern human warfare, the imbalance of power between groups determines conflict escalation [7] in chimpanzees. Chimpanzees show abilities to assess rivals’ numbers from a distance by not approaching when rival vocalizations indicate numerical superiority [8,33–35]. Whether chimpanzees buffer the costs of territorial disputes through tactical use of high elevation to preemptively gather information about rivals and use this information to make tactical decisions to approach or retreat from rivals, has so far not been addressed.
Here, we hypothesized first that a tactical use of elevated areas by chimpanzees assists in early detection of rivals. We first analyzed how the likelihood to stop at the highest hills located within the overlap border area of 2 neighboring chimpanzee groups is affected by the traveling direction, own numbers, and intercommunity distances. Stop events (resting for >5 min) at high elevation offer sensory advantages, potentially allowing to gather information about rivals. However, climbing hills may be energetically costly, so that their use must be carried out when the potential benefits of rival detection outweigh energetic costs. We predicted that Taï chimpanzees would be more likely to stop at peripheral hills when traveling towards the border than towards the territory center, when in low rather than in high numbers (as risks of an unfavorable imbalance of power are higher when in low numbers), and when rivals are less likely to have already been detected—namely for longer intercommunity distances. We also considered the number of peripheral hills used before and after each occurrence of hill use per day as this may impact the likelihood to stop on a particular location. If information on rivals has already been gathered from preceding hill tops, climbing more hills could be redundant and increase energetic expenditure. Here, we predicted that, as chimpanzees move towards the territory border, they will be less likely to stop at peripheral hills if they have already stopped on previous peripheral hills.
Second, we predicted that chimpanzees preferentially engage in activities that favor information gathering about rivals (resting rather than feeding or traveling) when in elevated border areas relative to elevated central areas. Changes of activity between central area and periphery are well known in chimpanzees [33], but not whether local elevation modifies activity budgets. Consequently, to account for the impact and interplay between elevation and territory location, we tested how the interaction between elevation and location (measured by kernel values) influences the likelihood to observe chimpanzees rest, feed, and travel. In tropical forest habitat, a low-visibility environment, early detection of conspecifics occurs primarily through auditory rather than visual channels. Vocalizations and buttress drums are among the first detected long-distance signs of rivals’ presence in 73% of neighboring encounters [4]. Chimpanzee pant hoots and buttress drums can be heard >1 km away [36] and elevation above the canopy level improves sound detection over long distances [37]. Thus, decisions to engage in activities (resting) that favor auditory attentiveness to distant sounds, particularly when on high elevation in dangerous border areas, could maximize acoustic detection of rivals. Activities differ in their potential to increase auditory attentiveness and in creating a suitable environment for information gathering about rivals. Feeding and traveling activities involve a particular focus on a goal and usually produce noise and often require stopping to focus on distant vocalizations (S1 Video). Resting offers the best conditions for information gathering from the surrounding auditory environment, especially to detect distant chimpanzee sounds.
Third, we hypothesized that early detection of rivals enables tactical use of information obtained in elevated border areas to make low-risk travel decisions. Riskier situations to the in-group occur when rivals are in high numbers while own numbers are low and when rivals are located close by (<2,000 m). We used simultaneous data on 2 neighboring chimpanzee groups to assess if potential information gathering about neighboring rivals during high elevation use within the overlap border area would lead to risk-reduction tactical movement decisions to retreat from or advance toward rivals. We expected that, if chimpanzees use hills in the territory border to gain information about neighboring rivals’ current numbers and location, chimpanzees’ subsequent travel decisions after hill use (but not after use of low-lying peripheral areas) will maximize benefits and reduce the risks of territorial engagement. We examined the tactical movement patterns within the 30 min following each stop (>5 min) event at peripheral hills (n = 304 occurrences) and low-lying peripheral locations (n = 321 occurrences) together, with a model assessing the likelihood to advance towards rivals’ location. Given that high hills may allow to gather information on both rival numbers and location, we tested in this model the effect of the three-way interaction between elevation, imbalance of power, and intercommunity distance. Fig 1 provides a graphical representation of the predictions based on this interaction. In these analyses, we assessed the imbalance of power by the arithmetic difference between adult party size of the location users and the closest adult party size of rivals. In Taï chimpanzees, the number of adult males determines territory increase while the whole group enables an efficient territory defense [38]. Since both adult males and females are involved in intergroup encounters [39] and border patrols [31,40], the competitive ability of Taï chimpanzees is best reflected by all adults rather than by only adult males. We measured intercommunity distance at the time of departure from the specific peripheral location.
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TIFF original image Download: Fig 1. Predictions for the likelihood chimpanzees advance towards or retreat from rivals’ location, depending on elevation, imbalance of power, and intercommunity distance. Sitting chimpanzee silhouettes: those gathering information (illustrated by the “ear” drawing); standing chimpanzee silhouettes: rival chimpanzees; black arc circles depict the auditory detection range that reaches further from higher elevation; gray circles depict the putative risk imposed by rivals (low, medium, high); red arrows: likelihood to advance towards rivals; purple arrows: likelihood to retreat from rivals; thickness of arrows: extent of the likelihoods.
https://doi.org/10.1371/journal.pbio.3002350.g001
We investigated these 2 hypotheses—tactical use of elevation for information gathering and tactical decisions based on information obtained when using high elevation—in the Taï western chimpanzees (Pan troglodytes verus). High elevation in the Taï forest is characterized by granitic inselbergs—isolated rocky hilltops with little canopy cover—which offer ideal listening locations for early detection of rivals. We simultaneously collected behavioral and GPS ranging data between December 2013 and October 2016 on 2 neighboring chimpanzee communities who share a territory overlap area, the South and East groups [41]. Human observers conducted daily focal follows of 58 habituated individual chimpanzees for 8 to 12 h/day (10,480 and 10,706 observation hours across 1,287 and 1,350 follow-days for South and East, respectively). We continuously recorded the individuals visible within the focal party, allowing us to assess adult party sizes. We reconstructed the topographical landscape across the groups’ territories using the GPS points accrued during each minute of focal sampling, recording location and elevation (range of elevation: 115 to 287 m above sea level, Figs 2, S1 and S2). Simultaneous focal follows of individuals from both neighboring communities allowed us to assess intercommunity GPS distances. Given the fission–fusion nature of chimpanzee societies, the intercommunity distance represents the minimum known distance between the neighboring groups.
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