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348 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY DF CRDCODILIANS No available data indicate that the hypothalamus can respond to changing blood temperatures by the secretion of releasing hormones. However, two distinct glycoproteins similar in molecular weight and amino acid composition to mammalian follicle stimulating hormone (FSH) and luteinizing hormone (LH) have been isolated from the pituitaries of Alligator mississippiensis (Licht et al., 1974, 1976). Both of these gonadotropins stimulate testosterone production by ovarian and testicular tissues, with LH being slightly more effective (Licht and Crews, 1976; Tsui and Licht, 1977). There are no data on circulating gonadotropin levels in any crocodilian. Follicular development can be stimulated artificially with pregnant mare serum gonadotropin, but artificial induction of ovulation has not been achieved (Cardeilhac et al., 1982). Alligators (unlike turtles) show a marked response to the synthetic decapeptide luteinizing hormone releasing hormone (LH RH) (Lance, 1983, 1984). Alligator LH-RH is similar to that of mammals but differs from it in having at least one amino acid substitution in the 8 position of the molecule (Lance, 1983, 1984). This is similar, if not identical, to avian LH-RH (King and Millar, 1982). Tissue levels of between 816 and 2780 pg/mg protein have been estimated for the alligator hypothalamus (Lance, 1983, 1984). Footnotes for Table IfI UThc figures quoted are poolt'd maximum, minimum, ilnd i'l\Trage villul's frpm Ihe \'i1riou~ publications cited Older accounts \,",'ith obViously exaggerated figures, e.g., Boake 11870) have been omitted. 'Species classification b.:lscc! on Densmore (1981) and Groombridge (1982) dClarke (I88Ba, b, 1891); Reese (1915b); McIlhenny 0934, 1935); Giles and Childs (1949): POpt> (1956); Jnolnen (196'J\; Ch,lDred< (197:3, 19(5); Joanen dlHj McNease (1975a, 1978, 1979f, :980); GoodWin .'lnd :\1anon (1978). loanen (personal communication); Chu (1957); Chu-Chcn (1982); Behler dnd Joanen (19tl2). tCasas and Guzman (1970); Del Toro (l974). gHagmdnn (1906, 1909); Schmidt (1928); Del Taro (1969); Hunt (1969); Neill (1971); Staton dnd Dixon (1977); Corzula (197R) hFort Worth Zoo (personal communication); Mcdem (1960); Cuy;gisber~ (l\i72); Crawshay.,' and Schaller (980). 'Neill (1971); Del Taro (1974) 'Neill (l q 71); Groombridge (1982). kHagmann (1902a, h, 1909); Recse (1923); Mellem (1963); Croornbridge (19~2). IRec5e (1931 a); Medcm (1971, 1972). m Ander50n (lB7S); Butler (1905); Werner (1933); Singh and Bustard (1977); Singh (1979); Bu"tard (1lJ79, 1980a, 1984); Acharjyo et cd (1975). TlMiil1er (1838); De Rooij (1915); Butler (1905): Sudharfna (1976) °Guib(' (19701); Neill (1971); Tr"'on (980) PDescourtilz (1809); Schmidt (1924); Werner (143.1); Rand (196H); Neill (1971); Carrick and Lln/; (1977); Ogden (1978); CasaS' .'lnd Cuzrn,1n (1970). QWaitkuwail (1982). TBlohm (1982); Medem (1976). 'Dunn (1977); Webb (1977); Webb e, al. (1983e). I Croombridge (1982). UHunt (1973,1975); Del Toro (1975); Pcrcz-IJigdredd (1%0). l'VoeHzkow (1899); Bigalke (1931); Cott (1961, 1969, 1(75); Pooley 0962, 19fil}il, 1971. 1977); Modh
348 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCODILIANS lay in August. Thus the interval between copulation and egg laying could be as short as one month. Detailed autopsies to check for the state of ovarian development in different species are required to determine whether the interval between copulation/ovulation and egg laying varies as widely as the present data suggest. Because crocodilians range over wide areas with drastically varying climatic conditions, a prolonged interval between copulation and egg laying may offer some advantages (Webb et aI., 1983e). During these time intervals, there may be (1) sperm storage in the female and delayed fertilization or (2) a very slow rate of embryonic development while the eggs are in the oviducts (embryonic hibernation). Sperm storage occurs in birds (R. Bellairs, 1971), in many lizards and snakes (Saint Girons, 1962; Cuellar, 1966; R. Bellairs, 1971), and in some turtles (Barney, 1922; Ewing, 1943; Smith, 1956). In artificial insemination experiments, sperm from Alligator mississippiensis has been stored successfully for many days (Joanen and McNease, 1973, 1975b; Larsen, 1981; Larsen et aI., 1982). The length of time that sperm remain viable within the female reproductive tract is unknown. No sperm storage structures can be identified in the female reproductive tracts of A. mississippiensis or Crocodylus Iziloticus from Zimbabwe. Embryos are stored in some turtles (Oliver, 1955; Ewert, 1979) and in the tuatara (Sharell, 1966), but this phenomenon is undocumented for crocodilians. Embryos of Alligator mississippiensis