A group of Issa Valley chimpanzees navigate an open woodland crown to forage on new leaves.
Image by Rhianna C. Drummond-Clarke/Greater Mahale Ecosystem Research and Conservation (GMERC)
What is particularly striking is the complete absence of any doubt among the scientists that evolution, driven by natural selection, is the correct framework for interpreting these observations. At no point do they resort to supernatural explanations or even hint that evolution might be insufficient to explain the data. On the contrary, their conclusions seamlessly integrate with the existing evolutionary narrative, demonstrating how behaviours seen in modern chimpanzees provide a living window into the adaptations of our shared ancestors. This directly undermines the creationist claim that mainstream biologists are “abandoning” evolution in favour of unproven religious explanations—a claim that has no basis in reality.
Creationist dogma insists on static, unchanging “kinds” and appeals to unverifiable supernatural causes. Yet, studies like this show that every aspect of our evolutionary past—anatomical, genetic, and behavioural—can be explained through natural processes, without the need for divine intervention. The evidence for a shared ancestry between humans and other primates grows with every new study, while creationism remains stuck with no predictive power and no scientific methodology.
In short, this research reinforces the power and universality of the Theory of Evolution. The scientists involved didn’t set out to “disprove creationism”; they simply applied rigorous observation and analysis, and the results—once again—fell squarely on the side of evolution. Far from being abandoned, the ToE continues to thrive as the backbone of modern biology, while creationism, with its untestable supernatural entities, offers no explanation at all.
Background: The Issa Valley Chimpanzees. The Issa Valley, located in western Tanzania’s Uvinza District, lies within the Greater Mahale Ecosystem near the eastern edge of the chimpanzee range. It is part of a savannah–woodland mosaic — a patchwork landscape of open grassland, woodland, and riparian forest. This habitat closely resembles the environments thought to have been occupied by early hominins during key stages of human evolution.The researchers from the Department of Human Origins, Max Planck Institute of Evolutionary Anthropology, Leipzig, Germany, GMERC, Ltd., Mpanda, Tanzania and the Department of Anthropology, University College London, London, United Kingdom, have explained their findings and its significance in a Frontiers news release.
The Issa chimpanzees (Pan troglodytes schweinfurthii) are part of a long-term research project managed by the Ugalla Primate Project (UPP), established in 2001. These chimpanzees are not habituated to human presence in the same way as those in sites like Gombe or Mahale, which makes direct observation more challenging—but also helps preserve more natural behaviour.
What makes the Issa Valley population especially valuable is their environment: unlike the dense tropical forests inhabited by many chimpanzees, the Issa chimps live in a much more open, seasonal habitat. This allows researchers to ask key evolutionary questions—such as how early hominins might have behaved in similar settings. Their locomotion, use of trees, and occasional upright walking offer critical insights into how and why bipedalism evolved.
These chimpanzees have shown that arboreal life remains essential even in open landscapes, which directly challenges the assumption that early hominins abandoned trees once they ventured into the savannah. The Issa Valley thus serves as a kind of living laboratory for understanding our evolutionary past.
How much time did our ancestors spend up trees? Studying these chimpanzees might help us find out
A new study on savannah-living chimpanzees could help answer some of paleoanthropology’s oldest questions about the origins of walking on two legs. Some theories suggest that the spread of savannahs pushed hominins to walk upright on the ground, a more efficient way to move in this environment. But many early hominins are still well-adapted for climbing. The study found that chimpanzees living in a savannah-mosaic environment continued foraging in trees, and spent most time in trees foraging for the most abundant or nutritious foods. These results suggest that those arboreal adaptations remained critical for early hominins’ survival after bipedalism appeared. They might even have contributed to the development of walking on two feet.
It’s hard to tell when — and why — our ancestors got down from trees and started walking on two legs. Many early hominins capable of bipedal walking were also well-adapted for climbing, and we lack fossil evidence from a key period when climate change turned forests into open, dry woodland called savannah-mosaic, which might have pushed hominins onto the ground. Now a study on modern chimpanzees could help fill in the gaps. Scientists observing chimpanzees in the Issa Valley, Tanzania have shown that despite living in a savannah-mosaic, they frequently climb trees for valuable food — potentially explaining why early hominins kept their arboreal adaptations.
For decades it was assumed that bipedalism arose because we came down from the trees and needed to walk across an open savannah. Here we show that safely and effectively navigating the canopy can remain very important for a large, semi-arboreal ape, even in open habitat. Adaptations to arboreal, rather than terrestrial, living may have been key in shaping the early evolution of the human lineage.
