Is Gopherus agassizii a Desert-Adapted Tortoise, or an Exaptive Opportunist? Implications for Tortoise Conservation

– The desert tortoise (Gopherus agassizii) has traditionally been viewed as an archetypal desert-adapted vertebrate. However, evidence from historical ecology, phylogenetics, anatomy, physiology, and biogeography qualifies this view significantly. Ancestors of G. agassizii stabilized as an essentially modern morph some 17-19 million yrs ago ago, perhaps 12 million yrs 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. Prominent among these are slow metabolic rates and high tolerances for osmotic flux in body fluids. Other functional characteristics for accommodating contemporary aridity are exaptations shared with forest-dwelling batagurid and manourine chelonian antecedents, originally evolved for terrestrialism, not aridity. Large brittle-shelled eggs, herbivory, and a generalized and expansive digestive tract may all be among these symplesiomorphies, at least relative to the gopherine clade. Other anatomical and behavioral features are associated with a fossorial life style which may have developed in sandy habitats within grasslands and along forest edges, where microclimates were semi-arid, but at a time North American landforms had not yet experienced desert aridity. Burrow excavation may have evolved in response to the stress of intense insolation in exposed scrub, grasslands, and meadows, only later serving as protection against cold, heat, and predators. Modern climate and vegetation typical for contemporary populations of G. agassizii have only developed episodically during perhaps the most recent 1% of its 3–5 million yrs history as a distinct species, and especially during the last 7000 yrs. Biogeographically, neither the testudinids as a group, nor G. agassizii as a species, 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 lowland deserts of the Baja California Peninsula, Sahara, Atacama, the Choco, and most of the Arabian Peninsula. Both extant and fossil G. agassizii range well beyond the limits of deserts ecologically into thornscrub, woodland, and grassland habitats. Ecologically, Gopherus tortoises generally, and Mojave 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. Sclerophyllous vegetation, so characteristic of extreme desert habitats, is largely absent from the diet. The desert tortoise functions well in some, but not all, undisturbed desert landscapes. Its survival is contingent upon a combination of ancient exaptations and contemporary adaptations which resist drought and locally dry microclimates and soils, but evolved long before their desert habitats themselves. Semi-arid steppe vegetation, such the mesquite grasslands of the Tamaulipan Plain may combine habitat attributes that are optimal for the G. agassizii (“Xerobates”) species group, as evidenced by the continuing high densities of group member G. berlandieri. Nutritionally, G. agassizii is an opportunistic generalist, shuttling through temporally and spatially patchy forage. As a consequence, G. agassizii appears to be able to accommodate a wide range of environmental changes. Yet 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 significantly compromised.