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

Detoeuf-Boulade, Audrey Simone (2006-12)

Thesis (MSc)--University of Stellenbosch, 2006.

Thesis

ENGLISH ABSTRACT: The reproductive cycle of female and male Crocodylus niloticus was investigated in the Okavango Delta, Botswana. Plasma samples collected from crocodiles larger than 110 cm in total length (TL) were analysed for: (i) estradiol-17P (E2), testosterone (T) and progesterone in females, and testosterone in males by means of enzymatic immunoassays (EIA); (ii) vitellogenin (Vtg), by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis; and (iii) triacylglycerol (TAG), total cholesterol (CHO) and phospholipids (PL) by means of calorimetric enzyme assays. In addition, spem1 samples were collected from males larger than 180 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 each year. Reproductively active females produced plasma vitellogenin during winter and early summer (June-September), when elevated plasma steroid E2 concentrations were detected. Sex steroid hormone concentrations were comparable between reproductively and nonreproductively active females, however they did not follow the same cyclicity. Nonreproductively active females showed elevated hormone concentrations during August instead of Jw1e, and did not produce significant levels of plasma Vtg. One male of 192 cm TL was producing spennatozoa, therefore, male C. niloticus may reach sexual maturity at this size. However this size class may not be able to successfully reproduce because of competition with larger males. Spermatozoa were present in the semen samples from June, when plasma T started rising, throughout October. Plasma T peaked in August during the mating season. Plasma TAG, CHO and PL, neither varied seasonally, nor conelated with seasonal variation in any plasma steroid hormone concentrations or reproductive events either in females or males. Variation in plasma sex steroid hormone and lipid concentrations in different size classes was also investigated. In addition, four body and three head measurements were analysed for Sexual Size Dimorphism (SSD). The head shape of adult males and females differed significantly, males having a broader and shorter head than females. Baseline infommtion on growth form and equations predicting Snout-Vent Length or TL from any trait measurements are also available.

AFRIKAANSE OPSOMMING: Die voortplantingssiklus in wyfies en mannetjies van Crocodylus niloticus in die Okavangodelta, Botswana, is ondersoek. