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338 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCODILIANS 15J6'fl JANUARY ~.\\ 15 I !": (J~I'F ,~I> f"s. ) ~""~ Q""(,""~ ~ / " ~ ~.: ,.;:- $" '; - ~ ~& ~ $5 '" '6'~ '9. '" ./ ~!(J i ~ / t.\l~~\~'t.\'I:i ----.........." .0'" 'i\ ~,t~ \l'l:i'''' Femele Inhabits r:.f;\~ ~\l\t \ ~ tl' Ir Isolated den-type \ ...~o\t ~,,-.j, ..V:l~~"'0 Light -.J w .oleroreos, \ 0(\0">' 0'>1. 0 \\"" bellowing LOCi' 0:' ~LO mole prefers ~o \'0 U deep open -a- = - f- « '" o .oler arees " -r,O\c'\I\lI'1QS II'I\\\O\! IttQlflq ..... ',. ,,~,t.~\\"'~~ \l\'i- ~i -- ~~~ " \l~ ~~~ i _.-~\ '.,~ / ;,,;. ~ 't-'rjiJ~,,+\\\l"'/ /' / '1..- ' ~~ ('q,~-& ~J'" 4.0' ff ~ '0 Y', t'; J'~v('~o~ ~...,,; g ~ 0""-%",; ~ ~~ ~o" ~I' ~ ~0'::>, +~"- §~ \ ':i:, ,~ ~~~ "';\ -~ \\ \~ -
340 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCDDILIANS mone biosynthesis) are present (Lance, 1983, 1984). However, mature sperm do not appear in the seminiferous tubules until late April/early May. Living sperm first appear in the penial groove in early May and are present for only a 6-week period, with maximum output (coincident with female ovulation) during a 2-week period in late May/early June (Joanen and McNease, 1970, 1972, 1973, 1975a, b, 1979a, 1980). Jenkinson (1913) states that crocodilian sperm are small (20-30 jJ-m), but Lance (1983) reported a mean length of 60 to 70 jJ-m for alligators. Mature males have sperm concentrations of 0.5 to 1 billion per ml of semen (Larsen, 1981). Sperm storage occurs principally in the ductus deferens as the epididymis is poorly developed (Lance, 1983). Dead, immotile sperm were first noted in mid-June, and by June 20th, 90% of spermatozoa were dead (Joanen and McNease, 1970, 1973, 1975a, b, 1980). Sperm production varies significantly amongst reproductive size classes (i.e., 1.8 m and above), and large males (over 2.7 m) produce living spermatozoa for a longer time period than do smaller males (Joanen and McNease, 1980). This may explain the reproductive superiority of the large bulls, and why social order favors breeding by males over 2.8 m in length. During testicular regression, the diameter of the seminiferous tubules decreases, they contain progressively fewer spermatozoa, ~5-3J3-HSD activity is minimal and the interstitium is composed of connective tissue (Lance, 1983). There is no evidence for over winter sperm storage in the males; indeed, the epididymides (the usual site of sperm storage in postnuptial reptiles, Lofts, 1977; Moll, 1979) are not enlarged even at the height of the breeding season (Lance, 1983). Plasma testosterone levels show a similar seasonal fluctuation to that observed for testicular weight (Fig. 2) with a peak of 90 ng/ml in April/May (Lance, 1983, 1984) and rapidly decreasing thereafter. The association of elevated testosterone levels with the peak of testicular weight, spermatogenic activity, bellowing intensity, and mating behavior (Fig. 2) suggests that these functions are androgen dependent, although this remains to be tested experimentally. The mechanism initiating gonadal regression is unknown although Lance (1983) discusses several possibilities. Clearly the topic is worthy of investigation not only for its biological interest, as it occurs at a time when temperature and photoperiod appear to be optimal, but also for its importance in alligator farming. The plasma testosterone rises slightly in September, when the alligator testes are fully regressed (Lance, 1983). This is difficult to explain, but it does not coincide with mating behavior, spermiogenesis or renewed spermatogenesis. The profile is, however, typical of postnuptial spermatogenic cycles (Lance, 1983) and may represent the curtailing of another breeding cycle imposed by the climatic habitat of Alligator mississippiensis, the most northerly ranging crocodilian. In this regard, comparison between annual testosterone levels in this species with those in more tropically located crocodilians (particularly those suspected of having two breeding seasons, e.g., Crocodylus niloticus, C. palustris, C. porosus, and C. johnsoni) would be profitable. REPRODUCTIVE BIOLOGY The reproductive system of females shows seasonal variation. Each ovary contains at least three size classes of follicles:
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