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Burkart, J. M.

Publications and source records attributed to Burkart, J. M..

At least 19 recordsLinked to original sources
bioRxiv2026

"Mine" or "yours"? Acoustic structure of food calls predicts sharing decisions in marmosets

Foraging and feeding are often associated with vocalisations across animal species. Food calls may function to claim ownership of resources or recruit others to feeding sites, yet their potential role in facilitating food sharing remains poorly understood. This study investigates the relationship between food calls and food-sharing behaviour in common marmosets (Callithrix jacchus), focusing on adult-infant interactions, where food transfers are most frequent. We analysed over 1000 food interactions across six groups, categorising events as non-sharing (without begging or despite begging) or sharing (proactive, resisted, or facilitated). We examined the spectro-temporal structure of food calls produced immediately before each interaction. Food transfers were more likely to occur when preceded by a food call, suggesting a functional link between food calls and sharing. Random forest models reliably classified food calls according to interaction type, while generalized linear mixed models identified acoustic variation associated with sharing outcomes. Our results suggest that food calls encode information to either encourage or deter conspecific (infant) approach, influencing food transfer outcomes. These findings highlight the role of food calls in modulating social interactions around food in marmosets, advancing our understanding of vocal communication and its integration with cooperative behaviours in primates.

animal behavior and cognition↗
bioRxiv2026

Supra-regular recursion in primate vocal sequences

Complex syntax is the widely accepted demarcation between animal and human communication. Although animal vocal systems occasionally contain pockets of syntax-like structures, their grammars are thought to be at best regular, whereas human language is supra-regular, i.e., including at least context-free computational complexity. Leveraging a new likelihood method for probabilistic formal grammars, we show that the call sequences of three phylogenetically diverse nonhuman primates (bonobos, olive colobus monkeys, common marmosets) include parts that are most likely generated by supra-regular recursive grammars, similar to human language. These findings challenge syntax as the major transition in hominid evolution and opens up prospects for alternative evolutionary scenarios.

animal behavior and cognition↗
bioRxiv2026

Norm-like vocal behavior in a nonhuman primate

Human interlocutors have strong expectations that guide when, how, and what is communicated, but the evolutionary origins of such normative expectations remain poorly understood. Vocal accommodation in marmosets, where individuals modify their calls to match partners, offers a potential model. Within an interactive closed-loop playback paradigm, marmosets experienced partner calls that either converged toward or diverged away from their own call structure. Marmosets responded affiliatively to convergence, whereas diverging calls elicited protest vocalizations. Importantly, the protests stopped when the calls converged again. Effects were partner-specific, indicating that marmosets expect bond partners, but not strangers, to converge during a conversation. Furthermore, individuals modified their calls to restore similarity to dissimilar partner playbacks. These findings identify marmoset vocal accommodation as a socially regulated behavior with normative properties.

animal behavior and cognition↗
bioRxiv2026

What is encoded in a marmoset phee call? Food context beyond arousal and valence

Animal vocalisations can convey information about external events, but whether this goes beyond reflecting the emotional state elicited by these events is debated. To explore this, we studied the acoustic structure of common marmoset (Callithrix jacchus) phee (long-distance contact) and ek (alert/mobbing) calls produced in five treatments varying in the emotional valence and arousal they elicit (internal state), as well as food and social context (external events). We measured changes in arousal via nasal temperature and analysed both basic acoustic parameters and Mel-frequency cepstral coefficients (MFCCs) of the calls. Support Vector Machines combined with Linear Mixed effect models revealed that phee calls encode both external events and internal states, while eks reflected predominantly arousal. Notably, an acoustic signature related to food context was present in phees both when provided (positive valence) and teased with highly preferred food items (negative valence), and even when food was not physically present (food call playback treatment). This suggests marmoset long-distant phee calls encode external information beyond emotional arousal and valence, and independently of the presence of an immediately triggering stimulus.

animal behavior and cognition↗
bioRxiv2026

An Automated Wireless Seesaw System Enabling Spatial Separation of Action and Reward in Group-Housed Marmosets

