This project uses a long-term study of individually-marked Black Brant Geese in Alaska to understand how producing young in a given year affects an individual's future ability to produce young. Female geese expend substantial nutrients to produce and incubate a clutch of eggs and to care for their broods. Because geese store and deplete nutrients on time scales of several months, use of nutrients for one reproductive attempt may reduce their availability for future reproductive efforts. Nutrient use during reproduction may also reduce future adult survival. Studies of senescence suggest that use of nutrients during reproduction inhibits maintenance at the cellular level and results in earlier senescence. This study relies on substantial numbers of individuals older than 20 years to study impacts of reproductive effort early in life on senescent decline.
The number of Black Brant Geese breeding in Alaska has declined over the past four decades. This study provides principal data used by managers to understand factors influencing population size and dynamics. Data from this project have been influential in management of this species on the Yukon Delta National Wildlife Refuge and throughout their range. Investigators also work closely with local Yupik native residents of western Alaska and employ young people from Chevak, the nearest village.
Our project explores questions about whether decisions that animals make about how much to invest in their offspring are the results of evolution of these decisions or whether the patterns that we observe represent constraints imposed on individuals by their histories and their environments. We have taken two approaches to these questions. First, we manipulate the number of offspring that parents raise by exchanging offspring among families. Second, we follow known individuals throughout their lives to see if investments they made in their offspring or other factors early in life affect the rate at which they undergo senescent decline. We explore all of these questions in a breeding colony of Black Brant Geese on the spectacular Bering Sea coast in south western Alaska (Figs. 1,2). Brant have been studies at this site off and on since the late 1940s, continuously since about 1980. We keep track of individual brant using engraved plastic leg bands (Fig. 3), many of which we apply while the birds are still young, so we know how old they are. We have banded about 50,000 brant since 1984. Each year we encounter about 2,000 marked brant, either at their nests or when we capture them in banding drives in late summer. These latter captures occur when the adults have lost all of their flight feathers (which is characteristic of all waterfowl) and the goslings have not yet attained flight. We have approached our experiments by vising nests of marked brant right after laying is complete and exchanging eggs between nests. In this way, we either increase or reduce the size of the clutches females must incubate. We do similar exchanges right at hatch, when we increase the sizes of some broods while reducing the sizes of others. We have found some somewhat surprising results. We expected that we might find that reducing the number of offspring adult brant had to rear, would result in an increase in the ability of adults to invest in offspring the next year because they didn’t have to work as hard. What we found was exactly the opposite. For female brant that laid 3 or 4 eggs, reducing the sizes of their reduced the likelihood they would nest the next year (Fig. 4). We believe this result may stem from the fact that in geese larger families are socially dominant to smaller families. When we reduced family size, we, therefore, may have reduced a family’s social status, making it harder for them to compete for the nutrient’s they needed to complete breeding the next year. Our analysis of senescence has benefitted from dozens of individuals surviving to at least 20 years old, with one individual making it to 29 years old. We found substantial declines in both survival and percentage of females breeding that began after age 10 (Fig. 5). Interestingly, the rate of senescent decline was more rapid before the mid 1990s than since. We aren’t yet sure what has led to this difference but it may be due to the fact that there was more social stress in the earlier time period because the nesting colony was much more dense then.
