Author ORCID Identifier

https://orcid.org/0000-0001-7906-9075

Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department/School/Program

Biology

First Advisor

Dr. Vincent M. Cassone

Abstract

In vertebrates, melatonin is a hormone that is produced and secreted at night and inhibited by light. This unique “darkness-only” expression profile makes it an intellectually appealing candidate for a means of transmitting temporal information to an individual, both time of day and time of year.

In passerine birds, “time of day” information is certainly transmitted via melatonin secretion. The primary producer of systemic melatonin in this family of birds is the pineal gland, and surgical removal of it causes a bird to become arrhythmic in constant conditions. I find that as pinealectomized house sparrows (Passer domesticus) become behaviorally arrhythmic in extended constant darkness the molecular clock in their peripheral tissues largely continues to cycle absent its hypothesized synchronizing cues from the pineal gland. This suggests that the peripheral tissues are either autonomous or that there is a possible secondary circadian oscillator that synchronizes these tissues, such as the avian suprachiasmatic nucleus.

In passerine birds, “time of year” information is not transmitted via melatonin secretion to the primary gonads, unlike the case in seasonally breeding mammals. The duration of melatonin, longer in the winter and shorter in the spring as the photoperiod changes with the seasons, does affect secondary sexual characteristics, such as the vocal behavior of birds and the size of the associated nuclei in the brain. I find that long durations of melatonin are sufficient in preventing the photoperiodic expansion of vocal state in male house sparrows. This vocal state change in males consists of the development of a dawn and dusk chorus, as well as a switch from a vocal subtype associated with the wintertime birds flocking together to one of mate attraction and territory defense. This dynamic was independent of the size of the gonads, which were consistent those of photostimulated males. I also investigate this vocal state change in outdoor captive sparrows. This vocal state change is also present in female house sparrows, although unlike in male birds, the presence of absence of the pineal gland does not affect the timing of their vocalizations.

Further, I investigate the ability for rhythmic presentations of vocalizations to influence the circadian clock, in zebra finches, Taeniopygia guttata. Aural cues have been shown to act as a weak external cue for entraining a passerine bird’s circadian clock. I presented various permutations of zebra finch vocalization: a single song played repeatedly, the same song played reversed, random tones, and live monitoring of a breeding colony entrained to a light-dark cycle. The live monitoring was the strongest aural cue, and more generally it appears that novelty and context enhance the effect of audio cues on the circadian clock.

These experiments in sum suggest a role of melatonin in gating the seasonal expression of vocalization behavior in house sparrows. In male birds, a component of the seasonal dynamic is the development of a multimodal rhythm to when during the day the birds vocalize as the days lengthen consistent with spring, suggesting that the circadian clock may be involved with this diel variability. Additionally, this vocal behavior can also feedback onto the circadian clock. The pineal gland and its primary hormone melatonin function, in part, to regulate complex behavioral rhythms in passerine birds.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.478

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