Reproductive cycle and sexual size dimorphism of the Nile Crocodile (Crocodylus niloticus) in the Okavango Delta, Botswana

[摘要] ENGLISH ABSTRACT: The reproductive cycle of female and male Crocodylus niloticus was investigated in theOkavango Delta, Botswana.Plasma samples collected from crocodiles larger than 110 cm in total length (TL) wereanalysed for: (i) estradiol-17P (E2), testosterone (T) and progesterone in females, andtestosterone in males by means of enzymatic immunoassays (EIA); (ii) vitellogenin (Vtg), byone-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis; and (iii)triacylglycerol (TAG), total cholesterol (CHO) and phospholipids (PL) by means ofcalorimetric enzyme assays. In addition, spem1 samples were collected from males larger than180 cm TL and observed under a microscope for the presence or absence of spermatozoa.Females reach sexual maturity at 230 cm TL. Only 60 % of adult females reproduced eachyear. Reproductively active females produced plasma vitellogenin during winter and earlysummer (June-September), when elevated plasma steroid E2 concentrations were detected.Sex steroid hormone concentrations were comparable between reproductively and nonreproductivelyactive females, however they did not follow the same cyclicity. Nonreproductivelyactive females showed elevated hormone concentrations during August insteadof Jw1e, and did not produce significant levels of plasma Vtg.One male of 192 cm TL was producing spennatozoa, therefore, male C. niloticus may reachsexual maturity at this size. However this size class may not be able to successfully reproducebecause of competition with larger males. Spermatozoa were present in the semen samplesfrom June, when plasma T started rising, throughout October. Plasma T peaked in Augustduring the mating season.Plasma TAG, CHO and PL, neither varied seasonally, nor conelated with seasonal variationin any plasma steroid hormone concentrations or reproductive events either in females ormales.Variation in plasma sex steroid hormone and lipid concentrations in different size classes wasalso investigated.In addition, four body and three head measurements were analysed for Sexual SizeDimorphism (SSD). The head shape of adult males and females differed significantly, maleshaving a broader and shorter head than females. Baseline infommtion on growth form andequations predicting Snout-Vent Length or TL from any trait measurements are also available.

[发布机构] Stellenbosch University