Fetal growth of northern fur seals: life-history strategy and sources of variation

Abstract

Sex-specific growth curves are described for northern fur seal fetuses, Callorhinus ursinus. The relationships between body length, body mass, and gestational age are derived by regression analysis based on 7000 fetuses collected during 1958–1974 as part of a joint Canadian–American pelagic research effort. Male fetuses grow faster and larger than female fetuses. Length approaches an asymptote with time, but the increase in fetal mass appears exponential until parturition. The size of the fetus is influenced by the age, size, and reproductive history of the mother. Primiparous females produce smaller pups than multiparous females. This difference in fetal size is presumably due to physiological changes associated with having been previously pregnant and is not explained merely by differences in the size and age of the different parities. Older and larger females produce progressively larger fetuses until reaching their reproductive prime at about the age of 10–11 years. Adult females continue to grow beyond this age, but there is a senescent decline in the length and mass of the young they carry. There is no indication that the sex ratio differs from unity either between months, across years, or between mothers of different ages and parities.