Milking strategy in subantarctic fur seals Arctocephalus tropicalis breeding on Amsterdam island: Evidence from changes in milk composition

Milk composition was investigated throughout the 10-mo pup-rearing period i n subantarctic fur seals (Arctocephalus tropicalis) breeding on Amsterdam I sland. The mean milk composition was lipid, protein, and 42.8% +/- 5.7% 12. 1% +/- 1.5% 42.6 +/- 7.3% water. Subantarctic fur seals breeding on Amsterd am Island produced one of the richest milks ever reported in otariids (20.4 +/- 2.9 kJ/g), with lipid content contributing 85% of total gross energy. The high lipid levels measured in the milk of subantarctic fur seals breedi ng on Amsterdam Island is consistent (i) with the relatively long time lact ating females spend at sea, due to the relatively poor local trophic condit ions near the colony that necessitate that they travel long distances to re ach the foraging grounds, and (ii) with the consequently short time mothers spend with their pups ashore. Milk composition changed according to the ti me mothers were fasting ashore: milk produced during the first 2 d spent as hore, when more than 80% of milk transfer occurred, had higher levels of li pids, proteins, and gross energy than milk produced later during the visit ashore, suggesting that the pups were fed with two types of milk during a s uckling period. Throughout the year, mothers in good condition produced mil k of higher lipid content than others, suggesting that individual foraging skills contribute to enhance milk quality. Milk lipid and gross energy cont ent varied with pup age, according to quadratic relationships, increasing d uring the earlier stages of lactation before reaching asymptotic values whe n pups were 180 d old. The stage of lactation appears to be a better predic tor of milk lipid content than the duration of the preceding foraging trip, suggesting that either changes in the nutritional requirements of the pup and/or seasonal changes in trophic conditions act on milk composition. Thes e changes in milk quality may also be related to changes in maternal care; lactating subantarctic fur seals apparently reallocate their body reserves toward gestation rather than lactation at the end of the pup-rearing period .