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12 Tortoises are thought to have inhabited North America for 50 million years (Le et al. 2006) and the genus Go­ pherus has been around for at least 30 million of those years (Bramble 1974; chapters 1 and 2). Although assemblages of tortoises in the past apparently harbored two or more species involving one large genus (Hesperotestudo, Manouria, or Stylemys) and one small genus (Gopherus; Le et al. 2006), all modern species belong to Gopherus and occupy areas in which they are the sole representative of this turtle radiation (Morafka 1982). So, tortoises seem to have evolved within assemblages in which they interacted with larger competitors and these interactions seem likely to have resulted in behavioral characteristics of living taxa that are adaptations to competition that no longer exists. Additionally, the genus is characterized, in part, by the presence of subdentary glands that are used in chemical communication (Le et el. 2006), a diagnostic feature that opens the door to the possibility that the earliest members of the genus were as highly social as the living descendants that will be discussed in this chapter. Because living and fossil members of the radiation show a strong affinity for grassland savannas and deserts, the development of extensive social structure in North American tortoises appears to be an adaptation of these creatures to dry open habitats. In this chapter, we review social behaviors of living tortoises and synthesize them with a view toward understanding their historical origin. Similar reviews have been published (e.g., Berry 1986a) and these invariably have noted the great degree to which all species share behaviors. We will focus, therefore, on the shared features of tortoise behavior and not on the relatively minor differences among species. We will point out the unique behavioral features that characterize adaptations of G. agassizii to the extreme aridity of the Mojave Desert, however, as well as shared behavioral features associated with the unusually deep burrows that characterize G. flavomargina­ tus and G. polyphemus. North American tortoises live in aggregations that can be measured from the physical locations of individuals or of their burrows, generally over a season of activity (e.g., Styrsky et al. 2010). A variety of terms is available to describe this feature of tortoise biology, but the one most frequently encountered is the term “colony.” Because burrows are relatively easy to discover and map, delimitation of colonies is frequently based on these structures. Designating tortoises as colonial creates an obvious comparison of tortoises to other colonial vertebrates, such as ground squirrels, that are presented in most ethology texts as classic examples of colonial social organization. Members of a tortoise aggregation typically have the potential to socialize, as evidenced by direct observations or by mapping the spatial arrangement of burrows and using estimates of long distance movements to infer which sets of burrows contain animals that are likely to socialize with one another. Because tortoise aggregations are so similar to ground squirrel colonies, we accept colony as a reasonable term for tortoise social structure. We note, however, that populations of tortoises are also delimited by spatial aggregations of burrows (McCoy and Mushinsky 2007, Styrsky et al. 2010), placing the terms colony and population in jeopardy of becoming synonyms. It would be preferable to have a colony represent some identifiable component of a population. Fortunately, advancements in the analysis of social networks provide a mechanism that allows examination of colony attributes in a fashion that separates them from attributes of the population to which the colonial organisms belong. Colonial social organization in vertebrates has benefits that outweigh costs (Alcock 2009). These benefits generally derive from reduced predation associated with an increased ability of a group to detect approaching predators or the increased diversity of mating opportunities. North American tortoises are capable of recognizing predators and responding aggressively toward them (Barrett and Humphrey 1986), and individuals can vocalize in ways that some have interpreted as alarm calls (Nichols 1953). No tortoise species has been demonstrated to exhibit behaviors consistent with the types of behaviors used by colonial mammals, like ground squirrels, to detect a Craig Guyer sharon M. Hermann Valerie M. Johnson Social Behaviors of North American Tortoises Craig Guyer, sharon M. Hermann, and Valerie M. Johnson Social Behaviors of North American Tortoises 103 to the visual stimulus of an approaching dominant male by turning and walking away at a faster speed than the dominant male (Rose and Judd 1982). In other cases, the subordinate male retracts his head and limbs, a signal...

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