The research, conducted by Flora Felix Manyama of the University of Dodoma, focused on four habituated baboon troops, two of olive baboons (Papio anubis) and two of yellow baboons (Papio cynocephalus), within the 871-square-kilometre reserve that straddles the Kondoa and Chemba districts of the Dodoma region. Swagaswaga is a classic semi-arid savannah ecosystem: rolling hills, rocky outcrops and miombo woodlands punctuated by drainage systems such as the Makati River, where annual rainfall of 600 to 1000 millimetres falls almost entirely between November and April. From May to October, the landscape is defined by hot, sunny days, cooler nights and steadily vanishing water sources, an environmental rollercoaster that makes the reserve an ideal natural laboratory for studying how animals respond to resource scarcity.

The fieldwork was demanding in its precision. Across 67 observation days totalling 440 hours, baboon troops were followed on foot from six o’clock in the morning, before they left their sleeping sites, until they settled into a new sleeping site around half past six or seven in the evening. Observers, working at an average distance of five metres, recorded troop movements continuously using hand-held Garmin 520Hcx GPS units, a technique considered among the most reliable for quantifying ranging behaviour in primates. Data collection ran through the wet season from January to March 2024 and the dry season from June to August 2024, allowing direct seasonal comparisons for the same troops under radically different ecological conditions.

The headline result is stark. When both species were pooled, baboons travelled an average of 3.01 plus or minus 0.07 kilometres per day in the dry season, against 1.06 plus or minus 0.03 kilometres in the wet season, a difference that proved statistically significant under a Mann-Whitney U test. The explanation, the study argues, lies in water. During the wet months, ephemeral water sources are replenished and vegetation flourishes, meaning baboons rarely need to venture far for a drink or a meal. When the rains stop, those sources dry up, forcing the troops to expand their search radius and, in doing so, cover three times the ground they would under favourable conditions.

The two species, however, did not respond identically. Olive baboons consistently outwalked their yellow cousins in both seasons, averaging 3.27 kilometres per day in the dry season and 1.17 kilometres in the wet, while yellow baboons logged 2.76 and 0.95 kilometres respectively. Although the overall difference between the species did not reach statistical significance in the Kruskal-Wallis test, the pattern was consistent. The most plausible driver, the study suggests, is group size: olive baboons live in larger troops than yellow baboons at Swagaswaga, and larger groups exhaust food patches faster and face greater within-group feeding competition, which compels them to travel farther and more frequently between resource patches.

Group dynamics themselves shifted with the seasons in ways that illuminate the fundamental trade-offs of social living. Average troop sizes were significantly larger in the wet season, at 16 individuals, than in the dry season, when they dropped to 9. Olive baboons formed the biggest groups, at 18 in the wet season and 11 in the dry, compared with 14 and 7 for yellow baboons. Importantly, the study notes that these fluctuations were not driven by births or deaths but by fission and fusion, the splitting and merging of groups that is common among primates and governed by both social and ecological pressures. When resources are plentiful, the costs of crowding are low and the benefits of group living, including dilution of predation risk and improved vigilance, tip the balance in favour of large aggregations. When water and food grow scarce, smaller groups become energetically advantageous.

This logic follows a well-established framework in behavioural ecology: as time spent travelling increases, a threshold is eventually reached at which the energy cost of moving becomes so high that small groups outcompete large ones. In semi-arid environments like Swagaswaga, where prolonged dry seasons can sharply curtail food and water availability, an increase in group size directly expands the area a troop must cover to meet its collective needs. Individual baboons in larger groups must therefore travel farther and burn more energy than they would in smaller aggregations, a cost that the dry season amplifies to a breaking point. The observed seasonal fissioning is the animals’ solution to that arithmetic.

Perhaps the most consequential finding for conservation is what happens at the reserve’s edge. Although baboons spent roughly 85 percent of their observation time inside Swagaswaga, both species regularly moved beyond the borders, where human settlements and cultivated farms offer alternative food sources. Olive baboons, with their larger troops and longer daily ranges, spent more time outside the reserve than yellow baboons, and both species ventured out most often during the dry season, presumably driven by the same resource shortages that inflated their travel distances. Baboons are omnivorous and opportunistic, and the study notes they will eat cultivated crops and even domesticated animals such as goats, sheep and poultry, behaviour that places them in direct conflict with farmers.

