Appropriate timing of reproduction is crucial to animals’ reproductive success and fitness. Animals living in unpredictable environments do not constrain their breeding to one season, but rather breed any time of the year in which good conditions occur. These animals, termed “opportunistic” breeders, cannot predict when good conditions will occur based on the seasons, and must instead respond quickly to proximate environmental factors to successfully breed and raise young. Animals may respond to a number of different cues in the environment to decide when is the best time to breed, and thus the brain must integrate these varying sources of information to produce an appropriate decision.
Cue integration in the brain takes place in the hypothalamus located at the base of the brain and, if conditions are favorable, will result in a physiological reproductive cascade. In stimulatory conditions gonadotropin-releasing hormone (GnRH) is released from the hypothalamus, which stimulates the production and release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland. These two hormones act on the gonads to increase production of testosterone, estradiol and gametes, leading to an increase in fertility. These changes in physiology are associated with an increase in reproductive behavior, including courtship and song behavior. Release of GnRH from the hypothalamus is considered the gateway for reproduction, so research on the mechanisms of reproductive timing often focus on how information is transmitted to the GnRH system and hormones that influence GnRH release.
The goal of this dissertation is to increase our understanding of the mechanisms underlying reproductive timing in response to social cues, stress, and food availability in an avian model of opportunistic breeding, the zebra finch (Taeniopygia guttata). Social cues and food availability stimulate and stress inhibits reproduction in zebra finches, but the endocrine underpinnings of these phenomena are undefined. Chapter 1 provides an overview of the current scientific knowledge regarding the effects of social cues, stress, and food availability on reproductive physiology. The remaining chapters detail experiments I conducted to further elucidate the mechanisms responsible for the effects of these cues on reproductive physiology and behavior, and the dissertation concludes with a discussion chapter.
Chapter 2, “Social stimulation, reproductive behavior and the thyroid hormone deiodinase system in an opportunistically-breeding songbird”, examines the role the presence of an available mate plays in activating reproduction. Isolated males were compared to those receiving a female stimulus or a male stimulus animal. I measured key genes in various tissues of the reproductive axis, activation and protein content of the hypothalamus, and reproductive behaviors. My findings suggest that the hypothalamus is indeed active in response to an available mate and appropriate reproductive behaviors increase, but more work needs to be done to determine how GnRH release might be governed by social cues.
Chapter 3, “Differential response of GnIH in the brain and gonads following acute stress
in a songbird”, looks at stress as a potential mechanism by which to time reproduction. Stress is a known inhibitor of reproduction across vertebrate taxa, and thus can be used to time breeding by increasing the stress hormone corticosterone (resulting in decreased reproductive axis function) during times when environmental conditions are poor. This chapter focuses on gonadotropin-inhibitory hormone (GnIH), a hormone known to inhibit reproduction at all levels of the reproductive axis, in the hypothalamus and gonads. While previous research has suggested a role for GnIH in stressed-induced reproductive inhibition, this is the first experiment to show a differential response of the GnIH system in the hypothalamus and the gonads in response to stress. This study provides more information about the role of GnIH, particularly that the GnIH response to stress may not be as conserved throughout vertebrate evolution as previously thought.
Chapter 4, “Corticosterone measurements differ in plasma sampled from a wing vein and
trunk blood”, is concerned with the methodology used to measure hormones in laboratory and field settings and the potential sources of intra-individual hormone variability. When comparing corticosterone levels between wing blood, a source commonly used in field studies, and trunk blood, a source commonly used in terminal studies, we found differences in corticosterone levels within an individual. This finding illustrates that the source of blood collection for hormone assay is an important source of variability and must be considered in experimental design and when comparing results across studies.
Chapter 5, “Perception of food availability determines reproductive response to food restriction in the zebra finch Taeniopygia guttata”, investigates the role of perception in food-induced reproductive suppression and activation. Animals that were food restricted decreased reproductive parameters (GnRH, general activity), while animals that were food restricted but could see food during restriction maintained higher levels of GnRH and activity similar to controls. This suggests that the visual cue of food is enough to maintain reproductive activity even in conditions of starvation, and that perceptual information as well as metabolic information from food is important in these reproductive activity considerations. Additionally, measurement of expression of the orexigenic peptide neuropeptide Y (NPY) and its correlation with GnRH across groups indicates a potential novel role for this peptide in governing reproductive activation in response to the perception of food. Taken together, these studies indicate the importance of brain integration of social cues, stress, and food availability to initiate breeding during favorable conditions in opportunistic breeders.