The precise function of the modified first toe of males is unknown, although it is reasonably safe to assume that the modification is closely associated with clasping during coition. In the matings that I observed, the inturned first claw of the male secured a hold on the female's rump or just beneath her legs, whereas the remaining three toes gripped the edge of her plastron. The combined hold, on shell and skin, clearly affords the male a more secure position during coitus (whether the female clasps his legs with hers or not) than would a hold on skin or shell alone. Possibly intromission can be maintained in this position even when the female is attempting to escape. In males the plastron is less concave in T. ornata than in T. carolina. Furthermore, males of T. ornata are, on the average, smaller than females, whereas the reverse is true in T. carolina. Possibly the ability of the male to secure an especially firm grip on the female enhances the probability of small males mounting and inseminating larger females, whereas successful matings might otherwise be limited to pairs in which the male was the larger member.
It is worthy of note that turtles of the genus Terrapene are seemingly the only North American emyids that carry out the entire process of mating on land; other, semiterrestrial emyids (for example, Clemmys insculpta and Emydoidea blandingi) return to water for actual coition, although the precoital behavior sometimes occurs on land.
Nearly all gradations from a fully developed zygomatic arch to a greatly reduced arch can be observed in skulls of the various species of Terrapene ([Fig. 2]) (Taylor, 1895:586, Figs. 2-7). The highest degree of reduction is achieved in T. ornata and T. klauberi, both of which lack the quadratojugal bone and have no zygomatic arch whatever (except for an occasional, poorly defined anterior vestige formed by the postfrontal, the jugal, or both). Reduction of the zygoma clearly represents modification of a more generalized, complete arch. As yet there is no clear evidence that reduction of the zygomatic arch is of adaptive value. It is noteworthy, however, that similar reduction of the arch has occurred independently in a number of emyid and testudinid groups, nearly all of which have terrestrial or semiterrestrial habits. Although discussion of phyletic lines in Terrapene is beyond the scope of this report, I tentatively suggest that reduced zygomatic arches have arisen independently in more than one group of Terrapene and that similar reduction of the arch in two species of the genus does not necessarily indicate an especially close relationship of such species.
In a recent survey of cloacal bursae in chelonians, Smith and James (1958:88) reported T. ornata and T. mexicana to be among the few emyids that lacked these structures; in the opinion of the authors (op. cit.:94) cloacal bursae evolved in chelonians that required an accessory respiratory organ for long periods of quiescence (hibernation or aestivation) under water, and were secondarily lost in terrestrial forms that hibernated on land. The assumption is a reasonable one, at least in regard to emyids and testudinids. Lack of cloacal bursae in T. ornata and in all testudinids, can be correlated with the completely terrestrial habits of those turtles. Cloacal bursae seem to be vestigial in the species of Terrapene possessing them and to be of little or no use as respiratory structures (except perhaps in T. coahuila).
In most of the species of Terrapene the carapace has a pattern of pale markings on a darker background; however, unicolored individuals are the rule in certain populations (for example, at the western edge of the range of T. carolina and in T. ornata luteola) and occur as occasional variations in other populations (in T. yucatana, T. mexicana, and, throughout the range of T. carolina, albeit more commonly in the southeastern part of the range). Personal observation of interspecific and ontogenetic variation of color patterns of box turtles has convinced me that a basic pattern of more or less linear radiations is the one from which all other patterns (including spots, blotches, rosettes, and the unicolored condition) can be derived, and, that the radial pattern is generalized and primitive for Terrapene (possibly for all emyids and testudinids as well). In the light of this conclusion, the radial pattern of T. ornata may be considered generalized. I suspect, however, that the pattern of a living species most closely approaching that of the primitive ancestral stock of Terrapene is the pattern of fine, wavy, dark radiations (on a paler background) present in young examples of T. coahuila.
Box turtles in general have lower reproductive potentials (as indicated by fewer eggs and longer prepuberal period) than do most aquatic emyids. This low potential seems to be compensated for by a lower rate of postnatal mortality (especially in adults) due to the protection afforded by the closable shell and the ability to recover from serious injury. Terrapene o. ornata and T. c. carolina are the only box turtles the life histories of which are known well enough to permit significant comparison. The reproductive potentials of T. o. ornata and T. c. carolina seem to be much the same.
PLATE 15
Aerial photograph of Damm Farm (July, 1954).