Milk Intake of Elephant Seal Pups: an Index of Parental Investment C. Leo Ortiz, Burney

The time and energy parents spend producing and rearing offspring are key variables in the evolution of sex ratios (Fisher 1930) and sex differences inmating behavior (Bateman 1948; Williams 1966). Trivers (1972), expanding earlier ideas, argued that the relative parental investment (PI) of the sexes in offspring governs the operation of sexual selection, influences parent-offspring relations, and plays an important role in the evolution of sexual reproduction. His model has stimulated much research (Wittenberger 1981; Barash 1982) even though the key variable, PI, cannot be measured precisely because it requires evaluating diverse nonadditive costs of one offspring (e.g., primary sex cells, feeding, transportation, and defense) with respect to the parent's ability to invest in another (Ralls 1976). We measured an important aspect of PI, the transfer of all milk nutrients from mother to young during lactation, in free-living orthern elephant seals, Mirounga angustirostris. Milk intake of elephant seal pups is a good index of the energetic aspects of PI from birth to weaning because: (1) the sole source of food and water for pups is mother's milk; (2) all milk is derived from the mother's body stores while she fasts from food and water during lactation; and (3) the nursing period is brief (X = 28.5 + 4.5 days) and conclusive, ending abruptly when the mother abandons her pup and returns to sea (Le Boeuf et al. 1972). We obtained an index of the energy mothers invest in suckling pups by estimating milk transfer from measurements of tritiated water turnover (Pace and Rathbun 1945; Lifson and McClintock 1966; Nagy 1975; McFarlane et al. 1969), measuring mass gain of pups from birth to weaning, and from data obtained in a previous study detailing changes in milk composition during nursing (Riedman and Ortiz 1979).