Annual cycle of activity, reproduction, and body mass in Mexican ground squirrels (Spermophilus mexicanus)

The striking patterns in seasonal activity and mass change exhibited by hibernating species of ground squirrels (Spermophilus) are thought to be shaped by climatic conditions and seasonal peaks of environmental productivity. I examined the annual cycle of Mexican ground squirrels (Spermophilus mexicanus) in southeastern New Mexico to test the prediction that because of the moderate winters and summer peaks in precipitation typical of the Chihuahuan Desert, the low-latitude, desert-dwelling Mexican ground squirrel would have a longer active season, a delayed reproductive season, and reduced seasonal changes in body mass compared to species that live at higher latitudes and elevations. I found that adult Mexican ground squirrels are active for roughly 5 months of the year, which is similar to other long-season ground squirrels. Litter emergence and prehibernation fattening in adults and juveniles coincide with the summer peak in precipitation and plant productivity, occurring later in the year than in most Spermophilus. The mating season is delayed after spring emergence and is long relative to other Spermophilus, lasting 30-71 days. Similar to other ground squirrels, mass change in adults is characterized by gains before hibernation, and loss in males during the mating season. However, adult males emerge from hibernation at a very low mass and exhibit an extended period of mass gain after emergence. These findings provide support for the hypothesis that the environment is important for the evolution of annual patterns in Spermophilus.