- Understanding the factors that contribute to or limit reproductive success is a fundamental objective of the field of ecology, providing insight into the ways ecosystems function and facilitating better management of natural resources. Behaviors that benefit offspring often increase costs to parents, and thus parents must adjust their level of investment under different conditions to maximize their fitness. Investigating the ways in which individual animals vary parental investment in response to changing environmental conditions is critical to predicting population responses to natural or anthropogenic changes in environmental conditions.
I studied the relationships between provisioning, growth, and survival of Adélie Penguins at a very large, high latitude breeding colony where intraspecific competition for food is thought to limit reproductive rates and colony growth through density dependent processes. I measured three aspects of provisioning, which previous research has shown or suggested might represent variation in parental investment, with increased presumed benefits to chicks coming at an energetic cost to parents. These measurements of parental provisioning effort included: 1) the overall amount of food delivered; 2) the type of food delivered; and 3) the frequency of food delivery. I measured mass and skeletal growth, determined chick sex via molecular methods, and evaluated daily survival probability using quantitative methods which allowed for imperfect resighting probability. I collected data during two breeding seasons, one of which appeared to be relatively typical in terms of growth rates and breeding success (Austral summer of 2012-13; "2012"), and one characterized by apparent food limitation and depressed chick growth rates and reproductive success (Austral summer of 2013-14; "2013"). I compared provisioning, growth and survival at the level of individual penguin families in order to gain a more direct perspective of how trade-offs in parental investment may impact offspring growth and survival, and ultimately how they affect reproductive success.
I found that 1) male penguin chicks gained on average 15.6 g d⁻¹ more, and grew bill length 0.05 mm d⁻¹ faster than female chicks. Growth rates of flipper, tibiotarsus, and foot length were similar between the sexes, and these overall patterns in growth rate were consistent between the two years of the study. In addition 2) the relationships between growth rates and the amount and type of food delivered were different for different morphological features (mass vs. skeletal growth), and also between the sexes. Across most morphological measurements, growth rates of males during 2013 (believed to be the more challenging year) were more sensitive to variation in the provisioning parameters. These differences led me to conclude that during 2013, when there was a greater interval between feedings, parents had to deliver a higher quality (lipid-rich) diet and a greater quantity of food to male chicks in order to maintain their faster growth rate. Contrary to many previous findings for this species, 3) survival rates declined throughout the entire chick-rearing period for both sexes and during both years. This temporal pattern in survival could be related to competition for food at the very large colony where this study took place, with competition increasing throughout the season as chicks grew and require more food. Patterns of decline in survival probability were different between the sexes, but the differences were not consistent between two years of study. In both years the probability of a male surviving from about 10 days post-hatch to the end of the chick provisioning period was similar, at around 0.36; for females however, this probability was 0.68 in 2012 but only 0.17 in 2013. Finally, 4) survival probability could not be predicted directly by growth rates, but was predicted by the size of chicks when they made a critical transition from the guard to the crèche stage at approximately 2-3 weeks old (higher survival with larger size at crèching). Chick size when making this transition was best predicted by growth rates and the age when chicks transitioned to the crèche stage.
These results increase our understanding of the relationships between trade-offs in parental investment and offspring growth and survival. The Southern Ross Sea is a relatively simple ecosystem and Adélie penguins are constrained to breed within the short austral summer, yet my results suggest that alternative parental investment strategies may exist under different environmental conditions for this long-lived seabird. These results also provide critical information to help facilitate sound management of the Ross Sea ecosystem as that system undergoes natural and anthropogenic changes.