Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

The history of climate and vegetation for the last 60 million years provide insight into the ecology and physiology of the modern Gopherus agassizii. Both Gopherus and evidence for tropical deciduous forest appeared in the fossil record in the Eocene (55 mya, million years ago). Climates were tropical with only wet and dry seasons and little temperature difference. Tortoises were likely active in warm-wet periods, inactive in warm-dry periods, ate fresh and dried understory warm-season herbs and grasses, and primarily used sheltersites to avoid predators. The uplift of the Rocky Mountains and the Sierra Madre Occidental in the late Oligocene-middle Miocene (25-15 mya) established the modern biotic and climatic provinces in North America. New climate limited biomes (thornscrub, grassland, conifer forests, tundra, etc.) formed along environmental gradients. The modern subgenera (genera) diverged primarily involving enhanced burrowing behavior in Gopherus compared to the more conservative Xerobates. With the development colder temperatures at higher latitudes and elevations, temperate winter and a four-season climate developed. In these areas, tortoise seasonal activity patterns, diet and behavior were little changed except that part of the extended inactive period due to cold (hibernation) and sheltersite were increasingly important environmental buffers. The drying trend that resulted in the formation of the Sonoran Desert (by 8 mya) resulted in the modern five-season climate (winter, spring, foresummer, summer, fall). Selection likely intensified spring inactivity and foresummer estivation. The reversal to more tropical climates in the Pliocene (5-2.4 mya) and the northward expansion of the Gulf of California (Bouse Formation) would not have resulted in major life history evolution. In the Pleistocene, new climatic regimes developed with much cooler summers and the cool season greatly expanded in spring and fall resulting in a general contraction (for the first time) of monsoonal rainfall from the tropical oceans and widespread expansion of Mediterranean climates and biotas. For 85-90% of the last 2.4 mya, glacial climates allowed woodland and chaparral to expanded into desert elevations; Mohave Desert species such as Yucca brevifolia (Joshua tree) were widespread. The Mohave Desert climate can be viewed as a reversal to the original Eocene two-season regime (wet, dry) with two devastating differences: (1) the rains fall in the cool season instead of the warm season leaving the summers very hot and dry and (2) winter temperatures are cold. Tortoises adapted to this new environment in several important ways: (1) building more extensive burrows to buffer extreme cold, heat, and aridity, (2) shifting primary activity period from hot-wet summer to relatively-cool spring rainy season, (3) shift of diet from tropical subshrubs, herbs and grasses to spring annuals, mostly of temperate origins, and (4) a related shift from general consumption to general avoidance of dried plant foods. In this deep historical context, it is clear that the Mohave desert tortoise is the most recently evolved North American tortoise and likely the only one to have adapted to a winter rainfall climate.