Honors Theses

Date of Award

5-2026

Document Type

Undergraduate Thesis

Degree Name

BS

Department

Biology

Faculty Mentor

Jonathan H. Perez, Ph.D.

Advisor(s)

Jason L. Strickland, Ph.D. and Jackie Howell

Abstract

All vertebrates, regardless of breeding strategy, utilize the Hypothalamic Pituitary Gonad (HPG) axis to regulate reproduction. In seasonal breeders, changes in deiodinase enzymes altering local T3 signaling in the hypothalamus are strongly tied to reproduction, but the role of deiodinase enzymes and local T3 signaling in regulating opportunistic species has been poorly studied. Prior work has shown that in female zebra finches, when breeding is repressed via water deprivation, hypothalamic deiodinase 2 expression increases. Deiodinase 2 is the primary enzyme that locally converts prohormone T4 into the bioactive hormone T3, suggesting that T3 plays a role in inhibiting reproduction. This study tested the hypothesis that an increase in T3 directly inhibits reproductive physiology and behavior in the opportunistically breeding Zebra Finch. Using subcutaneous, slow release T3 implants we aimed to establish a causal relationship between elevated T3 levels and suppression of reproduction. Breeding pairs of finches given ad libitum resources, including nesting material, were subject to either a T3 implant or sham implantation. Finches were observed for three and a half weeks before euthanasia and tissue collection. Males in the control group had well-developed nests, while treatment group males had either not constructed a nest or produced incomplete nests. Males maintained large gonads consistent with previous findings, but did show suppression of reproductive behavior (i.e., nest building). Females showed a trend towards reduced follicles and ovarian mass following T3 treatment. Our data supports the modulation of neural T3 signaling as an upstream regulator of the HPG axis to opportunistic breeders.

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