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.

Life history consequences of variation in age at primiparity in northern elephant seals

“…1978;Le Boeuf and Bonnell 1980;Cooper and Stewart 1983). The relatively young modal age at primiparity (age 4) at relatively new peripheral colonies, such as Afio Nuevo and the nearby Farallon Islands (Huber 1987), is consistent with a growing population. As some colonies (e.g., Isla de Guadalupe) reach carrying capacity and become crowded at peak season, we expect that the social context of limited breeding space operating through social interactions will increasingly come to favor deferred breeding.…”

“…During a 4-year period on Afio Nuevo Island, under crowded conditions, only 20% of the primiparous 3-year-olds and 35% of primiparous 4-year-olds weaned pups successfully (Reiter et al 1981). Young females of ages 3 and 4 were more successful breeding in the less crowded conditions at the Farallon Islands, weaning 33 and 64% of their pups, respectively (Huber 1987), as well as on the Afio Nuevo mainland (Reiter et al 1981). Moreover, the reproductive success of primiparous females may be poorer, relative to older females, than the weaning success data indicate.…”

“…Risks associated with depleted condition are not restricted to mortality (Stewart and Lavigne 1984). A significantly lowered nutritional plane decreases subsequent reproductive performance in various mammalian species (Clutton-Brock et al 1982;Testa 1987;Huber 1987). Among elephant seals, young females, age 2-5 years, have lower weaning success with sons, which are 8% heavier than daughters, but their survival and reproductive performance the following year is no different than that of older females (Le Boeuf et al 1989).…”

“…Most relevant to the present study is the finding by Huber (1987) on the same species breeding on the Farallon Islands in central California. Survival of females to the year after breeding, as reflected by resighting on the rookery, was significantly lower among primiparous 3-year-olds than primiparous 4-and 5-year-old.…”

“…Because of a report that more P3 females than P4 females skipped pupping the year following primiparity (Huber 1987), we calculated the frequency of all individuals we failed to observe on the rookery in the year following primiparity for females subsequently confirmed to be alive. These unverified absences could have resulted from failure to identify a tagged individual, emigration that was not reported to us, or failure to appear (a true skip in pupping).…”