Natality and weaning success in relation to age of first reproduction in northern elephant seals

Abstract

Reproductive histories of female northern elephant seals (Mirounga angustirostris) tagged as pups at the Farallon, Año Nuevo, San Miguel, and San Nicolas islands were followed on the Farallon Islands, California, from 1975 to 1983. Age of primiparity ranged from 3 to 6 years. Females that first reproduced at age 4 or 5 had significantly higher subsequent natality and weaning success than did females first giving birth at 3 years. However, no difference in weaning success was evident between primiparous and experienced 4- and 5-year-olds. The number of primiparous 3-year-old animals that skipped pupping the following year was significantly higher than the number of primiparous 4-year-old animals that skipped the following year. Overall subsequent natality by parous females (both known-age and females tagged as adults) followed for 4 consecutive years averaged 0.86. On average, 14% (range 8 to 20%) of parous females did not give birth each year. Some of the females that skipped pupping hauled out in the fall with immature animals and some were present at breeding rookeries during the season of their missed pupping only to copulate. From 1974 to 1977, the number of pups born increased an average of 60% each year; however, from 1978 to 1983 the rate decreased to 25%. Most population growth was due to immigration. The range in age of parous females increased from 3 to 5 years in 1975 to 4 to 13 years in 1983.