Energetics of Reproduction in Males

The information presented below on weight changes of males is an interim and incomplete presentation of our data. We include it here to complement the data on northern elephant seal bulls (Deutsch et al., this volume).

Male elephant seals on South Georgia may become sexually mature at 5 to 6 years of age, although most harem bulls are probably 9 to 12 years old (McCann 1981). As is true among females, a very wide size range of animals is present on the breeding beaches. Bulls occupying positions within harems typically weigh 1,500 to 3,000 kg, and weights up to 3,700 kg have been reported (Ling and Bryden 1981). Bulls peripheral to harems may weigh as little as 1,000 kg. The duration of the bulls' stay in and around harems is also very variable. In our sample, the longest stay was 73 days; the average was 58 days. Males lost weight at 12 to 15 kg per day while ashore, although one male lost weight at a prodigious 20 kg/day. Typical males lose around 700 kg during their breeding fast, and some lose up to 1,000 kg.

When compared to lactating females, the rate of weight loss and total

weight loss are much greater and more variable. Females lose around 8 kg/day for 23 days or about 180 kg. Much of this loss is material, largely fat and protein, transferred to their pup. Male losses are largely metabolic, probably mostly fat consumed to provide energy for breeding activity. While males are 2 to 8 or more times the size of breeding females, their total losses are only about 4 times as great, even though they may spend double the time breeding and fasting.

These rates of weight loss are larger than similar estimates for northern elephant seal bulls (Deutsch et al., this volume). This is not unexpected, since southern elephant seal bulls are somewhat larger. The spatial organization of harems on South Georgia is also quite different from that on most northern elephant seal beaches. Harems on South Georgia tend to be linearly arrayed along the shoreline and are often separated by unoccupied beach or rocky headlands. This distribution may require a swim if males from one harem are to visit another. The number of interactions between animals could therefore be quite different when compared to northern elephant seals among which harems often spread back from the beach and are more crowded. This might well influence the energy used during breeding.