Dr Rhianna Drummond-Clarke, lead author
Max Planck Institute for Evolutionary Anthropology
Leipzig, Germany.
Habitats and hunger
Issa Valley is divided between a small amount of thick forest surrounding riverbanks and open woodland. The chimpanzees forage more in the woodland during the dry season, when it offers more food. Their habitat and diet are comparable to those of some early hominins, which means their behavior might offer insights into those extinct hominins’ lives.
Our previous research found that, compared to chimpanzees living in forests, Issa Valley chimpanzees spent just as much time moving in the trees. We wanted to test if something about how they foraged could explain their unexpectedly high arboreality. Savannah-mosaics are characterized by more sparsely distributed trees, so we hypothesized that adapting behavior to forage efficiently in a tree would be especially beneficial when the next tree is further away.
Dr Rhianna Drummond-Clarke.
Researchers monitored the adults of the Issa community during the dry season, watching how they foraged in trees and what they ate there. The size, height, and shape of the trees were recorded, as well as the number and size of branches.
Issa chimpanzees mostly ate fruit, followed by leaves and flowers — foods found at the ends of branches, so the chimpanzees needed to be capable climbers to reach them safely. They spent longer foraging in trees that were larger and offered more food. The longest foraging sessions, and the most specialized behaviors to navigate thinner terminal branches, were seen in trees with large open crowns offering lots of food: perhaps abundant food justified the extra time and effort. A similar trade-off between the nutritional benefits of specific foods and the effort of acquiring them could also explain why chimpanzees spent longer in trees while eating nutritionally-rich, hard-to-access seeds.
Fast food
Because they are relatively large, chimpanzees move within trees not by climbing on thin branches but by hanging under them, or standing upright and holding on to nearby branches with their hands. Although these ‘safe’ behaviors are traditionally associated with foraging in dense forest, these findings show they’re also important for chimpanzees foraging in a savannah-mosaic.
We suggest our bipedal gait continued to evolve in the trees even after the shift to an open habitat. Observational studies of great apes demonstrate they can walk on the ground for a few steps, but most often use bipedalism in the trees. It’s logical that our early hominin relatives also engaged in this kind of bipedalism, where they can hold onto branches for extra balance. If Issa Valley chimpanzees can be considered suitable models, suspensory and bipedal behaviors were likely vital for a large-bodied, fruit-eating, semi-terrestrial hominin to survive in an open habitat.
Dr Rhianna Drummond-Clarke.
However, the researchers say that we need more fossil evidence and more studies on different aspects of chimpanzee foraging to test this idea.
This study only looked at foraging behavior during the dry season. It would be interesting to investigate if these patterns remain during the wet season. Analyses of the nutritional value of foods and overall food availability are also needed to test our hypothesis that a strategy of foraging for longer in large trees on certain foods is energy-efficient in an open habitat. Importantly, this is also only one community of chimpanzees. Future studies of other chimpanzees living in such dry, open habitats will be vital to see if these patterns are truly a savannah-mosaic signal or unique to Issa.
Dr Rhianna Drummond-Clarke.
Publication:
A mother chimpanzee looks out from the central tree to her breakfast (seeds!) at the end of thin branches.Rhianna C. Drummond-Clarke/Greater Mahale Ecosystem Research and Conservation (GMERC)
The thing that clearly emerges once again from this research is that scientists are not abandoning evolution and adopting creationism, as creation cult leaders have been assuring their credulous followers for the best part of half a century. On the contrary, studies like this one show that evolutionary theory continues to predict, explain, and unify findings across genetics, anatomy, behaviour, and ecology. The researchers behind this chimpanzee study didn’t invoke magic, miracles, or divine intervention — they followed the evidence, and it led, as it always does, to evolution.Introduction: The association between an open habitat (e.g., savannah-mosaics) and increased terrestriality is central to hypotheses of hominin evolution, especially the emergence and evolution of bipedalism, as well as ape evolution as far back as the early Miocene. However, the selective pressures that act on apes in an open habitat remain poorly understood. Observations of chimpanzees that live in savannah-mosaics, analogous to some reconstructed hominoid palaeohabitats, can provide valuable insight into the behavioural adaptations of a large-bodied, semi-arboreal ape to an open habitat, characterised by sparsely distributed food sources and a broken canopy. We previously showed that savannah-dwelling chimpanzees in the Issa Valley, western Tanzania, maintain a high level of arboreality, and particularly suspensory behaviour, largely associated with foraging. Here, we investigate how chimpanzee foraging strategy in a savannah-mosaic may drive a high frequency of arboreal behaviours despite reduced arboreal pathways. Specifically, we hypothesized that Issa chimpanzees would spend more time foraging (and moving) per tree to maximize utilization of food in a sparse landscape. This foraging strategy would be facilitated by foraging in trees with large crowns and abundant terminal-branch foods, which are characteristic of miombo woodlands. However, the link between foraging positional behaviour and tree structure remains understudied.