These crop-raiding excursions have tangible costs on both sides. Human-wildlife conflict around Swagaswaga typically centres on baboons raiding farms just beyond the reserve boundary, and prolonged conflict threatens both local livelihoods and the long-term tolerance that conservation depends on. The study argues that quantifying how much time baboons spend outside the protected area, and under which seasonal conditions, gives wildlife managers a practical tool: interventions can be timed and targeted for the dry season, when the risk of encounters peaks. Because baboons are widely regarded as agricultural pests and are classified as a species of least concern, they rarely attract conservation funding, yet their ecology makes them a bellwether for how semi-arid protected areas function.

The broader implications stretch into a warming future. Climate change is expected to make rainfall in semi-arid regions more erratic, potentially prolonging dry seasons and intensifying water shortages, which would push baboons and other wildlife into even longer daily movements and more frequent excursions beyond reserve boundaries. Understanding the precise relationship between seasonality, ranging and group structure, as this GPS-based study does, is therefore a first step toward predicting and managing those pressures. The research calls for further work on human-baboon conflict around Swagaswaga to inform management strategies, and its data are held at the University of Dodoma for future study. For now, the image it leaves is vivid: on the hottest, driest days, when the ephemeral pools have cracked and the acacias stand bare, baboon troops tighten their ranks, shrink their numbers and set out across the savannah on journeys three times longer than any they make in the season of plenty.

Subject of Research: Effects of seasonal variation on daily travel distances and group sizes of olive and yellow baboons in a semi-arid Tanzanian game reserve

Article Title: Effects of seasonality on baboons’ (Papio cynocephalus and Papio anubis) daily movement patterns and group sizes in semi-arid environment at Swagaswaga game reserve, Tanzania

Article References: Effects of seasonality on baboons’ (Papio cynocephalus and Papio anubis) daily movement patterns and group sizes in semi-arid environment at Swagaswaga game reserve, Tanzania. (n.d.). https://doi.org/10.1007/s44353-025-00058-8

Image Credits: AI Generated

DOI: 10.1007/s44353-025-00058-8

Keywords: baboons, seasonality, Swagaswaga Game Reserve, Tanzania, olive baboon, yellow baboon, daily travel distance, group size, semi-arid, human-wildlife conflict, GPS tracking, primatology

Functionally referential signals — vocalizations that reliably inform listeners about specific external events or objects — have long fascinated comparative psychologists because they blur the boundary between animal communication and human language. Vervet monkeys emit distinct alarm calls for leopards, eagles and snakes, and recipients respond appropriately to each. Prairie dogs embed information about predator features in their calls, and some birds encode details about food quality. But the question of just how fine-grained these references can be has remained open. Most documented referential systems distinguish broad categories: predator versus non-predator, high-value versus low-value food. Whether a signal can pick out individual food types within a category — whether a chimp’s grunt for bread sounds meaningfully different from its grunt for mango — had never been empirically tested for chimpanzee food calls.

Chimpanzees produce rough grunts when finding and eating food, and earlier work had established that the acoustic structure of these calls varies with the preference value of the food that elicits them. Grunts given to highly preferred foods differ systematically from those given to less desirable items, and subsequent research showed that listening chimpanzees can use this variation to guide their own foraging decisions. That established a meaningful baseline: the calls encode relative preference. What remained unknown was whether the specificity stopped there. If two foods are both highly preferred — say, bread and mango — do the grunts they elicit sound the same to a listener, or do they carry information distinguishing one food from the other? And critically, does that fine-grained information actually matter to receivers, or is it merely acoustic noise that no chimpanzee attends to?

To answer these questions, the research team, whose work was supported by the Biotechnology and Biological Sciences Research Council and by the Swiss National Science Foundation through the NCCR Evolving Language programme, combined two complementary approaches. The first was a careful acoustic analysis of food calls recorded from chimpanzees at the Wolfgang Köhler Primate Research Centre in Leipzig, Germany, where calls elicited by foods of matched preference values could be compared. The second was a playback experiment — a technique in which recorded vocalizations are broadcast to an animal while researchers observe how it responds — allowing the team to test whether the acoustic distinctions they detected were actually meaningful to a listening chimpanzee rather than incidental by-products of arousal.

