Seasonal Inactivity of the Desert Box Turtle, Terrapene ornata luteola, at the Species' Southwestern Range Limit in Arizona

Abstract

At its southwestern range limit in southeastern Arizona, the Desert Box Turtle Terrapene ornata luteola, takes refuge in subterranean mammal burrows, primarily those of the Kangaroo Rat Dipodomys spectabilis, from November through June. I inferred temporal and physiological characteristics of turtles in burrows from temperatures collected with miniature data loggers buried in the soil and attached to turtles from August 2001 through June 2003. In the fall, entrance into hibernation varied among individual turtles and between years from 18 October to 6 December. Mean daily body temperatures at the beginning of hibernation were about 14-188C. From December through February, body temperatures averaged about 98C and were slightly less than and highly correlated with the 30-cm soil temperature. By April, body temperatures of turtles approached levels exhibited by surface-active turtles (minimum approximately 188C for moving turtles) but turtles did not emerge from burrows until the summer monsoons began about three months later. Thus, the five-month hibernation period of T. o. luteola was contiguous with a three-month estivation period for a total annual period of subterranean refuge of about eight months. Except for extending into warm months, the annual inactive period of T. o. luteola in southeastern Arizona was not remarkably different from that in other parts of the species' range. In arid southeastern Arizona, populations of Terra- pene ornata, primarily a prairie-adapted form (Legler, 1960; Dodd, 2001), are at the species' southwestern range limit and may be active for only a few weeks out of the year. In seasonally cold or dry environments, reptiles commonly spend a substantial portion of the year awaiting favorable weather in hibernation or estivation (Gregory, 1982). Furthermore, in areas such as southeastern Arizona, contiguous periods of cold and hot dry seasons may produce an extended period of weather conditions unfavorable for surface activity. With low annual reproductive output and the lowest field metabolic rates reported for any reptile, Terrapene have the potential to be highly successful in unpredict- able resource environments (Penick et al., 2002). Legler (1960) hypothesized that extreme southwestern pop- ulations of T. ornata, formally recognized as Terrapene ornata luteola, were likely to be physiologically adapted for living in rigorous arid environments. However, activity and thermal biology of T. o. luteola during the activity season in southeastern Arizona did not differ substantially from that in more central prairie portions of the species' range (Plummer, 2003). In this paper, I show that certain temporal and thermal aspects of hibernation of T. o. luteola in southeastern Arizona are also not exceptional; however, hibernation is followed by an estivation period that results in a major portion of the annual cycle being spent inactive in underground refugia.