Biology and Conservation of North American Tortoises

16 Demography of North American Tortoises

16 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. A Note on Tortoise Size “Size” of a tortoise typically is indicated by one or more shell measurements. In most cases, the principal measurement is midline carapace length (MCL), which is the straight line measurement from the anterior edge of the nuchal scute to the caudal edge of the most posterior scute (supracaudal or 12th marginal). For most species, MCL and straight carapace length (SCL) are synonymous; but, for some species, such as G. berlandieri, they are not. The supracaudal scute curls under in males of G. berlandieri so much that MCL can be shorter than SCL. When the carapace is hinged and movable, other measurements, such as curved carapace length (CCL) or midline plastron length (MPL, from the gular notch to the anal notch) must be taken; but, this is not an issue for North American tortoises. Height typically is the measurement from the bottom of the plastron at its midpoint (junction of the abdominal and femoral scutes) to the top of the carapace and width is the measurement of the carapace at its widest point. A Note on Tortoise Age Because tortoises grow at different rates throughout their lifespans, and the rates at which individuals grow are further modified by the productivity of the environment, aging them is a difficult process. Individuals of the same size in two different locations may be quite different in age. One way that aging may be accomplished is through the use of plastral scute annuli. These are annual growth rings, similar to those in trees. Scute annuli tend to become indistinguishable as individual growth slows with age, however, and annuli often are obscured because of abrasion with sandy substrates. Individuals also may lay down more than one growth ring per year. Thus, one must be cautious about interpreting age from growth rings (Wilson et al. 2003). The use of scute annuli for aging is confined to relatively young individuals, for example, up to 12–15-year-olds in G. polyphemus (Mushinsky et al. 1994, Aresco and Guyer 1999b) and up to 18-year-olds in G. berland­ ieri (Hellgren et al. 2000). GOpHERuS FlAvOMARGinATuS Gopherus flavomarginatus females reportedly become sexually mature at a mean age of 13.9 years, based upon Richards’s growth model and measures of scute annuli of live and preserved animals (Germano 1994). The mean carapace length of adult females from the Mapimi Biosphere Reserve (MBR) is 352 mm (SD = 15.1, range = 315–385, n = 57), and the Earl D. McCoy Gustavo aguirre L. Richard t. kazmaier C. Richard tracy Demography of North American Tortoises Earl D. McCoy, Gustavo aguirre L., Richard t. kazmaier, and C. Richard tracy Demography of North American Tortoises 135 trade-off in these variables. No evidence for such a trade-off in G. flavomarginatus has been demonstrated; no correlation between egg mass and clutch size or between egg length or width and clutch size has been shown. It has been suggested that in other reptiles, the relationship can be obscured by high variability in...