Cooperation in social species is shaped by ongoing social relationships, partner choice, and group interactions, demanding experimental systems that preserve the social context in which these behaviors unfold. Here we introduce the e-Seesaw, a wireless system for automated liquid reward delivery designed to support home-enclosure experiments on reward access distribution in common marmosets (Callithrix jacchus) with minimal human intervention. The apparatus combines a modular peristaltic pump with a Bluetooth-controlled trigger, allowing spatial separation between the site of action and the site of reward delivery while preserving group housing. We provide detailed design files, software, and assembly instructions to support reproduction and adaptation. In a proof-of-concept deployment across seven families, animals readily engaged with the device, producing a median of ~87 trigger activations per session. Engagement was concentrated early within sessions and remained largely stable across repeated deployments, including under increased action-reward separation. These results established the e-Seesaw as a flexible and reproducible platform for automated reward-delivery experiments in animals tested within their social groups, while reducing human involvement and avoiding fixed dyadic testing.

animal behavior and cognition↗
bioRxiv2026

A phylogenetically informed comparative analysis of sexual testosterone dimorphism across mammals in relation to paternal care and sexual size dimorphism

Within species, male testosterone is often linked to mating competition and paternal care, suggesting that sex differences in endogenous testosterone values across mammals may covary with broader reproductive strategies. Using a structured literature search, we compiled 63 studies, spanning 31 non-human species and 9 human populations, reporting endogenous, non-experimentally manipulated testosterone values for both adult males and females within the same population and context. From these studies, we calculated male-to-female testosterone ratios, and analysed these data using Bayesian phylogenetic multilevel models. We tested whether testosterone dimorphism was associated with paternal care and sexual size dimorphism while accounting for sampling matrix, assay method, breeding context, and wild versus captive setting. Across non-human mammals, neither paternal care nor sexual size dimorphism (indexing competition) showed a clear association with testosterone ratios, and the same pattern emerged in the primate-only subset. By contrast, sampling matrix was consistently associated with testosterone dimorphism across all analyses, with lower male-to-female ratios in non-blood than in blood-based measures. In primates, testosterone ratios were also lower in captive than in wild populations, although this pattern was not clearly supported in the broader non-human dataset. In the human-only analysis, testosterone ratios did not clearly differ between industrialized and small-scale societies, whereas the matrix effect remained evident. Overall, our results suggest that sampling matrix is a major source of variation even for ratio-based measures, highlighting the need for caution when inferring between-species endocrine differences from studies using different substrates. More broadly, directly comparable, non-experimentally manipulated testosterone data for both sexes remain rare across mammals, limiting comparative inference.

evolutionary biology↗
bioRxiv2026

Animal collocation revisited: intercohort comparison and a case study comparing call combinations between sexes in common marmosets

Many animals communicate using sequences of signals, but identifying recurrent, non-random signal combinations remains methodologically challenging. Collocation analyses are increasingly popular approaches for detecting which signals animals combine at rates greater than expected by chance. However, existing methods for animal collocation analysis face several limitations that reduce their statistical rigour: they lack uncertainty estimates, fail to control for non-independence in sampled data, and do not account for inflated family-wise error rates when identifying attraction among many different signal types. These limitations restrict the broader applicability of animal collocation analysis, including preventing robust comparisons of signal combination strength between cohorts (e.g. populations, sexes or age classes). We adapt a novel form of Multiple Distinctive Collocation Analysis using Pearson residuals (MDCA-Pr) that addresses these statistical limitations, and validate its use in animal communication research in three ways: first, using numerous simulated datasets of different sizes and levels of signal recombination; second, using simulated data to evaluate the performance of MDCA-Pr in intercohort comparisons, and third, by demonstrating how MDCA-Pr can be applied to compare the vocal sequences produced by male and female captive-living common marmosets (Callithrix jacchus). MDCA-Pr shows high sensitivity, including at small sample sizes, and generally low false-positive rates, which we further reduce by applying additional criteria for identifying attraction between signals. During intercohort comparisons, MDCA-Pr is conservative, with low false-positive rates, and statistical power increases with sample size. MDCA-Pr is a robust method for evaluating signal attraction in animal communication and enables accurate intercohort comparison of animal signal combinations. Significance StatementBy assessing the performance of MDCA-Pr on simulated animal-like data, we demonstrate that this method reliably detects signal combinations within and across animal cohorts, while overcoming statistical limitations of previous collocation analyses. We present an analytical pipeline for applying MDCA-Pr to animal signal data, including for intercohort comparisons, enabling identification and comparison of combinatorial strategies across entire signal repertoires. We illustrate this approach by comparing call combination strategies of male and female common marmosets when presented with food under experimental conditions, finding similar combinatorial strategies between sexes. MDCA-Pr therefore permits rigorous characterization of animal signal combinatoriality and opens avenues for investigating how demographic, social, and group-level factors influence combinatorial patterns.

