Revisiting our primate roots in infants grooming

TL;DR

The study finds that infants display grooming-like behaviors, which may reflect evolutionary roots shared with primates and decline as gestural communication emerges.

Contribution

The study identifies grooming-like behaviors in preverbal infants as a vestigial motor pattern with potential evolutionary significance.

Findings

Infants display grooming-like behaviors directed at caregivers' hairy skin, resembling non-human primate grooming.

Grooming behaviors decline by 15 months, coinciding with the emergence of gestural communication.

Grooming frequency peaks between 2pm and 4pm, aligning with a circadian dip in beta-endorphins.

Abstract

Grooming is a common behavior across many animal species, including non-human primates (NHP), and serves a variety of functions, such as cleaning hair, alleviating stress, and enhancing social bonds. This study aims to explore whether grooming-like behaviors are present in humans during early developmental stages, before the emergence of gestural and language communication. Through the observations of 67 preverbal infants, we identified frequent manual behaviors, including grasping, holding, and behaviors resembling grooming, particularly directed toward caregivers’ hairy skin. These behaviors were analyzed and validated by twelve independent primatologists, who confirmed that behavioral sequences and their kinematics closely resembled grooming behaviors seen in NHPs, while also distinguishing them from other types of manual actions such as holding or grasping. Longitudinal analyses demonstrated a significant reduction in infants’ grooming behaviors beginning at 8 months, with these behaviors no longer observed by 15 months, a developmental shift that coincided with the emergence of more sophisticated gestural communication. Interestingly, grooming frequency, but not other actions, peaked during a specific time window, between 2pm and 4pm, which corresponds with a well-documented circadian dip in beta-endorphins, a neurochemical associated with stress regulation and social bonding. This alignment points toward a potential physiological underpinning for the timing of these behaviors. These findings suggest that infant grooming behavior represents a vestigial motor pattern, likely reflecting conserved evolutionary mechanisms shared with non-human primates. This behavior may represent a primitive form of early social interaction, highlighting the role of ancient motor programs in shaping prelinguistic communication.

The online version contains supplementary material available at 10.1038/s41598-026-39909-2.

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