Change in plasma cortisol and metabolites during the attendance period ashore in fasting lactating subantarctic fur seals

Lactating fur seals (Arctocephalus tropicalis) alternate foraging trips at sea and pup attendance periods ashore. During the onshore nursing periods, lactating females do not have access to food and meet both their own metabolic requirements and milk production from their body reserve. Blood and milk samples were collected from females captured soon after their arrival ashore from a foraging trip and before their departure. Milk lipid but not milk protein content was positively related to the body condition index (BCI) of the female. During the 4-day attendance period ashore, females lost body mass, and plasma cortisol levels increased, whereas plasma urea concentration decreased and beta-hydroxybutyrate (beta-OHB) remained unchanged. The increase in cortisol level took place while blood urea concentration decreased and beta-OHB remained at a low level suggesting that it was independent from the transition from phase II to phase III that is indicative of the depletion of lipid body store as described in penguins. Thus, our results suggest that the increase in cortisol level in relation to decreasing BCI may either contribute to the mobilization of protein stores to ensure milk production when easily mobilized stores are used and/or could act as a re-feeding signal which is triggered well before females have depleted their body store.