Demography of North American Tortoises

Life-history traits include those characters ultimately influencing the growth and persistence of populations in the face of environmental change. In general, the traits of males, as well as the sex ratios of populations, may not be as important as the traits of females to population growth in North American tortoises, because females of most tortoise species are known to store sperm. The overriding importance of female traits may be especially strong when sperm can be stored for multiple years with little loss of ability to fertilize eggs, which has been demonstrated for Gopherus agassizii (Palmer et al. 1998). Females should be able to produce eggs even when they mate infrequently, so traits of females may be key to predicting population growth. The important female traits include age of first reproduction, clutch size, number of clutches produced per year, and age-specific survivorship. Environmental factors influence all of these traits. Here, we present an outline of the demographic data available for females of the species of North American tortoises. We also present comparative data for males. The type and extent of data vary among species, often reflecting the biases of the researchers gathering the data. Geographical and environmental variation in the data are illustrated, but the actual extent of such variation probably is greater than reported. As demography depends on many rates, such as birth rate and growth rate, information from other chapters (e.g., chapters 7 and 13) can supplement the information presented in this chapter