Milking Strategy in Subantarctic Fur SealsArctocephalus tropicalisBreeding on Amsterdam Island: Evidence from Changes in Milk Composition

Abstract

Milk composition was investigated throughout the 10-mo pup-rearing period in subantarctic fur seals (Arctocephalus tropicalis) breeding on Amsterdam Island. The mean milk composition was 42.8% +/- 5.7% lipid, 12.1% +/- 1.5% protein, and 42.6% +/- 7.3% water. Subantarctic fur seals breeding on Amsterdam 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 breeding on Amsterdam Island is consistent (i) with the relatively long time lactating females spend at sea, due to the relatively poor local trophic conditions near the colony that necessitate that they travel long distances to reach the foraging grounds, and (ii) with the consequently short time mothers spend with their pups ashore. Milk composition changed according to the time mothers were fasting ashore: milk produced during the first 2 d spent ashore, when more than 80% of milk transfer occurred, had higher levels of lipids, 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 suckling period. Throughout the year, mothers in good condition produced milk of higher lipid content than others, suggesting that individual foraging skills contribute to enhance milk quality. Milk lipid and gross energy content varied with pup age, according to quadratic relationships, increasing during the earlier stages of lactation before reaching asymptotic values when pups were 180 d old. The stage of lactation appears to be a better predictor 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. These 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.

Foraging ecology of subantarctic fur seals Arctocephalus tropicalis breeding on Amsterdam Island: seasonal changes in relation to maternal characteristics and pup growth

“…The autumn peak in milk gross energy (Georges et al 2001) may compensate for longer trips undertaken by the females during that period. However, although pups are adapted to long-term fasting periods , the concurrent decreasing energy content of the milk (Georges et al 2001) and increasing requirements of pregnancy (Oftedal 1985) are likely to reduce a female's capacity to fulfill the requirements of its pup during winter. Furthermore, the greater mean seasonal trip duration (see Fig.…”

“…In this context, body length represents an important component of the body's reserve strategy, especially since foraging zones are located far away from the rookery. Female subantarctic fur seals breeding on Amsterdam Island have the largest body size of this species (Bester & Van Jaarsveld 1994, Robinson et al 2002, and the milk they produce is among the richest of all otariids (Georges et al 2001). This suggests a phocid-like strategy (Trillmich 1996); that is, they may rely more on their fat reserves than any other otariid species.…”

“…Maternal foraging ranges are thus limited by the pup's requirements as well as by the female's own swimming and diving ability. The spatial and temporal separation between the feeding zones and the rookery present some energetic constraints during the pup-rearing process: a seal has to catch prey located sometimes far from the rookery, store the energy acquired and then transfer it to its pup as lipid-rich milk (Gentry & Kooyman 1986, Georges et al 2001, Arnould & Hindell 2002. The connection between the marine environment and reproductive performance therefore occurs through resource acquisition and allocation processes, and imposes important nutritional and energetic costs.…”

The comparative energetics and growth strategies of sympatric Antarctic and subantarctic fur seal pups at Îles Crozet

“…Goldsworthy and Crowley (1999) suggested that the difference in growth rates could reflect either a higher milk consumption rate in Antarctic fur seals or greater metabolic expenditure by subantarctic fur seals. However, the limited information on pup metabolic rates for the species is restricted to their allopatric sites (making comparisons difficult) and there is no information on their milk consumption rates at sympatric sites (Arnould et al, 1996a(Arnould et al, , 2001Beauplet et al, 2003;Georges et al, 2001;Guinet et al, 1999). Furthermore, while mass gain differs between the species, it is not known whether the divergent lactation strategies influence the composition of growth and development (Owens et al, 1993;Spray and Widdowson, 1950).…”

Milk fatty acids predict the foraging locations of the New Zealand fur seal: continental shelf versus oceanic waters

“…Bathymetry was used to determine when a seal left continental shelf waters. Bathymetry values were calculated for each seal at 15 min intervals along each interpolated satellite track and were obtained from the General Bathymetric Chart of the Oceans (GEBCO) 1 × 1 km World Bathymetry Grid (2.1).Milk FA can originate from recent dietary intake and/or from mobilization of stored body fat, depending on when the seal last fed (Georges et al 2001, Staniland & Pond 2005, Budge et al 2006 fore reflect a combination of the most recent dietary intake and a longer dietary history. To ensure the foraging trip used to classify foraging location (i.e.…”