Master of Science
Bruce A. Kingsbury
Date of Award
Despite broad research into the physiological adaptations that animals use to survive the winter there are still relatively few tools available to predict when animals become active. This is true of many reptiles in which important aspects of their life history are poorly understood. Research that better describes these events is important because reptiles are quickly declining throughout the world. Through an increased understanding of when these animals become active it should be possible to better plan anthropomorphic activities to mitigate human impacts on remaining wild populations. Turtles are especially vulnerable to human activities since small rates of adult annual mortality can lead to the loss of populations.
Eastern Box Turtles, Terrapene c. carolina, can serve as a model organism for this group as they are long lived, and many aspects their biology are well understood. Box turtles were once common throughout eastern North America but are rapidly declining across their known range despite their ability to successfully use a variety of habitats and food resources. Major threats to this species include habitat loss, and human induced mortality caused by roads and pet collection. Even some activities, such as prescribed burns which are used in an effort to restore habitats, can cause direct mortality and population declines. The over-wintering biology of box turtles may present an opportunity to mitigate the impacts of human activities on this species because these animals seek refuge from winter by digging burrows. Previous work has demonstrated that underground turtles are protected from surface activities such as fire.
In order to determine when turtles emerge, becoming vulnerable to surface activities, and which environmental variables can be used to predict behavior, temperature loggers were deployed. Devices were attached the carapaces of monitored animals, as well as, at varying soil depths near to where animals over-wintered. As ectotherms box turtles are obligated to their environment for thermal energy and to regulate metabolic activity. Because of their close association with the ground in winter, the temperature of the soil is likely key to determining when these animals can become active. Soil temperatures were found to better determine when turtles became active than either date or air temperatures. In addition, turtles were found to consistently surface prior to fully emerging in spring and that both of these events could be described using soil temperatures. Surfacing was best described by sub-surface surface soil temperatures while surface soil temperatures were best able to determine emergence.
Because shell temperatures and average soil temperatures would be practically difficult to measure in most populations of Eastern Box Turtles, a method using growing degree days is reported here in place of direct measures of temperature to predict spring behavior. Growing degree days have long been used in agriculture to determine when to plant and harvest crops. Recently they have been used with other ectotherm groups, fish and invertebrates, to accurately predict growth and different life stages. In this study we found that GDD could be used to accurately determine when box turtles become active. This is a novel approach for reptiles, and may be useful for determining when other herpetofauna become active in spring.
Christopher L. Woodley (2013).
Predicting Spring Emergence in a Northern Population of the Eastern Box Turtle (Terrapene c. carolina).