animal behavior and cognition↗
bioRxiv2026

Sequence-level vocal convergence in common marmosets

Vocal accommodation is the process by which individuals adjust their vocalizations to resemble those of social partners. This phenomenon is widespread in social animals and can reinforce affiliation, signal group identity, and facilitate coordination. Most studies of vocal accommodation have focused on convergence in the acoustic structure of individual calls. Whether social partners also converge in how calls are arranged into sequences remains largely unknown. We examined vocal convergence during pair formation in common marmosets (Callithrix jacchus) by recording phee sequences from nine dyads composed of three males and three females before pairing and again four months after, in two audience contexts: when individuals interacted vocally with their partner or with an opposite sex stranger. We quantified similarity between individuals in call sequence-structure using transition probabilities, bigram frequencies, repeat-length distributions, and local alignment, and quantified similarity in acoustic structure using spectral parameters, MFCCs, and dynamic time warping. We found vocal convergence on a sequence level. After pair formation, partners became more similar in sequence structure when calling to strangers, whereas no change was detected in partner directed sequences. In contrast, call acoustic structure did not change in either context. Because vocal repertoires are constrained by anatomy and physiology, reorganizing existing call types into different combinations may provide a flexible route for modifying signals without altering the acoustic structure of individual calls. Our results provide evidence that social bonds can drive sequence level vocal convergence in a non-human primate, suggesting that vocal flexibility may arise not only through changes in acoustic structures but also through changes in how calls are organized over time.

animal behavior and cognition↗
bioRxiv2026

Who says what when? Patterns in captive common marmoset (Callithrix jacchus) volubility

Volubility - the number of vocalizations per unit of time - is an understudied aspect of animal communication, potentially crucial in cooperatively breeding species that depend on coordinated behavior. Callitrichid monkeys such as common marmosets (Callithrix jacchus) are often characterized as highly vocal, yet variation in their calling rates remains poorly quantified. We recorded 69,538 vocalizations from 45 captive marmosets tested in dyads across six experimental conditions. Total call rates varied with condition but not with caller sex or status. Alertness call production depended on caller status: breeders showed context-appropriate responses to an ambiguous object, whereas helpers responded more generally. Food calls were tightly coupled to feeding rather than food availability, with males calling more than females. Dyadic analyses revealed call-type-specific covariation between subjects and partners, consistent with vocal contagion for alertness calls and recruitment for food calls. These patterns indicate flexible, call-type-specific volubility and information sharing beyond simple arousal contagion.

animal behavior and cognition↗
bioRxiv2025

Common marmosets (Callithrix jacchus) excel in a one-trial spatial memory test, yet perform poorly in a classical memory task

When quantifying animal cognition, memory represents one of the most tested domains and is key to understanding cognitive evolution. Memory tests thus play an important role in comparative cognitive research, yet slight variations in the experimental settings can substantially change the outcome, questioning whether different memory tests tap into different memory systems or whether they test memory at all. Here, we first assessed memory performance of 16 common marmosets (Callithrix jacchus) in two distinct paradigms varying in their format and delay. First, we examined marmoset memory in a 24-hour delay memory test (24h-DMT) in which they could freely explore an environment with three novel objects of which one contained food. We examined their retention the day after, and the procedure was iterated cumulatively with previous objects remaining in the enclosure until the marmosets had to choose the correct out of 30 objects. Second, we administered a classical delayed response test (DRT) in the same animals with three objects and a maximum delay of 30 seconds. In the DRT, marmoset performance was poor and not better than chance after 15 seconds already. However, individuals excelled in the 24h-DMT, performing above chance level after 24h even with tenfold the number of objects to choose from compared to the DRT. Moreover, individual performances in the two tests were not correlated, and typical age effects on memory could not be detected in both experiments. Together, these results suggest that the two tests explore different domains, and that the 24h-DMT examines long-term memory. The outcome of the DRT is more difficult to assign to memory since individuals performed only moderately even in the 0-second delay condition. This puts into question whether this task design indeed tests memory or other cognitive processes.