Methods: We collected data on arboreal foraging behaviour and corresponding tree structural characteristics over five months in the dry season, and used generalized linear mixed models to test for any effect of food type and tree structural characteristics on (1) duration of foraging bouts, (2) frequency of locomotion, and (3) use of suspensory behaviour.
Results: We found that food types and tree structures found in woodland vegetation are associated with more time spent in foraging trees, a higher rate of locomotion, and the use of suspension in particular.
Discussion: Our results suggest that arboreal, and especially suspensory, locomotion can be advantageous for foraging in a savannah-mosaic and not just closed forest habitats. These findings have implications for reconstructing hominoid positional behaviour from the fossil record and provide a model for how arboreality, and specifically suspensory behaviour, could have been an important part of the hominoid niche in savannah-mosaic habitats.
1 Introduction
Increased terrestriality, and correspondingly a reduced dependence on arboreal locomotor behaviours, has been considered an adaptive response to more open habitats in the evolution of some primate taxa, including cercopithecoids (e.g., McCrossin et al., 1998), capuchin monkeys (e.g., Falotico, 2011) and hominoids (i.e., living apes, including humans, and their extinct ancestors; e.g., Doran and Hunt, 1994; Sockol et al, 2007; Rodman and McHenry, 1980). This association between terrestriality and open habitat has formed the basis of hypotheses seeking to explain the divergence of hominins (the human lineage), typically defined, at least in part, by morphological adaptations to habitual bipedalism, from panins (chimpanzees and bonobos) in the late Miocene (e.g., the savannah hypothesis, reviewed in Domínguez-Rodrigo, 2014). However, the porous hominin fossil record between 7–4 Ma (reviewed in Stamos and Alemseged, 2023) together with the virtual absence of an African ape fossil record (McBrearty and Jablonski, 2005; Suwa et al., 2007.1), means the evolutionary trajectory of hominins and panins from their last common ancestor, and the selective pressures that acted on them, remain poorly understood. Whilst palaeoenvironmental reconstructions support a shift from closed (e.g., tropical forest) to more open, heterogeneous environments (e.g., wooded, mosaic landscapes, hereafter “savannah-mosaic”) early in hominin evolution (Levin et al., 2008; Cerling et al., 2011.1; Blumenthal et al., 2017; Negash et al., 2024; for review, see Su, 2024.1), there is mounting fossil evidence suggesting arboreality remained a vital part of the hominin adaptive niche (Alemseged, 2023.1; Cazenave and Kivell, 2023.2; but see Ward, 2002). Several extinct hominins have curved phalanges, relatively long upper limbs, and/or morphological features typically associated with arboreality, ranging from the late Miocene (e.g., Orrorin tugenensis, Senut et al., 2001; Ardipithecus kadabba, Haile-Selassie, 2001.1; Sahelanthropus tchadensis, Daver et al., 2022) to late Pleistocene (e.g., Homo naledi, Berger et al., 2015; Homo floresiensis, Larson et al., 2009). Furthermore, although there are limitations in paleodietary reconstructions, carbon isotope and dental microwear studies suggest certain hominins retained a diet with substantial arboreal food sources in environments with increasing C4 vegetation (i.e. more open habitats) (Ungar and Sponheimer, 2011.2; Levin, 2015.1). In some cases, hominin diets are reconstructed as similar to those of extant chimpanzees (e.g., Ardipithecus ramidus, White et al., 2009.1; Australopithecus anamensis, Alemseged, 2015.2; Australopithecus sediba, Henry et al., 2012).
