Age-Trajectory of Mother-Infant Relationships in Wild Assamese Macaques

Maternal care is ubiquitous in mammals, yet its degree and duration vary across taxa. In primates, mothers provide extended care for young and follow similar developmental transitions in the mother-infant relationship, yet at different paces of change. Since ecological pressures shape life-history traits including female reproductive rate and timing of infant independence, research is needed on mother-infant relationships in wild populations exposed to energetic constraints and predation risk. Assamese macaques (Macaca assamensis) of the study population are seasonal breeders living in an unpredictable environment, where fluctuating food availability imposes energetic challenges on mothers and infants. We quantitatively describe how maternal care and offspring independence develop throughout infancy. Using continuous focal observations on 59 infants, we model the nonlinear age-trajectories of mother-infant proximity and transitions from dependent to independent feeding and locomotion, and estimated sex differences in these trajectories. Newborns were fully dependent on their mothers for feeding and transport, with mothers maintaining close proximity. A transitional phase emerged between 1 and 3 months of age, marked by reduced maternal proximity and increasing infant independence. During the second half of infancy, infants achieved near-complete locomotor and feeding independence, while residual proximity and body contact persisted. No sex differences were detected in the mother-infant relationship trajectory. Collectively, the timing of maternal investment aligns with the breeding strategy of this seasonal species, with females balancing investment in current and future reproduction. This study establishes a baseline for examining how ecological variability affects the timing and pace of mother-infant behavioral transitions.