Evolutionary History of the Antelope Jackrabbit,Lepus alleni

Abstract

The antelope jackrabbit, Lepus alleni occurs from south-central Arizona, southward to northern Nayarit, Mexico. In Arizona and the northern part of Sonora the species is sympatric with Lepus californicus. Lepus alleni belongs to the white-sided jackrabbit group of the genus, which separated from a common ancestor to Lepus californicus ∼1.2 MYBP. Genetic evidence indicates that this white-sided jackrabbit group separated into at least three species during the Pleistocene, while L. californicus expanded its range. We contend that L. alleni is a tropic-subtropic species restricted to remnant savannas and those more mesic portions of the Sonoran Desert where summer precipitation and humidity are adequate, and to areas of maritime dew lacking L. californicus. Not only is L. californicus better adapted to aridity, this species is also able to tolerate relatively cold temperatures. L. alleni is confined to habitats having mean annual temperatures between 20°C and 60°C, and goes into hypothermia when body temperatures drop to 28°C. The adaptive advantages of the antelope jackrabbit are its larger size (553–670 mm) allowing for greater heat retention, mobility, feeding on taller vegetation, and ability to peer over taller cover. These morphological and physiological adaptations of L. alleni to climatic conditions and environmental changes are closely related to its evolutionary history. This has restricted this species to habitats with moderate environmental characteristics.