Immune responses and resource allocation in Northern elephant seal pups: Effects of fasting and sexual dimorphism.

Mammalian parental care varies widely in investment and duration. While many species transition gradually from dependence on maternal care, others, such as the Northern elephant seal (Mirounga angustirostris), are abruptly weaned and endure prolonged fasting, relying solely on resources stored during nursing. Limited resources can lead to trade-offs between costly processes like growth and immune responses, which influence survival and fitness. Resource allocation reflects individual condition and species life-history, and may differ by sex if larger size benefits male lifetime fitness. We investigated inflammation and bacterial inhibition in Northern elephant seal pups immediately after weaning and one month into fasting, assessing whether responses were influenced by sex, growth, fatty reserves (body condition and blubber thickness), available energy (blood levels of glucose, triglycerides and total protein), nutritional stress (creatinine and δN), concurrent infections, and differences in maternal foraging (inferred from isotopic signals in pups' whiskers). Newly-weaned pups had stronger immune responses than later in fasting: bactericidal ability decreased by 8.76 % (95 % CI: 5.16-12.47) and inflammation by 36.37 % (95 % CI: 16.39-53.07), as resources became depleted. Although body length and mass did not differ by sex, females maintained higher body condition (mass per unit length) throughout fasting. Newly-weaned females inhibited bacterial growth better than males, but this advantage disappeared after a month. Fatty reserves and plasma proteins were associated with immune responses later in fasting, but maternal feeding site and trophic level were not. However, endogenous resource use (measured as δN values) was positively correlated with bacterial inhibition. Neutrophil counts and blood iron levels explained variation in inflammation. Our study shows that innate immune responses in a rapidly growing, capital-breeding mammal depend on maternal resources and reflect early sex differences and concurrent physiological challenges. Allocation of depleting resources during fasting drives trade-offs between immune function, growth, and development.