Annual Cycle, Population Dynamics and Adaptive Behavior of Citellus Tridecemlineatus

A marked population of Citellus tridecemlineatus, averaging 131 individuals per year, was studied for 3 years on a 113-acre tract in northern Texas. Emergence from hibernation was from 12 March to 5 April. Adults began hibernating in July and juveniles in August and September. Adults averaged 135 days of dormancy and 238 days of aboveground activity. Mating occurred aboveground in April, May and June. Litters were born in May, June and July. Some females older than one year produced two litters per reproductive season. Size of litters emerging from the nest burrow ranged from an average of 4.9 for young females to 7.0 for old females. The sex ratio of juveniles was 1:1. The resident adult population varied from 24 individuals in 1965 to 34 individuals in 1964 with a sex ratio of 1 male: 2.8 females. Mature females tended to survive longer than mature males. Home ranges averaged 11.7 acres for males and 3.5 acres for females. Population size was mostly regulated by dispersal of the juvenile part of the population in late summer and early fall. Additional adjustment of population size and sex ratio through dispersal and mortality occurred in the spring. Colonies of thirteen-lined ground squirrels in northern Texas probably result from attraction to a particular habitat rather than attraction of one squirrel to another. Because the species does tend to occur in colonies, both social and individual behavior have evolved. The adaptive significance of vocal signals, defense and care of young, burrow construction and escape behavior are discussed.