Physiological and Behavioral Ecology of Juvenile Gopher Tortoises

Thomas Andrew Radzio

doi:10.17918/etd-7758

Few terrestrial vertebrate ectotherms are strictly herbivorous, and those that are tend to inhabit tropical or warm desert environments, presumably reflecting thermal constraints on digestion of plant matter via fermentation. However, the imperiled gopher tortoise (Gopherus polyphemus) of the southeastern United States is solely herbivorous and often occupies shaded forests. Its ancestral environment is hypothesized to have consisted largely of warmer semi-open canopy longleaf pine (Pinus palustris) forests, but fire suppression and other factors have led to grand scale conversion of this highly biodiverse ecosystem to more shaded habitats. I investigated thermoregulatory strategies, thermal constraints on growth, and antipredator behavior of neonate, hatchling, and juvenile gopher tortoises in a longleaf pine forest of southwest Georgia. My findings indicate young gopher tortoises exploit most thermal opportunities available at burrows, basking increases growth rates, and low environmental temperatures likely constrain growth throughout the active season. Extensive basking begins at the neonate stage, where young exhibit a high-basking, rapid-growth strategy that exposes them to predation risk during thermoregulatory activity, but is presumably optimal overall. Although young bask extensively, their body temperatures may be particularly influenced by shade because they limit this activity to just in front of burrows. The field and laboratory data also suggest that tortoise body temperatures and thermosensitivity of growth are such that small changes in environmental temperature, including those that could be incurred by increased shading or climate change can substantially impact growth rates and time spent at small body sizes, where individuals are most susceptible to predators. Although young tortoises can increase growth rates by basking, surface activity also increases exposure to predators. Individuals appear to manage predation risk by limiting most basking to burrow aprons and entrances, remaining vigilant, and quickly hiding belowground in response to potential predators. Simulated predator approaches on basking hatchlings and juveniles revealed very long flight initiation distances, which increased strongly with size/age, and apparent use of vibrations (aerial or ground) to detect and avoid danger, providing a possible ecological function for uniquely large otolith ear bones characteristic of this species. Consistent with the hypothesis that young exploit most thermal opportunities available at burrows to maximize growth, disturbed individuals hid for short durations, especially when using cool burrows. Similarly, surface activity during the hour following disturbance correlated negatively with burrow temperature. Tortoises raised in captivity during the first year of their lives for thermal physiology experiments also exhibited a high-basking, rapid-growth strategy and generally normal antipredator responses following hard release back into the field. Taken together, behavior, physiology, and environmental temperatures indicate warmer environments can reduce neonate-juvenile exposure to predators by increasing growth rates and, at least during certain times of the year, decreasing surface activity.

Physiological and Behavioral Ecology of Juvenile Gopher Tortoises

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Physiological and Behavioral Ecology of Juvenile Gopher Tortoises

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