25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts

Is Gopherus agassizii a Desert-adapted Tortoise or an Exaptive Opportunist? Implications for Tortoise Nutrition

David J. Morafka¹ and Kristin H. Berry^2
1Department of Biology, California State University, Dominguez Hills,
Carson, California 90747-0005
²U. S. Geological Survey, Western Ecological Research Center, Box Springs Field Station,
Riverside, California 92507-0714

While the desert tortoise (Gopherus agassizii) traditionally has been viewed as an archetypal desert-adapted vertebrate, evidence from historical ecology and phylogenetics, anatomy and physiology, and biogeography qualifies this view significantly. Ancestors of G. agassizii achieved a modern and stable morphology some 18 million years ago (mya) ago, perhaps 12 my before the formation of major regional deserts in North America. Some physiological mechanisms for avoiding or accommodating desert stressors may be symplesiomorphies, primitive character states common to most ectothermic amniotes, such as slow metabolic rates and high tolerances for osmotic flux in body fluids. Other functional characteristics for accommodating aridity were either exaptations shared with forest-dwelling batigurid and manourine antecedents. Large brittle-shelled eggs, herbivory, and a generalized and expansive digestive tract may all be among these synapomorphies. Other anatomical and behavioral features are associated with a fossorial life style which apparently developed in sandy habitats within grasslands and along forest edges, where microclimates were semi-arid, but a time major landforms had not yet experienced desert levels of aridity. Modern climatic and vegetational settings typical for contemporary populations of G. agassizii have only developed episodically over the last 1% of its history.

Biogeographically, neither the testudinids, nor the species G. agassizii, are confined to deserts. Both track more reliably with warm temperate to tropical climates, and appear to be excluded from the extremely arid zones with less than 50-80 mm mean annual precipitation, such as the deserts of the Baja California Peninsula, Sahara, Atacama, and Arabia. Both extant and fossil G. agassizii range well beyond the limits of deserts ecologically into thornscrub, woodland, and grassland habitats.

Ecologically gopher tortoises generally, and G. agassizii, in particular, exploit a wide variety of food resources. Preponderant components of the diet are succulent, herbaceous vegetation ranging from cactus fruit to a variety of grasses and forbs. Even carrion and insects can constitute a small portion of the diet. Sclerophyll vegetation, so characteristic of extreme desert habitats is largely absent from the diet.

The desert tortoise functions well in modern but natural desert landscapes. Its survival is contingent upon a combination of ancient exaptations and contemporary adaptations which resist drought and locally dry microclimates, but evolved long before deserts themselves. Nutritionally it is an opportunistic generalist, shuttling through temporally and spatially patchy forage. When anthropogenic desertification of a pre-existing desert impoverishes the landscape floristically and depletes forage, the opportunities for continued tortoise survival and recruitment may be compromised significantly.