Milking It: Repeated Postweaning Suckling Events in Galápagos Sea Lions ( <i>Zalophus wollebaeki</i> )

AbstractMilk is not cheap. The energetic cost for mammalian mothers to provide and sustain milk production makes it a finite resource. Offspring are therefore expected to wean before sexual maturity and reproductive activity, whether on their own or through termination by their mothers. Weaning delays should result in a reproductive trade-off for the mother: the possibility of begetting a fitter pup at the cost of a longer interbirth interval. Using 20 years of data, we show the occurrence of repeated suckling events between female Galápagos sea lions (GSLs; <i>Zalophus wollebaeki</i>) and their adult (≥5 years) biological offspring well beyond the average age of independence and when the offspring are themselves already reproductively active. This behavior, "supersuckling," suggests that GSL mother-offspring relationships are more complex and longer lasting than previously thought. To our knowledge, this is the first long-term documentation of known mother-offspring pairs repeatedly performing this behavior in any marine mammal species.