Date of Graduation

8-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Biology (PhD)

Degree Level

Graduate

Department

Biological Sciences

Advisor/Mentor

Beaupre, Steven J.

Committee Member

DuRant, Sarah E.

Second Committee Member

Willson, John D.

Third Committee Member

Tipsmark, Christian K.

Keywords

Crotalus; Ecdysis; Modeling; Physiological ecology; Rattlesnake; Shedding

Abstract

The semi-frequent replacement of the epidermis (ecdysis) is a characteristic trait of reptiles. Whereas all reptiles regularly engage in some degree of skin shedding, skin morphology in snakes necessitates the synchronous replacement of the entire epidermis and facilitates the subsequent removal of the old layer as a single sheet. To date, the ubiquitous process has garnered little attention from researchers because snakes shed with unpredictable timing and frequency and are exceedingly cryptic during ecdytic cycles; previously impeding detailed physiological or ecological investigations of the process in the clade. Because of the lack of study, ecdysis is often viewed as a maintenance function; occurring whenever change in body size necessitates the generation of a new epidermal layer. However, recent observations that skin shedding plays a role in conspecific sexual signaling in some snakes suggest that the predominate view of ecdysis as a growth function may be overly simplistic. By studying population-scale patterns of shed, I was able to describe the timing and frequency of ecdysis in a population of Timber Rattlesnakes, solving a long-standing problem in continued study of ecdysis; predicting the occurrence of shed events. Coupling my knowledge of patterns of shed timing with novel methodologies for inducing shed, I was able to induce ecdytic cycles in a laboratory setting and herein provide the first measurements of the metabolic effort and duration of shedding in any reptile. I integrated data on the frequency, duration, and metabolic effort of shed into an individual-based computer model of the Timber Rattlesnake to address larger questions about the selective pressures that may shape patterns of shed in snakes. I found that Timber Rattlesnakes shed infrequently (1-2 times per year) and often in close proximity to the mating season regardless of sex. However, the physiological conditions that best correlated to shed frequency differed between males (body condition) and females (reproductive condition). Each shed event required a significant metabolic (3% of the total annual energy budget) and temporal (~28 days at 25⁰C with ~50% of that including some degree of visual limitation from occluded spectacles) investment. In my computer simulations, I found that time spent in shed limited lifetime energy budgets (decreasing lifetime resource acquisition via foraging) and that the energetic effort of ecdysis may serve to limit shed frequency in low resource environments. In my observations of patterns of shed in the wild and through simulations of expected female fecundity under alternate shed frequencies, I found evidence that ecdysis may play a vital role in the reproductive biology of rattlesnakes. Ecdysis is a vital and omnipresent feature of reptilian biology. My data are the first to demonstrate that the frequency of the process is constrained in a population. I provide evidence for the role of growth and body condition, time-energy budgets, environmental conditions, and reproductive events in dictating patterns of shed. As such, patterns of shed may be population specific and serve as an indicator of the important environmental and biophysical forces which shape life histories across populations and species.

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