animal behavior and cognition↗
bioRxiv2024

The ontogeny of play in a highly cooperative monkey, the common marmoset

Play is mostly observed in juveniles in mammals, and the type of play (social, locomotor and object play) tends to mirror adult function. In some species, also adults play with immatures, in particular if same-aged play partners are lacking and adults also invest in caretaking. We studied the ontogeny of play in cooperatively breeding common marmoset groups composed of parents and twin offspring between the age of two to six months. Social play was by far the most prevalent and increased with age. Adults were important play partners: Before 19 weeks old, the play partners of immatures was an adult in 54% of the time spent playing socially. After week 19, this proportion decreased to 29%. The rest of the social play time was spent playing with their twin. Thus, despite the constant presence of a twin, adult-immature play remained considerable, with equal contributions by mothers and fathers and no trade-offs with other care-taking behaviours (i.e., carrying and food sharing) for either of the parents. Notably, parents avoided playing simultaneously, presumably to avoid periods when no one could be vigilant. Together, these results resonate strongly with the highly interdependent and cooperative lifestyle of common marmosets. HighlightsO_LISocial play is the predominant form of play in marmosets and increases between the age of 2 to 6 months C_LIO_LIParents are important play partners for immatures during ontogeny C_LIO_LIThe odds of playing with the twin rather than a parent increases with age C_LIO_LIMothers and fathers play at similar amounts with the immatures C_LIO_LIMothers and fathers take turns and almost never play simultaneously with the immatures C_LI

animal behavior and cognition↗
bioRxiv2024

Echoes of self: Understanding acoustic structure and informational content in common marmoset (Callithrix jacchus) phee sequences

Communication in social animals relies heavily on acoustic cues, yet many species possess limited vocal repertoires. To compensate, animals often produce vocalizations in sequences, potentially enhancing the diversity of transmitted information. However, the significance of repeated elements within these sequences remains poorly understood. Here, we investigated the spectro-temporal structure of elements within common marmoset (Callithrix jacchus) phees, a long-distance contact call, often produced in sequences. Employing machine learning techniques (random forests) and linear mixed effects models, we explored how elements varied based on their position within sequences and assessed their ability to encode identity and sex information. Additionally, we examined similarities between elements occupying the same position in different sequences. Our results reveal structural differences both within and between sequences, with variations observed in phees at different positions within the same sequence, yet similarities found between first elements of distinct sequences. Notably, all phees encoded caller identity and sex information, with varying accuracy across positions within sequences, indicating a higher encoding of sex information at the beginning of the sequence and a greater emphasis on identity in subsequent elements. These findings suggest that repeated sequences may be functionally diverse structures, enriching the complexity of animal communication systems.

animal behavior and cognition↗
bioRxiv2024

Same data, different results? Evaluating machine learning approaches for individual identification in animal vocalisations

Automated acoustic analysis is increasingly used in behavioural ecology, and determining caller identity is a key element for many investigations. However, variability in feature extraction and classification methods limits the comparability of results across species and studies, constraining conclusions we can draw about the ecology and evolution of the groups under study. We investigated the impact of using different feature extraction (spectro-temporal measurements, linear and Mel-frequency cepstral coefficients, as well as highly comparative time-series analysis) and classification methods (discriminant function analysis, neural networks, random forests, and support vector machines) on the consistency of caller identity classification accuracy across 16 mammalian datasets. We found that Mel-frequency cepstral coefficients and random forests yield consistently reliable results across datasets, facilitating a standardised approach across species that generates directly comparable data. These findings remained consistent across vocalisation sample sizes and number of individuals considered. We offer guidelines for processing and analysing mammalian vocalisations, fostering greater comparability, and advancing our understanding of the evolutionary significance of acoustic communication in diverse mammalian species.

animal behavior and cognition↗
bioRxiv2024

Short-term memory, attentional control and brain size in primates

Brain size variability in primates has been attributed to various domain-specific socio-ecological factors. A recently published large-scale study of short-term memory abilities in 41 primate species [1] did not find any correlations with 11 different proxies of external cognitive demands. Here we found that the interspecific variation in test performance shows correlated evolution with total brain size, with the relationship becoming tighter as species with small sample sizes were successively removed, whereas it was not predicted by the often-used encephalization quotient (EQ). In a subsample, we also found that the sizes of brain region thought to be involved in short-term memory did not predict performance better than did overall brain size. The dependence on brain size suggests that domain-general cognitive processes underlie short-term memory as tested in [1]. These results support the emerging notion that comparative studies of brain size do not generally identify domain-specific cognitive adaptations, but rather reveal varying selection on domain-general cognitive abilities. Finally, because attentional processes beyond short- term memory also affected test performance, we suggest that the delayed response test can be refined.

animal behavior and cognition↗
bioRxiv2023

Marmosets mutually compensate for differences in rhythms when coordinating vigilance