Among non-hominin apes, some large-bodied Miocene hominoids that show adaptations for arboreal locomotion have been placed in wooded, grassy habitats resembling savannah-mosaics as early as 21 million years (Morotopithecus, Peppe et al., 2023.3; Kenyapithecus, McCrossin et al., 1998). Together with hominin evidence, this brings into question the link between open habitat and the reduced importance of arboreal behaviour throughout hominoid evolution. Based on fossil evidence from Moroto, Uganda, MacLatchy et al. (2023.4) have further suggested that selective pressures associated with navigating open canopy to forage (rather than forest canopy with high connectivity, e.g., Ripley, 1979) may have driven the emergence of diverse orthograde arboreal locomotor behaviours in large-bodied apes.
In the absence of direct fossil evidence, extant chimpanzees (Pan troglodytes) that live in open and seasonal habitats analogous to palaeohabitat reconstructions for some hominoids can provide suitable models to test the “savannah effect” on ape behaviour (e.g., Moore, 1992, 1996; Pruetz and Bertolani, 2009.2; Lindshield et al., 2021, 2025; Drummond-Clarke et al., 2022.1, 2024.2; but see White et al., 2009.1). Chimpanzees (P. t. schweinfurthii) of the Issa Valley, western Tanzania, live in a savannah-mosaic habitat (Figure 1) that closely resembles the reconstructed palaeoenvironments of some late Miocene and early Pliocene hominoids (e.g., Morotopithecus, Peppe et al., 2023.3; Graecopithecus, Böhme et al., 2019; O. tugenensis, Pickford and Senut, 2001.2; Ar. ramidus, Levin et al., 2008, Negash et al., 2024; Australopithecus afarensis, Su and Haile-Selassie, 2022.2). Here, we investigated the role of foraging strategy and tree structure in an open habitat as drivers of arboreality and its associated positional behaviours, including suspension, in the Issa Valley chimpanzees.
Compared to forests, savannah-mosaics are generally considered to exert higher energetic stress on chimpanzees due to more sparsely distributed food sources (requiring more travel; Raichlen and Pontzer, 2021.1) and high seasonality (McGrew et al., 1981; Kortlandt, 1983; Pruetz and Bertolani, 2009.2). Although findings of lower dietary diversity in savannah—compared to forest-dwelling communities (McGrew et al., 1988; Hunt and McGrew, 2002.1; Pruetz, 2006.1; Piel et al., 2017.1)—lend support to this concept (Vogel et al., 2017.2), savannah chimpanzees are not under higher nutritional stress compared to forest-dwelling populations (Wessling et al., 2018). Furthermore, bonobos living in forest-savannah mosaic habitats showed no difference in dietary diversity compared to their forest-dwelling counterparts (Pennec et al., 2020). This evidence could suggest that chimpanzees adjust their foraging strategy to counteract the high energetic demands of foraging on sparse food sources. Optimal foraging theory predicts that an animal will invest more energy (i.e., via time and movement) into foraging on patches of higher value, with time spent foraging in a patch used as a measure of patch value (Slocombe and Zuberbühler, 2006; Kalan et al., 2015.3; Villioth et al., 2022.3, Villioth et al., 2023.6). Patch value itself is suggested to be dependent on two factors. First, the energy return rate of the landscape, with lower food availability increasing the value of patches and thus time invested per patch (Charnov, 1976; Vogel et al., 2017.2). This concept is supported by studies on spider monkeys (Ateles belzebuth belzebuth, Suarez, 2006.2.) and Himalayan langurs (Semnopithecus entellus, Sayers et al., 2010). Second, the energy and nutrition available within the patch influences its value (Stephens and Krebs, 1986), which could be a function of patch size (i.e., larger size = higher value), food type and density within the patch (i.e., high density = high value), and, for social species, the amount of intraspecific competition (e.g., more conspecifics means less food per individual, as found for Cebus imitator, Vogel, 2005.1; cercopithecids, Janson and Goldsmith, 1995; Pan troglodytes, Lindshield, 2014.1, Villioth et al., 2022.3, Villioth et al., 2023.6). A foraging strategy of investing more time to high value patches, such as large trees with abundant foods, could therefore be an efficient way to maximise energy intake while foraging on sparsely distributed food sources (Chapman et al., 1995.1; Villioth et al., 2022.3).Figure 1.Figure 1. The Issa Valley study site. (A) Location in Tanzania and vegetation map. (B) An example of typical riparian forest vegetation at Issa during dry season (note dry river bed but evergreen vegetation). (C–E) Examples of typical miombo woodland vegetation during dry season at Issa. Woodland trees lose their leaves (C, D), grass dries and is burnt by fires. The woodland undergoes regeneration with regrowth after fires (E). Photo credits RCDC/GMERC.
