The composition of the milk of antarctic (Arctocephalus gazella) and subantarctic (A-tropicalis) fur seals at Macquarie Island

The composition of milk collected from 36 antarctic (Arctocephalus gazella) and 17 subantarctic fur seals (A. tropicalis) breeding sympatrically at Ma cquarie Island was examined over the first 100 days of lactation in the 199 0/91 season. The mean composition of milk in A. gazella and A. tropicalis w as 41.3% and 44.6% water, 39.8% and 38.6% lipid, 18.1% and 16.1% protein, a nd the estimated gross energy content of milk was 19.9 and 18.9 kJ g(-1), r espectively. Neither the composition of milk nor its energy density differe d significantly between species, despite a difference of 4-6 months in lact ation length. Water content of milk could be used to predict lipid (r(2) = 0.67) and protein (r(2) = 0.57) content, but was most accurate at predictin g gross energy content (r(2) = 0.97). These relationships were the same for each species. The water content of milk decreased throughout the first 100 days of lactation in both species, while lipid, protein and energy content all increased. The addition of maternal mass into regression analysis with days post-partum increased the significance of models predicting the conte nt of lipid and proteins in the milk, but not those predicting the water or gross energy content. Milk collected on the first day of 2-day attendance bouts had, on average, 9% greater lipid content, and 5% greater protein con tent than milk collected on the second day. The growth rates of subantarcti c fur seal pups were significantly lower than those of antarctic fur seal p ups over the first month of growth, suggesting that (despite similar milk c omposition, attendance patterns and diet of the two species of fur seal) th e overall rates of energy transfer from mother to pup in subantarctic fur s eals is lower than in antarctic fur seals.