Maternal Care Strategies Differentially Optimize the Health and Immunity of Male and Female South American Fur Seal Pups

AbstractPinnipeds face increasing challenges that affect how maternal foraging and nursing strategies transfer key resources for immune function and ultimately the survival of their offspring. We evaluated how foraging strategies and maternal care in South American fur seals (<i>Arctocephalus australis</i>) affect male and female pup growth, physiology, and immune health. We used capture and recapture data for mothers and their pups and daily monitoring during the austral summers of 2015-2016, 2021-2022, and 2022-2023 to assess maternal care. We collected pup morphometric data, blood, and vibrissae to assess their growth rates, health, and immune competence. We found that mothers shifted their diet from demersal to more pelagic prey between 2015 and 2023 because of a decrease in their main demersal prey item, the southern hake (<i>Merluccius australis</i>). This shift in diet was associated with reduced maternal foraging time and increased nursing time, which differentially affected the health of male and female pups. Male pups grew faster, emphasizing energy reserves, especially in years with a higher abundance of demersal prey, while slower-growing female pups prioritized the development of the immune system, especially during years with a decline in demersal fish and an increase in the use of pelagic fish by their mothers, suggesting sex-specific adaptation for the long-term survival of offspring. This study suggests that prey scarcity could differentially impact the development and immune health of male and female offspring of marine top predators.