Relative growth of the major body componrnts of the southern elephant seal. Mirounga leonina (L.)

Abstract

Relative growth of the main body components (fat, muscle, bone, viscera, and skin) is dealt with in detail, particularly in animals which had not attained nutritional independence; a period of accelerated growth in these animals was followed by a complete fast of 5-7 weeks. Seal pups more than trebled their birth weight over the 23-day suckling period, a high percentage of this increase being attributed to deposition of the blubber. During the postweaning fast, muscle, fat, and viscera were lost at a rate similar to the rate of gain during the suckling period, whereas bone and skin did not alter significantly in weight during the fasting phase. After the pups attained nutritional independence, the main body components grew at the same rate as body weight, except that in males at puberty muscle weight increased relatively more quickly than the body weight, and at the expense of bone and viscera. Developmental patterns within the musculature were studied by dissecting the muscles individually, and grouping them according to anatomical location into 10 muscle groups. The development of the musculature during the suckling period tended to be reversed during the postweaning fast. During these two phases, elephant seal pups remained on land most of the time. Developmental changes in the musculature occurred after the seal entered the water, and were modified slightly in sexually mature bulls. These changes are associated with the ecology of this group of animals. Evidence is given that developmental changes within the musculature occurred in response to functional requirements, and it is proposed that this could be brought about by alteration of growth gradients, which are not necessarily reversible during body weight loss. Individually dissected bones were allotted to one of five major anatomical groups, and the weight of the bone groups expressed as a percentage of the total bone weight was used in assessing the results of the changes in bone weight distribution with age. The weight of the bone groups was compared with the corresponding groups in new-born animals. It was found that those bones which are most important structurally (enabling the seal to meet the demands placed on it by the force of gravity) grew most during postnatal life, and that developmental changes in the skeleton occurred in response to functional demands. The winter "rest" periods that immature animals spend ashore are apparently necessary to direct the development of the growing animal to meet these structural demands when it hauls out on land, which it must do later in life to moult and to breed.