Survival of juvenile Weddell seals Leptonychotes weddellii in McMurdo Sound, Antarctica was examined using 25 years of tagging and resighting data. Using mark–recapture models, we estimated age- and sex-specific apparent survival (survival confounded with emigration) rates of Weddell seals from birth to 6 years of age for 17 cohorts (1973–89). Survival in the first 2 years of life (0.429 and 0.635 for the first and second years, respectively) was lower than annual survival after 2 years of age (0.806), and was significantly lower for males than for females. First-year survival of the 1978 and 1983 cohorts was significantly lower than average first-year survival across cohorts. Reproductive rate of adult females was significantly correlated with first-year survival in concurrent years, but not when first-year survival lagged reproductive rate by 1 year nor with annual survival after 1 year of age. This correlation, however, was only significant in male first-year survival, possibly indicating that survival of males is more dependent on maternal or environmental conditions than survival of females. Average weaning mass varied significantly among cohorts and was nearly correlated to reproductive rate of adult females. This may indicate that maternal condition is reduced in years of low reproduction although more data are needed to test this conclusively.

Density-dependence in juvenile survival may be difficult to detect if survival is also affected by density-independent factors. We investigated the relationships among weather parameters, population density, and lamb survival of bighorn sheep with long-term data from a marked population where we manipulated population density. We distinguished neonatal survival and winter survival. Density interacted with weather variables to affect neonatal survival; spring and winter temperatures had a positive effect on neonatal survival only when population density was high. Neonatal survival was positively affected by spring precipitation independently of population density. Winter survival was positively correlated with temperature and precipitation during the previous spring, negatively correlated with density, and independent of winter temperature or snowfall. The effect of weather on lamb winter survival did not vary with density. Bighorn lambs are well adapted to harsh winter weather, but spring weather influenced survival of lambs at birth and during the subsequent winter, possibly through its effects on forage availability. Our study clearly demonstrates density-dependence in lamb survival. Some of the effects of weather on lamb survival are density-independent, others are mediated by an interaction with population density.