Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Doctor of Philosophy




Neff, Bryan D.


Parental care is essential for the survival of many young animals but presents significant costs to the caring parent. To mitigate these costs, parental care systems have evolved to optimize survival and fitness. According to parental investment theory, care allocation is influenced by the offspring’s value, which is often linked to their relatedness to the parent. In this thesis, I explore how hormones and gene expression influence parental care, focusing on bluegill sunfish (Lepomis macrochirus) and the hybrids they produce with pumpkinseed sunfish (Lepomis gibbosus). By manipulating direct and indirect paternity cues – swapping eggs between nests for the former and simulating cuckoldry for the latter – I investigated changes in parental investment via care behaviour. To do so, I examined variations in circulating endogenous 11-ketotestosterone, prolactin, and gene expression. I found that while 11KT levels in bluegill respond to paternity cues, they do not directly regulate parental care behaviour. Rather, these levels seem to indicate preparations for future reproductive events. In contrast, prolactin emerges as a critical hormone in fish parental care, with circulating levels correlating with nurturing behaviour and adjusting in response to perceived paternity. Additionally, I used whole-brain RNA sequencing to determine that gene expression associated with energy transport, immune response, and stress varies in response to paternity perception. Focusing on hybrids, known to provide care despite low genetic relatedness, I found they maintain distinct hormonal profiles and gene expression patterns. Specifically, they exhibit higher prolactin and lower 11-ketotestosterone levels compared to bluegills, pointing to a species-specific regulation of parental care, shaped by evolutionary and environmental factors. Overall, my thesis advances our understanding of parental care regulation in species with male-only care and complex reproductive systems. It underscores the significance of considering a range of endocrine, genomic, and environmental factors in understanding the evolution and maintenance of parental care, thereby enriching our knowledge within evolutionary biology and the neuroendocrine regulation of parental behaviour.

Summary for Lay Audience

Parental care is crucial for the survival of young animals but can be costly for the parent providing care. This thesis delves into parental care, focusing on a species of fish known as bluegill sunfish and their hybrid offspring with pumpkinseed sunfish. Parental male bluegill sunfish provide sole parental care for their brood and can determine which offspring are related to them. They use this information to adjust their level of parental care in response to their paternity. The objective of this research is to determine how hormones and genes influence how parental males adjust their parental care. To do this, I manipulated paternity swapping eggs between nests, or providing a visual cue that the offspring in the nest were not related to the male providing care. I found that a hormone called 11-ketotestosterone responds to cues about paternity. However, it does not seem to directly influence how bluegill care for their young – instead it may prepare them for future reproductive opportunities. In contrast, prolactin, another hormone, plays a crucial role. Prolactin adjusts according to how likely the parental male is to be the true father of the offspring, affecting how much care they give. Furthermore, I used genomic analyses to determine that genes related to energy transport, immune response, and stress vary in response to perceived paternity. Interestingly, I also observed unique traits in hybrid sunfish. Despite their low probability of paternity, they still provide parental care. Their distinct hormonal profiles and quality of parental care suggest a species-specific system of parental care, influenced by their genetic background and environmental factors. Overall, this research challenges traditional views on the regulation of parental care in fish, especially those with intricate reproductive systems. It highlights the importance of considering a range of hormonal, genetic, and environmental factors to understand the evolution and persistence of parental care. This work not only advances our knowledge within evolutionary biology, but also opens new avenues for future research in the neuroendocrine regulation of parental behaviour.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License