The composition of milk collected from 36 antarctic
(Arctocephalus gazella) and 17 subantarctic fur seals
(A. tropicalis) breeding sympatrically at Macquarie
Island was examined over the first 100 days of lactation in the 1990/91
season. The mean composition of milk in A. gazella and
A. tropicalis was 41.3% and 44.6% water,
39.8% and 38.6% lipid, 18.1% and 16.1% protein,
and the estimated gross energy content of milk was 19.9 and 18.9 kJ
g–1, respectively. Neither the composition of milk
nor its energy density differed significantly between species, despite a
difference of 4–6 months in lactation length. Water content of milk
could be used to predict lipid (r2
= 0.67) and protein (r2
= 0.57) content, but was most accurate at predicting gross energy
content (r2 = 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 content 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 content than milk collected on the second day.
The growth rates of subantarctic fur seal pups were significantly lower than
those of antarctic fur seal pups over the first month of growth, suggesting
that (despite similar milk composition, attendance patterns and diet of the
two species of fur seal) the overall rates of energy transfer from mother to
pup in subantarctic fur seals is lower than in antarctic fur seals.
Foraging ecology of subantarctic fur seals Arctocephalus tropicalis breeding on Amsterdam Island: seasonal changes in relation to maternal characteristics and pup growth