Doctor of Philosophy
Parental care is very critical for reproduction in species that provide it. Hormones such as prolactin and androgens play a crucial role in parenting and reproductive behaviours. In mammals and birds, prolactin’s role in parental care is well-established; it stimulates milk production and stimulates attachment to newborns by its release in the preoptic area of the hypothalamus (POA), among other functions. Androgens, on the other hand, are commonly associated with male secondary sex characteristics, territoriality, and aggressiveness in mammals and birds. There are similar reports of prolactin and androgen effects in fish, but there are fewer studies. I investigated if an androgen and prolactin have any similar effect on a paternal fish. Bluegill sunfish (Lepomis macrochirus) parental males build nests and wait for females to visit and lay eggs. The males exhibit aggressive behaviours toward any fish that comes close to the nest rim and also provide nurturing behaviours. In order to test the behavioural effects of androgens and prolactin, I artificially manipulated hormone concentrations using silastic implants: an androgen (11-ketotestosterone [11-KT]), an androgen antagonist (flutamide), prolactin, a prolactin-release inhibitor (bromocriptine), or castor oil (placebo). I found that prolactin increased the number of nurturing behaviours such as fanning the eggs, and 11-KT increased aggressive behaviours such as biting a nest intruder. I also found that there was a trade-off between these behaviours. My results support the behavioural effects of both hormones in fish. In order to test how the hypothalamus is involved in these reproductive behaviours, I also analyzed gene expression in the mesencephalon and diencephalon on a subset of these fish (control, 11-KT and prolactin), but there were no differences between the treatments. I discuss the possible reasons for obtaining a null result. It is possible that these two areas of the brain do not respond differentially to 11-KT and prolactin. Finally, I addressed the immunocompetence handicap hypothesis (ICHH), which posits a trade-off between aggressive behaviours, and thus androgen concentration, and the immune response. My results in bluegill do not support the ICHH, as there was no decrease in any measured immune responses to the androgen (11-KT). I also found that prolactin did not increase the immune response as it does in some other fish, birds and mammals. This work adds bluegill to the group of fish species in which prolactin and 11-KT have an effect on behaviours and weakens support for the ICHH, at least in fish.
Summary for Lay Audience
There are species where both or one of the parents care for the offspring after birth or after eggs are laid. Abandonment in such species means the offspring's death. Parental behaviours are, therefore, critical for fitness and are controlled by hormones. Prolactin, a hormone known for stimulating milk production, is also involved in different parental behaviours related to caring for the offspring. Another group of hormones is the androgens, commonly associated with male secondary sex characteristics, territoriality, and aggressiveness. These functions are well-established in mammals and birds but not in fish. In this study, I used bluegill (Lepomis macrochirus), a fish species in which parental males build nests and wait for females to visit and lay eggs. The males protect the nest and eggs and also provide nurturing behaviours making them a good species for testing parental care behaviours in fish. To test if androgens and prolactin have any effect on behaviour, gene expression and the immune response, I artificially manipulated hormone concentrations by increasing or decreasing the concentration of prolactin and androgens. I predicted that androgen would increase the number of aggressive behaviours and prolactin would increase the number of nurturing behaviours. The results confirmed my predictions and I also found that while aggressive behaviours are high, nurturing behaviours are low and vice-versa. I also analyzed gene expression in parts of the brain in a subset of these fish to understand which genes are involved in the hormones and behaviours. Finally, I addressed the immunocompetence handicap hypothesis (ICHH), which states that high androgen concentration decreases the immune response. My results in bluegill do not support the ICHH, as there was no decrease in the immune response to the androgen. I also found that prolactin did not affect the immune response as it does in mammals and birds. This work supports androgen’s and prolactin’s parental role in fish. The data presented here does not support the ICHH in fish.
Cunha, Adriano A P da, "Effects of 11-KT and Prolactin on Gene Expression, Parental Care Behaviour and Immune Response in Male Bluegill Sunfish." (2023). Electronic Thesis and Dissertation Repository. 9418.
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