The acoustic analysis produced a strikingly layered picture. The calls differed between foods of high and low preference value, replicating and extending the earlier findings that grunts track how much a caller likes what it has found. But the analysis went further: significant acoustic differences also emerged between individual food types within each of those preference categories. In other words, grunts elicited by different high-value foods could be statistically discriminated from one another, and so could grunts elicited by different low-value foods. The calls were not simply graded along a single dimension of excitement. Instead, they appeared to encode at least two distinct dimensions of information simultaneously — the general value of the food and its specific identity — a combination that suggests a far richer encoding system than anyone had demonstrated in chimpanzee feeding vocalizations.

Statistically detectable differences, however, do not automatically translate into communicative relevance. An acoustic signature can reflect physiological differences in arousal, jaw position, or food texture in the mouth without any listener ever extracting information from it. This is where the playback design proved decisive. The researchers broadcast recorded food calls to a chimpanzee subject and measured the responses, focusing on whether the animal’s behaviour indicated that it had interpreted the calls as information about what lay hidden at the playback location. By pairing calls recorded in one context with search opportunities in another, the experiment isolated the informational content of the vocalization itself from any direct sensory experience of the food.

The results were remarkable on all three fronts. The listening chimpanzee discriminated between calls elicited by high-value and low-value foods, confirming once again that preference information is accessible to receivers. More importantly, the subject also discriminated between calls elicited by individual types of high-value food, and between calls elicited by individual types of low-value food. The fine-grained distinctions that showed up in the acoustic analysis were not lost on the audience: the receiver behaved as though it knew not just that something good had been found, but what kind of good thing it was. This represents, the authors write, an unprecedented degree of specificity in the referential food calls of our closest living relatives — a level of detail previously identified only in alarm call systems, not in feeding vocalizations.

The theoretical implications ripple outward in several directions. For theories of language evolution, the finding narrows the perceived gap between primate vocal communication and human speech. One cornerstone of linguistic meaning is the ability of a signal to pick out specific referents rather than broad emotional states, and this study shows that chimpanzee calls can do something analogous with food types, not just food categories. It also challenges the long-standing division between ‘referential’ alarm calls and ’emotional’ food calls: if food calls encode specific identities in a way receivers use, the referential-emotional dichotomy looks increasingly like a human-imposed simplification rather than a real feature of chimpanzee communication. The study adds to a growing body of evidence that the vocal repertoire of great apes is far more structured than the behaviorist traditions of the twentieth century assumed.

The methodology deserves attention too. Combining acoustic analysis with playback experiments is the gold standard for demonstrating functional reference, because each approach compensates for the other’s weaknesses. Acoustic analysis can reveal structure that receivers ignore; playback can reveal comprehension of differences the analysis missed. By requiring both — statistically discriminable calls and demonstrably different receiver responses — the researchers built a case that is difficult to dismiss. The work was approved by the School of Psychology Ethics committee at the University of St Andrews, and the authors declare no competing interests. The study was published open access, with a citable, permanent DOI, meaning anyone can examine the evidence in full.

Questions naturally remain. The playback tested a single subject, so the generality of comprehension across individuals and communities will need confirmation. It is also unclear how the specificity arises — whether callers intentionally vary their grunts to inform others, whether the acoustic differences emerge as side effects of eating different foods, or whether listeners have simply learned to exploit regularities in the sounds. None of this diminishes the central result: chimpanzees listening to a distant group member’s grunts can apparently learn both how desirable a hidden food is and what that food actually is. For a species separated from our own lineage by roughly six million years, that is a strikingly sophisticated channel of information, and it suggests that the seeds of referential specificity run far deeper in the primate lineage than the study of alarm calls alone had ever revealed.

Subject of Research: Referential specificity of chimpanzee food calls tested through acoustic analysis and playback experiments

Article Title: Chimpanzee food calls provide information about the value and type of food: a playback study

Article References: Watson, S. K., Kaller, T., Cheng, L., Wathan, J., Townsend, S. W., Zuberbühler, K., Call, J., & Slocombe, K. E. (2026). Chimpanzee food calls provide information about the value and type of food: a playback study. Animal Cognition. https://doi.org/10.1007/s10071-026-02105-w

Image Credits: AI Generated

DOI: 10.1007/s10071-026-02105-w

Keywords: chimpanzees, food calls, functional reference, rough grunts, playback experiment, vocal communication, animal cognition, primatology, language evolution, acoustic analysis, foraging behaviour, referential signalling