Northern fur seal young: interrelationships among birth size, growth, and survival

Abstract

We investigated size at birth, growth, and early survival of northern fur seals (Callorhinus ursinus) from birth to weaning at Bering Island, Russia, over 8 breeding seasons from 1982 to 1989. One thousand and thirteen fur seals (565 males and 448 females) were measured in a longitudinal study and an additional 2697 animals were measured at birth. At birth, female pups were about 10% lighter and about 3% shorter than male pups. The coefficients of variation of mass (12.5 and 12.8%) and length (4.5 and 4.7%) were similar for the two sexes. We partitioned the lactation period into four time periods: 1, the perinatal period (ages 0-10 days); 2, the early development period (ages 11-40 days); 3, the period of intensive molting (ages 41-80 days); and 4, the preweaning period (ages 81-140 days). We investigated four measures of growth: absolute growth in mass (AGM) in grams per day, absolute growth in length (AGL) in millimetres per day, relative growth in mass (RGM) as a percentage per day, and relative growth in length (RGL) as a percentage per day. For both sexes, AGM was highest during period 4 (mean = 124.8 g/day, SE = 7.4 g/day, and mean = 109.6 g/day, SE = 6.8 g/day for males and females, respectively) and AGL was highest during period 2 (mean = 3.74 mm/day, SE = 0.18 mm/day, and mean = 3.42 mm/day, SE = 0.21 mm/day for males and females, respectively). RGM (mean = 1.06%, SE = 0.09%, and mean = 1.02%, SE = 0.11% for males and females, respectively) and RGL (mean = 0.53%, SE = 0.03%, and mean = 0.50%, SE = 0.03% for males and females, respectively) were highest during period 2 for both sexes. For both sexes, growth rates were slowest during the molting period. Sexual differences were detected in AGM in period 4 and for the combined data over periods 1-3. Sexual differences in AGL were detected for the combined data over periods 1, 3, and 4 only. No sexual differences in relative growth were found. Subsequent growth in mass and length was correlated with birth size. We found the greatest annual variation during the periods when growth was fastest. Condition indices were calculated using the allometric relationship between length and mass separately for neonates and pups older than 5 days. The condition indices at birth varied significantly annually. The condition index was lowest during the molting period (3). Animals that survived for at least 40 days were larger at birth and had a higher condition index than those that did not survive. In years of moderate or high pup survival rates, survival rates were higher in animals born later in the breeding season.