Synchronisation is widespread in animals, and studies have often emphasised how this seemingly complex phenomenon can emerge from very simple rules. However, the amount of flexibility and control that animals might have over synchronisation properties, such as the strength of coupling, remains underexplored. Here, we studied how pairs of marmoset monkeys coordinated vigilance while feeding. By modelling them as coupled oscillators, we noted that (1) individual marmosets do not show perfect periodicity in vigilance behaviours, (2) even then, pairs of marmosets developed a tendency to take turns being vigilant, a case of anti-phase synchrony, (3) marmosets could couple flexibly; the coupling strength varied with every new joint feeding bout, and (4) marmosets could control the coupling strength; dyads showed increased coupling if they began in a more desynchronised state. Such flexibility and control over synchronisation require more than simple interaction rules. Minimally, animals must estimate the current degree of asynchrony and adjust their behaviour accordingly. Moreover, the fact that each marmoset is inherently non-periodic adds to the cognitive demand. Overall, our study taps into the cognitive aspects of synchronisation and provides a mathematical framework to investigate the phenomenon more widely, where individuals may not display perfectly rhythmic behaviours.

animal behavior and cognition↗
bioRxiv2023

Dynamic vocal learning in adult marmoset monkeys

While vocal learning is vital to language acquisition in children, adults continue to adjust their speech while adapting to different social environments in the form of social vocal accommodation (SVA). Even though adult and infant vocal learning seemingly differ in their properties, whether the mechanisms underlying them differ remains unknown. The complex structure of language creates a challenge in quantifying vocal changes during SVA. Consequently, animals with simpler vocal communication systems are powerful tools for understanding the mechanisms underlying SVA. Here, we tracked acoustic changes in the vocalizations of adult common marmoset pairs, a highly vocal primate species known to show SVA, for up to 85 days after pairing with a new partner. We identified four properties of SVA in marmosets: (1) bidirectional learning, (2) exponential decrease in vocal distance with time, (3) sensitivity to initial vocal distance, and (4) dyadic acoustic feature synchrony. We developed a mathematical model that shows all four properties. The model suggests that marmosets continuously update the memory of their partners vocalizations and modify their own vocalizations to match them, a dynamic form of vocal learning. The model provides crucial insights into the mechanisms underlying SVA in adult animals and how they might differ from infant vocal learning.

animal behavior and cognition↗
bioRxiv2023

Human amygdala volumetric patterns convergently evolved in cooperatively breeding and domesticated species

PurposeThe amygdala is a hub in brain networks that supports social life and fear processing. Compared to other apes, humans have a relatively larger lateral nucleus of the amygdala, which is consistent with both the self-domestication and the cooperative breeding hypothesis of human evolution. MethodsHere, we take a comparative approach to the evolutionary origin of the relatively larger lateral amygdala nucleus in humans. We carry out phylogenetic analysis on a sample of 17 mammalian species for which we acquired single amygdala nuclei volumetric data. ResultsOur results indicate that there has been convergent evolution toward larger lateral amygdala nuclei in both domesticated and cooperatively breeding mammals. ConclusionThese results suggest that changes in processing fearful stimuli to reduce fear-induced aggression, which are necessary for domesticated and cooperatively breeding species alike, tap into the same neurobiological proximate mechanism. However, humans show not only changes in processing fearful stimuli but also in prosociality. Since cooperative breeding, but not domestication, is also associated with prosociality, a prominent role of the former during human evolution is more parsimonious, whereas self-domestication may have been involved as an additional stepping stone.

animal behavior and cognition↗
bioRxiv2022

Optimising source identification from marmoset vocalisations with hierarchical machine learning classifiers

Marmosets, with their highly social nature and complex vocal communication system, are important models for comparative studies of vocal communication and, eventually, language evolution. However, our knowledge about marmoset vocalisations predominantly originates from playback studies or vocal interactions between dyads, and there is a need to move towards studying group-level communication dynamics. Efficient source identification from marmoset vocalisations is essential for this challenge, and machine learning algorithms (MLAs) can aid it. Here we built a pipeline capable of plentiful feature extraction, meaningful feature selection, and supervised classification of vocalisations of up to 18 marmosets. We optimised the classifier by building a hierarchical MLA that first learned to determine the sex of the source, narrowed down the possible source individuals based on their sex, and then determined the source identity. We were able to correctly identify the source individual with high precisions (87.21% - 94.42%, depending on call type, and up to 97.79% after the removal of twins from the dataset). We also examine the robustness of identification across varying sample sizes. Our pipeline is a promising tool not only for source identification from marmoset vocalisations but also for analysing vocalisations and tracking vocal learning trajectories of other species.

animal behavior and cognition↗