Reproductive Biology of the Rock Squirrel (Spermophilus variegatus) in Southeastern Arizona

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Abstract
Rock squirrels (Spermophilus variegatus) were studied from July 1983 to June 1987 in an oak (Quercus sp.) savanna in southeastern Arizona. Adult males had testes in scrotal position from late March until at least early July. Estrous females were observed from 15 May (the earliest that summer research began in any year) to 16 June. Lactating adult females were observed from early June to early October, and, depending upon the year, the percentage of lactating adult females peaked from early July to late August. The percentage of adult females not lactating during the peak lactation period (16 June–15 September) ranged from 20 to 33% during a particular year. Yearling males had scrotal testes, and some (56%) yearling females lactated. Mating occurred from mid-April to early July. The mating period lasted approximately 9 weeks, and the timing of the mating period was associated closely with heavy summer rains, as juveniles first emerged shortly after the beginning of the rains and the concomitant increase in production of vegetation. Compared to many other ground-dwelling squirrels, rock squirrels in southeastern Arizona had a long breeding season that seemed to be associated with the extent of the summer rainy season and to the absence of a relatively early, cold winter that limits aboveground activity of many North American ground squirrels during autumn and winter.