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11 Home Range and Movements of North American Tortoises
- Johns Hopkins University Press
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11 Home range and movements of North American tortoises are influenced in both dramatic and subtle ways by climatic factors, topographical features, burrowing substrate, forage availability, social interactions, anthropogenic disturbances, and the physical structure of vegetation. Home range has been defined by Burt (1943) as “that area traversed by the individual in its normal activities of food gathering, mating, and caring for young.” A number of methods used in determining “home range” exist, and the merits of each have been discussed elsewhere (Calhoun and Casby 1958, Jennrich and Turner 1969, Anderson 1982). Descriptions of habitat characteristics for each species are covered in chapter 9, and details of basking, overwintering, social interactions, and egg-deposition sites are discussed in chapters 4, 12, and 13. Although we overlap with some of those topics, our objectives are to explain why, when, and how far tortoises move. Here, we discuss all North American tortoises (genus Gopherus), following nomenclature of Crother (2008) for G. polyphemus, G. flavomarginatus, and G. berlandieri. We use the recent division of the desert tortoise (Murphy et al. 2011) into G. agassizii and G. morafkai. home RanGe size anD DistanCes moVeD Although generalities do exist regarding movements of North American tortoises, considerable individual and site-related variation has been observed in home range size and distances moved for Gopherus polyphemus (McRae et al. 1981, Diemer 1992a, Wilson et al. 1994, Butler et al. 1995, RB Smith et al. 1997, Mitchell 2005), G. berlandieri (Auffenberg and Weaver 1969, Rose and Judd 1975, Judd and Rose 1983, Kazmaier et al. 2002), G. flavomarginatus (Aguirre et al. 1984), and both species of desert tortoise (Burge 1977, Hohman and Ohmart 1980, Turner et al. 1980, Medica et al. 1981, Barrett 1990, Martin 1995, Duda et al. 1999, Averill-Murray et al. 2002a, Franks et al. 2011, Nussear, unpublished data). Judd and Rose (1983) summed it up by noting: “Perhaps the single most striking aspect of the home range ecology of G. berlandieri is the great variability among individuals in home range size.” This behavioral plasticity and intersite variation are both intriguing and confounding, especially when trying to estimate minimum area requirements for preserves (Eubanks et al. 2002, Mitchell 2005). In general, both species of desert tortoise make longer movements and have larger home ranges than the other three species of North American tortoises; however, Kazmaier et al. (2002) documented relatively large home ranges for the diminutive G. berlandieri. Overlapping home ranges tend to be the norm for North American tortoises (McRae et al. 1981, Aguirre et al. 1984, Diemer 1992a, O’Conner et al. 1994b, Averill-Murray et al. 2002a, Kazmaier et al. 2002). On average, adult males of all five species tend to travel more frequently , move longer distances, and have larger home ranges than adult females (McRae et al. 1981, Aguirre et al. 1984, Diemer 1992a, O’Conner et al. 1994b, RB Smith et al. 1997, Averill-Murray et al. 2002, Kazmaier et al. 2002, Eubanks et al. 2003, Franks et al. 2011). This difference between sexes is not surprising, considering the intensive mate-seeking that males undertake (McRae et al. 1981, Douglass 1986, Diemer 1992a, Averill-Murray et al. 2002a, Kazmaier et al. 2002, Boglioli et al. 2003, Eubanks et al. 2003). An exception to the above trend has been documented at two sites in southern Nevada (Bird Spring Valley and Lake Mead), where adult female G. agassi zii had home ranges nearly double that of males (Nussear, unpublished data). Furthermore, some immature tortoises have larger home ranges than some adults (Aguirre et al. 1984, Diemer 1992a, Butler et al. 1995, Kazmaier et al. 2002, Pike 2006). Taking into account variations in habitat type, climatic factors, study duration, sample size, and data analysis, mean home ranges (minimum convex polygons) for adult and immature North American tortoises are shown in table 11.1. In some cases, tortoises moved only between two burrows Joan E. Berish Philip a. Medica Home Range and Movements of North American Tortoises Joan E. Berish and Philip a. Medica Table 11.1. Home range sizes (minimum convex polygons) of North American (Gopherus) tortoises Species Number by sex/size class Mean Home Range (ha) Minimum/Maximum Home Range (ha) Duration of Study (yr) State/Habitat type Reference G. polyphemus 8 M 0.45 0.06–1.44 900 m; Aguirre et al. 1984) and other western species. Emigrating G. polyphe mus, however, have been known to move 0.7–6.4 km over periods ranging from about four days to...