Summary.Observed changes in maternal investment due to an environmentally induced decrease in food supply (the 1983 E1 Nifio-Southern Oscillation) are compared with a priori predictions for the California sea lion (Zalophus californianus).Changes in behavior, growth and mortality of offspring were also examined. Data collected in the first two months postpartum for the years before (PRE), during (EN), and the two years after (POST1 and POST2) the 1983 E1 Nifio indicate that females initiated postpartum feeding trips earlier during the food shortage, and spent more time away on individual feeding trips in both the E1 Nifio year and the year after. Perinatal sex ratios (~:d') in the years PRE, EN, POST1 and POST2 were 1:1, 1.4:1, 1.1:1 and 1:1.4, respectively. Fewer copulations were observed during the E1 Nifio year, but this difference was not statistically significant. Pups spent less time suckling in the food shortage year and the year following, but attempted to sneak suckle more. Pups were less active and played on land less in the E1 Nifio and following year. Finally, maternal investment as measured by milk intake of offspring was decreased, pups grew more slowly, and suffered increased mortality during the food shortage year. Despite expected sex differences in maternal investment and pup behavior in response to food shortage, there were no sex-biased differences in response in either females or pups. As expected, the food shortage did not affect adult males since they migrate north during the non-breeding season where the environmental perturbation was less severe.

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