In the first characterisation of savannah chimpanzee positional behaviour, we previously found that chimpanzees of the Issa Valley were just as arboreal, and in some cases more so, than chimpanzees at more forested sites (Drummond-Clarke et al., 2022.1; Drummond-Clarke et al., 2024.3). What is more, Issa chimpanzees used comparably high frequencies of suspension and quadrupedal walking but less vertical climbing to navigate arboreal substrates, indicating horizontal movement on branches (rather than vertical movement on tree trunks as suggested by Senut et al., 2018.1) as an important component of their arboreal locomotor repertoire. These patterns raised questions as to why and how Issa chimpanzees maintain high levels of arboreality and, in particular, frequent locomotion on terminal branches, despite living in an open habitat with limited arboreal pathways. Notably, arboreal locomotion, and in particular locomotor (as opposed to postural) bipedalism, was used primarily for arboreal feeding at Issa (Drummond-Clarke et al., 2022.1). Suspension and assisted bipedalism are orthograde, horizontally-moving behaviours (Hunt, 1991; Hunt et al., 1996.1), that great apes can use in a continuum to navigate flexible terminal branches despite their large body size by distributing their weight across multiple supports (Thorpe et al., 2007.2b; Drummond-Clarke et al., 2022.1, Supplementary Video 1). Combined with a previous study showing the importance of forest architecture in shaping great ape locomotor behaviour (Pongo, Manduell et al., 2012.1), this supports a link between foraging positional behaviour and foraging tree structure (e.g., Crompton, 1984; Fleagle, 1979.1). We thus predicted that more time spent foraging on food items at the ends of branches specifically could drive the high frequency of terminal branch locomotor behaviours observed at Issa (Drummond-Clarke et al., 2022.1, 2024.3). However, whilst a basic link between substrate characteristics and ape locomotor behaviour is reasonably well established (e.g., suspension and assisted bipedalism on flexible substrates, climbing on vertical substrates; Hunt et al., 1996.1; Thorpe and Crompton, 2005.2, 2006.3; Manduell et al., 2012.1; Neufuss et al., 2017.3, 2018.2; Druelle et al., 2024.4; Drummond-Clarke et al., 2024.3), comparatively little is known about chimpanzee foraging strategy and substrate use in open habitats (reviewed in Lindshield et al., 2021), especially in relation to tree structure. Rather, existing studies on the relationship between tree structure and primate behaviour have focused only on tree size (Kalan et al., 2015.3; Villioth et al., 2023.6), nesting (e.g., Baldwin et al., 1981.1; Hernandez-Aguilar et al., 2013; Stewart and Pruetz, 2013.1; Samson and Hunt, 2014.2), or locomotor behaviour of other species (Galago, Crompton, 1984; Pongo, Manduell et al., 2012.1), severely limiting our understanding of how chimpanzee foraging behaviour and tree structure could be linked.
Here, we investigated chimpanzee arboreal foraging behaviour in relation to tree structure at Issa Valley to test the hypothesis that structural characteristics typical of woodland trees selects for the high levels of arboreality, and in particular arboreal locomotion and suspensory behaviour (Drummond-Clarke et al., 2022.1, 2024.3). Specifically, we predicted that foraging in larger trees with open and wide crowns, low connectivity and abundant terminal branch foods (characteristic of woodland feeding trees e.g., Brachystegia spp., Figures 1a, d) would not only drive longer foraging periods within tree crowns, but also more locomotion, and in particular suspensory behaviours, to navigate the terminal branch niche.
Drummond-Clarke RC, Reuben SC, Stewart FA, Piel AK and Kivell TL (2025)
Foraging strategy and tree structure as drivers of arboreality and suspensory behaviour in savannah-dwelling chimpanzees. Front. Ecol. Evol. 13:1561078. doi: 10.3389/fevo.2025.1561078
Copyright: © 2025 The authors.
Published by Frontiers Media S.A. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
Whatever led to early hominins adopting an upright, bipedal gait - and this research strongly suggests it was in order to reach food at the end of slender branches, the way modern chimpanzees do - it was in response to environmental change associated with the climate change that transformed forest into savannah - just as the Theory of Evolution predicts.
Meanwhile, creationism offers no testable predictions, no coherent framework, and no explanatory power — just outdated myths dressed up in pseudoscientific jargon. The more we learn about life on Earth, the clearer it becomes: evolution is not just a theory. It’s the only explanation that works.
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