cannot be arrested experimentally (or naturally) in their development at any time after fertilization. It is possible to slow development by cooling the eggs but temperatures below 26°C for prolonged periods are usually lethal (Ferguson, unpublished data). Likewise, freshly laid eggs of Crocodylus niloticus from Zimbabwe apparently die at temperatures below 26°C (Hutton, personal communication). Embryos of A. mississippiensis, C. johnsoni, C. niloticus, and C. porosus are laid at the same stage of development (Ferguson, 1984a), which is considerably advanced (see Section VI) as compared to avian or turtle eggs, which are laid at the late blastula and late gastula stages, respectively (Ewert, 1979). The stage of embryonic development at laying is not invariant, and egg retention within the body of females faced with adverse environmental conditions (e.g., drought, flooding, temperature variations) at the time of normal egg laying has been reported (McIlhenny, 1934, 1935; Reese, 1907, 1915a, 1931a). Females kept in artificial pens at high stocking densities exhibit elevated plasma levels of corticosteroids. These levels inhibit ovulation and hence delay egg laying; however, stressed females also retain eggs within their oviducts for abnormally long times. Such freshly laid eggs may contain embryos that are considerably advanced and have already become attached to the inner aspect of the shell membrane, so that there is an opaque band, or it develops shortly thereafter (see Section III). Such eggs are frequently deposited with the embryo on the underside which causes death (see Section II.F.). How long crocodilians can retain their eggs, the rate of development during this period as com- REPRODUCTIVE BIOLOGY 349 pared to the "normal" rate within the nest, and whether egg retention affects the viability of the embryo are all unknown. The stimuli for egg laying are presumably extrinsic factors, such as temperature, humidity, water levels, and photoperiod. Precisely how they are involved, how they are detected and how they produce their effects are all unknown. Turtles may be induced to deposit eggs prematurely by injections of one to three units of synthetic oxytocin per 100 g body weight (Ewert, 1979). An analysis of the circulating levels of this hormone in crocodilians around the time of egg laying would be useful, as would similar experimental injections into gravid females. If this technique were successful in "forcing" premature egg laying, it would facilitate obtaining the earlier stages of embryonic development, particularly in the rarer, endangered species of crocodilians. D. Nesting Biology An extensive literature on crocodilian nesting biology has accumulated, and detailed accounts are available for Alligator mississippiensis (Feilden, 1810, Clarke, 1888a, 1891; Reese, 1915a, 1931a; McIlhenny, 1934, 1935; Pope, 1956; Joanen, 1969; Joanen and McNease, 1971, 1973, 1975a, b, 1980; Neill, 1971; Fogarty, 1972, 1974; Goodwin and Marion 1978; Deitz and Hines, 1980), Crocodylus acutus (Ogden, 1978), C. cataphractus (Waitkuwait, 1982), C. jolmsOlzi (Webb, 1977a, b; Webb et aI., 1983e; Compton 1981). C. novaeguineae (Graham, 1981), C. niloticus (Voeltzkow, 1891, 1892, 1893, 1899; Cott, 1961, Guggisberg, 1972; Modha, 1967, 1968; Graham, 1968; Graham and Beard, 1973; Graham et aI., 1976), C. palustris (Symons, 1918; Deraniyagala, 1939; Whitaker and Whitaker, 1976a,b, 1977a, b, c; Yadav, 1969, 1979), C. porosus (Deraniyagala, 1936, 1939; Webb, 1977a, b; Webb et aI., 1977, 1983b; Magnusson et aI., 1978; Magnusson, 1980), Caiman crocodilus crocodilus (Del Toro, 1969; Hunt, 1969; Staton and Dixon, 1975, 1977), and Osteolaemus tetraspis (Tryon, 1980). The present account only highlights areas of embryological interest; strictly ecological data are omitted. Some crocodilians construct mound-like nests out of vegetation, whereas others dig holes in the ground (Table Ill). The phylogenetic (Greer 1970, 1971; Campbell, 1972) and ecological (Webb et aI., 1983e) implications of these two strategies have been debated. There is no well-documented example of both strategies occurring naturally in the same species; the behavior is maintained even in sympatric species (such as Crocodylus johnsoni and C. porosus in which a change of strategy from hole to mound by C. jolmsoni might decrease its embryonic mortality due to flooding). Eggs normally are laid during the night or early morning (Voeltzko w , 1899; Cott, 1961). The female positions her cloaca over the egg cavity and deposits the entire clutch at one time. Presumably, the eggs move down the oviducts and through a shell gland one at a time, so that the first
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Fig. 36. Alligator mississippiensis
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