The Antelope Ground Squirrel (Ammospermophilus leucurus) lives in desert scrub habitats across an enormous geographic range, from the tip of the Baja California peninsula in México to the northern Great Basin desert in southeastern Oregon. The four western desert regions represented by this 20-degree band of latitude (Figure 1) each contain distinctive vegetation, geological history, and climate. This Burke project investigates how adaptation to distinct local conditions might occur in populations of Antelope Ground Squirrels across their range.
Our field work has shown that reproductive cycles and ecology differ substantially over the range. Populations at the southern tip of the Baja peninsula have a breeding season of up to five months, probably due to the continuous availability of a diverse variety of perennial succulent plant foods that can support gestation and lactation by females and post-weaning feeding by juveniles. On the other hand, more northerly populations have a very restricted breeding season tied to so-called "winter annuals", which germinate in mid to late winter and sustain spring reproduction by rodents. Mating by Antelope Ground Squirrels occurs within a short two to three week window in eastern California and southeastern Oregon, and the timing of mating appears to be tightly coordinated to precede the abundant production of leafy green annual vegetation in late winter and early spring. Unlike the situation in the southern Baja California peninsula, the food supply in the Mojave Desert and Great Basin system has dried up by mid-summer, thus restricting the breeding season of the ground squirrels to late winter and spring.
Examination of genetic structure within Ammospermophilus leucurus also reveals differences between northern and southern groups, based on samples over the range from Oregon to the tip of the Baja California peninsula (Figure 2). A major break in DNA characteristics occurs between populations south of the Vizcaino Desert, at the middle of the Baja California Peninsula, and those to the north (Figure 3). We refer to these different northern and southern groups of populations as "clades". This genetic break suggests that the two clades were separated in the past, either by a physical geographic barrier or by inhospitable habitat that prevented dispersal. Examination of biological traits (perhaps related to reproduction or other important survival functions) may help us understand what evolutionary pressures led to the differences in the northern and southern populations and how the populations adapted to differential environmental conditions. This makes Antelope Ground Squirrels a good model for comparing patterns of genetic diversity and adaptation within a widespread species.
Josh Whorley is working toward his Ph.D. in Zoology and interested in ecological and evolutionary adaptation. His dissertation involves research on Antelope Ground Squirrels across their large geographic range.
Jim Kenagy is Curator of Mammals at the Burke Museum and Professor of Zoology in the Biology Department.
The Burke researchers are also collaborating with Mexican colleagues at the Centro de Investigaciones Biològicas del Noroeste, La Paz, BCS, México: Ticul Alvarez-Castañeda and Patricia Cortés-Calva.
J.R. Whorley, S.T. Álvarez-Castañeda and G.J. Kenagy 2004. Genetic structure of desert ground squirrels over a 20-degree-latitude transect from Oregon through the Baja California peninsula. Molecular Ecology 13:2709-2720.
The genetic structure of populations over a wide geographic area should reflect the demographic and evolutionary processes that have shaped a species across its range. We have examined the population genetic structure of antelope ground squirrels
G.J. Kenagy, J.R. Whorley, P. Cortés Calva, and S.T. Álvarez-Castañeda. 2004. Timing of reproduction in antelope ground squirrels, Ammospermophilus leucurus, near La Paz, Baja California Sur. Pp. 259-264. In: Contribuciones Mastozoólogicas en Homenaje a Bernardo Villa. V. Sánchez-Cordero & R. A. Medelln (eds). Instituto de Biología e Instituto de Ecología, UNAM, México.
Antelope ground squirrels in the Cape Region of Baja California are subject to a complex pattern of environmental seasonality and unpredictability in an ecosystem with diverse plant food resources. The squirrels showed a prolonged season of attempted breeding that was initiated during the dry season, in winter and spring, and extended at least into the beginning of the summer monsoonal rains. Males remained capable of mating for at least four months. Females showed considerable temporal variability (asynchrony) within the population in their attempted production of young. The fact that some elements of the flora reproduce and flourish during the rainy season, others during the dry season, and still others at irregular or continuous intervals suggests that the resource base for breeding by an omnivorous rodent such as