Suckling observations of Galapagos sea lions

Milk is not cheap. Mammalian offspring are expected to wean during the early juvenile stages, whether through their own mechanisms or their mothers’ instigation. To not do so would result in a reproductive trade-off for the mother: continued investment in the health and growth of her current offspring, over a shorter birth interval, and the possibility of begetting a fitter pup. Here we show, for the first time, using 20 years of data, repeated suckling events between female Galápagos Sea Lions (GSL) Zalophus wollebaeki and their fully adult biological offspring well beyond the expected 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 behavior has not been previously documented in any marine mammal species.

Methods
Data were collected between September 2003 and April 2023 on Isla Caamaño (0° 45’ S, 90° 16’ W) during 2 annual field seasons from October-December and February-April that cover the main reproductive period. Suckling observations were made through minimum twice daily rounds using non-invasive means. Morphometric measurements were collected during captures of individuals. All data collection was carried out with the knowledge and permission of the Galápagos National Park.