Biology and Conservation of North American Tortoises

5 Reproductive Physiology of North American Tortoises

5 The reproductive biology of each of North America’s tortoises has been studied at least to some degree in all five species. Gopherus agassizii and G. polyphemus have received the most attention from researchers, and these two species are known to vary from each other in important aspects of their reproductive biologies. The complete seasonal reproductive pattern is known for G. agassizii from the eastern Mojave Desert (Rostal et al. 1994b, Lance and Rostal 2002), G. flavomarginatus from the Chihuahuan Desert of Mexico (Gonzalez 1995, Gonzalez et al. 2000), and G. polyphemus from southwestern Georgia, U.S. (Ott et al. 2000), but it is only partially known for the other two species of Gopherus. Although some aspects of the reproductive biologies of G. morafkai and G. berlandieri can be included in this chapter, no endocrine data are available. Thus, some of the generalizations presented in this chapter may need to be revised as more information on these species is gathered. In this chapter, I define the basic reproductive traits of North American tortoises; and when possible, I make interspecific comparisons, particularly of male and female characteristics, the timing of reproduction, reproductive endocrinology, oviposition, and sexual dimorphism . The influences of upper respiratory tract diseases on reproduction also are discussed. maLe ChaRaCteRistiCs The male reproductive system of Gopherus spp. is similar to other turtles. All males possess a paired reproductive tract with testes located intraperitoneally (within the body cavity) and near the kidneys. The epididymides are coiled and are adjacent to the testes. Spermatogenesis (sperm formation) occurs in the seminiferous tubules of the testes, and sperm is stored in the epididymis until mating, which normally occurs in the late summer and fall. During mating, the male inserts its intromittent organ (or phallus; the equivalent of the penis ) into the female cloaca and semen is transferred internally to the female’s oviducts. The intromittent organ in tortoises tends to be round in shape when everted. The testicular cycle is most well studied in G. agassizii (Rostal et al. 1994b, Lance and Rostal 2002; fig. 5.1). A distinct seasonality exists for the cycle of this species. When male tortoises inhabiting the eastern Mojave Desert emerge from hibernation in April, the testes are fully regressed and the lumen of the seminiferous tubules contains only primary spermatogonia, sertoli cells, and cellular debris (the latter seemingly from the prior cycle). The epididymides, on the other hand, are fully packed with spermatozoa. The spermatozoa are quickly put to use, as the tortoises undergo an abbreviated mating period soon after emergence from hibernation. By May, the seminiferous tubules contain only Sertoli cells and spermatogonia, and the cellular debris is gone. In early July, the seminiferous tubules show active cell division of the spermatogonia and abundant spermatids. By September and October, spermatogenesis is complete, seminiferous tubules are at the greatest diameter, and the lumens are full of mature spermatozoa. Spermatogenesis is testosterone dependent and stimulation of gonadal recrudescence (recurrence) in the male appears to be influenced by temperature (Rostal et al., 1994b, Lance and Rostal, 2002). Seasonal testosterone levels are near their nadir (low point) in both male G. agassizii and G. polyphemus when they emerge from hibernation in late March and early April (fig. 5.2). Mating activity following emergence presumably does not generally fertilize the current year’s eggs, as females in most populations ovulate and produce eggs shortly after emergence or in some cases actually emerge from hibernation with fully shelled eggs in the oviduct. Male testosterone begins to rise as the summer temperatures increase. Male testosterone and corticosterone levels reach their maximum levels in August and September when spermatogenesis is complete and fall mating begins. Male–male aggression is also at its peak at this time. It may be a general rule for Gopherus David C. Rostal Reproductive Physiology of North American Tortoises David C. Rostal Reproductive Physiology of North American Tortoises 39 in the upper oviduct where the sperm is stored. The details of fertilization are not known. Following fertilization, membranes and albumin layers are laid down around the ovum. Once a developing ovum passes the isthmus of the oviduct, it enters the shell gland region where the calcified shell is laid down on the slowly rotating ovum. The completed eggs are then held in this region of the oviduct until nesting. In females that lay multiple clutches of eggs, large preovulatory follicles are present in the ovary, in addition to...