ORIGINAL PAPER A.R. Carlini á M.E.I. Marquez á G.A. Daneri á S. Poljak Mass changes during their annual cycle in females of southern elephant seals at King George Island Accepted: 5 September 1998 Abstract Mass changes in female southern elephant seals, sampled sequentially at dierent points through their annual cycle, were measured at King George Is- land, South Shetland Islands, during the 1995/1996 and 1996/1997 ®eld seasons. Females weighed after they had given birth showed an increase of 37 36 kg (mean SD), which represented 6.2 6.4% in rela- tion to their mass in the ®rst breeding season. During the ®rst aquatic phase, between the end of lactation and the beginning of moult, females gained a mean of 128 35 kg, (n = 18) (2.19 0.65 kg day )1 ), which represented between 27 and 83% of the mass they had lost during lactation. Nine females followed during moulting showed a mass loss rate of 5.0 0.4 kg day )1 , which was half the rate during lactation. Total mass loss during moulting (129 22 kg) was not signi®cantly dierent from mass gain for the same females between lactation and moult (135 37 kg). Furthermore, at the end of moulting, female mass was not signi®cantly dierent from the mass at the end of lactation. These masses represented 65 5% and 64 5%, respectively, of their initial mass after par- turition. During the second period at sea, from the end of the moult until females hauled out to give birth in the following breeding season, the estimated mass gain was 1.45 0.24 kg day )1 (n = 5), which was not sig- ni®cantly dierent to the rate of mass gain shown by the same females during the ®rst period at sea (2.26 0.70 kg day )1 ). Total mass gain during the second aquatic phase (364 63 kg) was not correlated with the mass at the end of moulting, but it was posi- tively related to the mass loss experienced by females from parturition until the end of the moulting period in the ®rst breeding season. Introduction During their annual cycle, female southern elephant seals (Mirounga leonina) have two main periods on land and two main periods at sea. The periods on land are similar in that females are fasting and draw on their reserves, accumulated mainly as blubber (Laws 1956; Costa 1991; Oftedal 1993), but these are associated with dierent aspects of their annual cycle. During lactation, females suckle their pups in- tensively, transferring all the energy required by the growing pup, as well as providing accumulated reserves for about 45 days after weaning (Arnbom et al. 1993). Lactation ends when females go to sea, about 23 days after giving birth and 27 days after their arrival at the breeding site. During the moulting period, females moult hair and the top layer of the epidermis (Ling 1968), a process that appears to be less expensive in terms of energy than lactation (Boyd et al. 1993). The periods on land are separated temporally, and between them females feed to replenish their reserves. The post-breeding aquatic phase lasts from the end of lactation to the time when females arrive for moult (the beginning of moulting taking place between December and January), and its duration is about 60 days at King George Island (Carlini et al. 1997) and 70 days for South Georgia and Macquarie elephant seals (Hindell and Burton 1988; Boyd et al. 1993). Records of diving ac- tivity suggest that females are foraging intensively dur- ing this period (Boyd and Arnbom 1991; Slip et al. 1994). The post-moult aquatic phase extends from the end of moulting to the next breeding season, from about February to September. During this phase, February to April would seem to be a more concentrated and active Polar Biol (1999) 21: 234±239 Ó Springer-Verlag 1999 A.R. Carlini (&) á M.E.I. Marquez á S. Poljak Departamento de Ciencias BioloÂgicas, Instituto AntaÂrtico Argentino, Cerrito 1248 (1010) Buenos Aires, Argentina e-mail: acarlini@netverk.comm.ar G.A. Daneri Museo Argentino de Ciencias Naturales, ``B. Rivadavia'' (MACNBR), Div. MastozoologõÂa, Av. Angel Gallardo 470 (1405) Buenos Aires, Argentina