Cory Thomas Miller

Prosocial behaviors, such as cooperation and food sharing, are critical for maintaining group cohesion in social species, yet the influence of transient physiological states on these behaviors remains poorly understood. This study investigates how short-term ecological factors impact social behavior in common marmosets ( Callithrix jacchus ), a highly prosocial nonhuman primate species. Specifically, we tested how food divisibility (small vs. large food items) and time since food access (TSFA; 0 h, 1 h, and 3 h) influenced food sharing behavior. Results revealed that larger food portions consistently promoted prosocial interactions, while longer durations since food access shifted behavior toward individualism. No significant interaction effect between TSFA and food size was observed. These findings suggest that marmoset prosociality is sensitive to immediate ecological conditions, reflecting flexible, context-dependent social decision-making.

Human and nonhuman primates rely heavily on vision to actively explore and navigate their environment. Although primate visual cortex has been studied extensively in head-fixed animals, little is known about how the primate visual system supports natural, active vision in freely moving animals. Here, we address this gap in the primary visual cortex (V1) by leveraging a head-mounted eye-tracking system while simultaneously recording the activity of ensembles of single V1 neurons in freely moving marmosets. Our results reveal that primate neural activity is tightly driven by visual input and organized by the temporal structure of natural gaze behavior, and these gaze-related responses are largely abolished in the absence of visual input. We further show that distinct phases of gaze movement, i.e. rapid redirection (gaze shift) and subsequent stabilization (fixation), engage separable suppression and enhancement of the V1 responses. The enhancement during fixation was clearly linked to visual input. These findings define the dynamics in V1 that link natural gaze behavior and stimulus-driven responses in freely moving primates. The work opens a previously inaccessible but fundamental regime of primate vision and establishes freely moving paradigms as a foundation for understanding real-world visual processing during ethologically relevant behaviors.

Prosocial behaviors, such as cooperation and food sharing, are critical for maintaining group cohesion in social species, yet the influence of transient physiological states on these behaviors remains poorly understood. This study investigates how short-term ecological factors impact social behavior in common marmosets ( Callithrix jacchus ), a highly prosocial nonhuman primate species. Specifically, we tested how food divisibility (small vs. large food items) and time since food access (TSFA; 0 hours, 1 hour, and 3 hours) influenced food sharing behavior. Results revealed that larger food portions consistently promoted prosocial interactions, while longer durations since food access shifted behavior toward individualism. A significant interaction between TSFA and food size indicates that the effect of food size on prosocial behavior varies with hunger state: increasing TSFA weakens the prosocial impact of large food portions. These findings suggest that marmoset prosociality is sensitive to immediate ecological conditions, reflecting flexible, context-dependent social decision-making.

Our understanding of how vision functions as primates actively navigate the real-world is remarkably sparse. As most data have been limited to chaired and typically head-restrained animals, the synergistic interactions of different motor actions/plans inherent to active sensing—e.g., eyes, head, posture, movement, etc.—on visual perception are largely unknown. To address this considerable gap in knowledge, we developed an innovative wireless head-mounted eye-tracking system that performs Chair-free Eye-Recording using Backpack mounted micROcontrollers (CEREBRO) for small mammals, such as marmoset monkeys. Because eye illumination and environment lighting change continuously in natural contexts, we developed a segmentation artificial neural network to perform robust pupil tracking in these conditions. Leveraging this innovative system to investigate active vision, we demonstrate that although freely moving marmosets exhibit frequent compensatory eye movements equivalent to other primates, including humans, the predictability of the visual behavior (gaze) is higher when animals are freely moving relative to when they are head-fixed. Moreover, despite increases in eye/head-motion during locomotion, gaze stabilization remains steady because of an increase in vestibularocular reflex gain during locomotion. These results demonstrate the efficient, dynamic visuo-motor mechanisms and related behaviors that enable stable, high-resolution foveal vision in primates as they explore the natural world.

Vocal turn-taking has been described in a diversity of species. Yet, a model that is able to capture the various processes underlying this social behaviour across species has not been developed. To this end, here we recorded a large and diverse dataset of marmoset monkey vocal behaviour in social contexts comprising one, two and three callers and developed a model to determine the keystone factors that affect the dynamics of these natural communicative interactions. Notably, marmoset turn-taking did not abide by coupled-oscillator dynamics, but rather call timing was overwhelmingly stochastic in these exchanges. Our features-based model revealed four key factors that encapsulate the majority of patterns evident in the behaviour, ranging from internal processes, such as particular states of the individual driving increased calling, to social context-driven suppression of calling. These findings indicate that marmoset vocal turn-taking is affected by a broader suite of mechanisms than previously considered and that our model provides a predictive framework with which to further explicate this natural behaviour at both the behavioural and neurobiological levels, and for direct comparisons with the analogous behaviour in other species.