UC Davis

Broadly, the goals of this research were to understand a) how early life differences influence adult phenotype for ducks living entirely in a captive setting and b) how physiological factors provide mechanistic linkages between early and adult phenotypes. Understanding how differences in adult phenotype develop can help us determine which characteristics may contribute to potential reproductive trade-offs and long-term fitness consequences. These questions were explored in two different contexts: domesticated Pekin ducks housed in commercial settings (Chapters 1 & 2) and Wood Ducks reared and living in captivity (Chapter 3). Although seemingly very different, the studies are grounded in a single conceptual framework involving how an animal’s early experience, which can be shaped by several factors both internally and externally, may affect their developmental trajectory through certain underlying mechanisms and then have a long-term effect on the way that they respond to their environment.There are several factors experienced throughout the lifetime of a duck that can influence its phenotype and long-term reproductive success. Factors from the external environment include maternal effects (e.g., incubation efforts as an egg or protection from predators and learning where and when to forage as a duckling), resource availability, weather conditions, and social factors. These factors can act on genetics and physiology (which can also act on one another) to affect phenotype. In adulthood, the responses to any given environment may be fixed due to genome, physiology, early experience, or the interaction between all three factors during development. Alternatively, these three factors may contribute to an animal’s potential for flexibility when exposed to new environmental experiences (social, resource, and weather-related). Understanding variation in adult phenotypes as a consequence of variation in early phenotypes can help reveal the value of particular developmental experiences on future reproductive success. In the first chapter, I studied domesticated ducks in a commercial setting. This chapter focused on if different social factors in the early rearing environment affect male Pekin duck reproductive behaviors in a commercial environment. Male ducks that had physical access to ducklings of the opposite sex in the early rearing environment were compared to males that did not have physical access to the opposite sex. There was no difference in rate of mounting behavior between groups, but I did find different rates across behavior sampling time points. Furthermore, I observed interesting individual differences in rates of mounting among drakes. Ultimately, this chapter provided novel information on Pekin drake reproductive behaviors in a commercial setting. The second chapter focused on the same commercial male Pekin ducks. However, this work had direct implications for management practices intended to be readily utilized by the commercial duck industry. The commercial ducks were from the same genetic strain, so there was likely little genotypic variation. Variation in physiological factors was studied to provide links between early environment and adult reproductive success. Testosterone levels in focal males were determined, and fertility was assessed as a more direct measure of reproductive success. There was no evidence of differences in fertility at the pen level between treatment groups, with fertility overall being rather high and no difference in testosterone levels between treatment groups. However, both fertility and testosterone levels varied across sampling time points. Additionally, I observed individual variation in testosterone secretion between drakes at each sampling point, except before birds were reproductively mature. This chapter linked commercial rearing practice with male duck testosterone levels and fertility to provide commercial producers with direct information on flock reproductive success. The third chapter pivoted away from a commercial setting and instead focused on wild ducks reared and living in captivity. This chapter contributes to behavioral ecology knowledge regarding how early experience affects long-term fitness outcomes. I made connections with life history theory to provide a foundation that can be used for wildlife management strategies related to Wood Duck populations. I assessed adult Wood Duck stress response and compared these measurements to adult body condition. Specifically, I measured adult hormone concentrations related to environmental challenges and energetic regulation that may be relevant during stressful life stages, such as breeding. Though no relationship was found between stress response and body condition, I did reveal repeatable patterns of individual variation in stress response and differences between series taken during and out of the breeding season (in Summer and Fall 2021, respectively). Adult Wood Duck body condition was measured and compared to duckling body condition through previously collected hatchling body morphometric data, and relationships between growth rates and adult body mass were discovered. I then discussed how early phenotype could potentially relate to fitness outcomes. In all, this chapter filled in gaps in our basic knowledge of how variation in early body type can have long-term effects on adult phenotype and the persistence of intra- and inter-individual variation in stress response. In the future, this knowledge of what contributes to individual fitness can be applied to wildlife conservation strategies utilizing artificial housing systems (e.g., captive breeding programs). In conclusion, though the studies addressed in my research were different, they both fell under the same central framework that examines how early experiences can shape adult responses in captive duck species. Across both studies, I was able to begin to reveal previously unknown and surprising levels of individual variation. Variation can begin to tell us which traits are important for these ducks in terms of survival and reproduction and what potential evolutionary consequences there may be for differential survival and reproductive success. Together, these chapters laid the groundwork for future longitudinal captive duck studies on sources of individual variation.