Weaning age and supersuckling in Galápagos Sea Lions (Zalophus wollebaeki): Maternal, offspring and environmental drivers of life-history strategies

Weaning marks a critical life-history transition in mammals, shaped by maternal investment strategies, offspring condition, and environmental constraints. In Galápagos sea lions (Zalophus wollebaeki), weaning age is highly variable, with some individuals continuing to suckle into adulthood (supersucklers). Using 20 years of mark-recapture data from 1890 individuals, we applied multi-state capture-recapture models to investigate the probability of weaning age, and logistic regression models to explore the drivers of supersuckling. We examined maternal effects (age, birth interval, and pup condition at birth), offspring traits (sex, early growth), and environmental variability (Oceanic Niño Index [ONI]). Weaning probability peaked at four years of age, with 95% of individuals weaned by this time. Longer birth intervals were associated with later weaning. Offspring sex had no influence on weaning probability. However, ONI affected juvenile survival, indirectly shaping weaning outcomes. No intrinsic predictors of supersuckling were identified, although low resource availability during an individual’s second or third year of life increased the likelihood of becoming a supersuckler. While weaning is typically driven by escalating mother-offspring conflict, the persistence of supersuckling challenges this paradigm. We propose that the regular occurrence of supersuckling in this species may therefore be socially driven, reflecting the maintenance of long-term mother-offspring relationships that contribute to a complex adult social structure. This study highlights the interplay between environmental unpredictability and social behaviours in shaping life-history strategies.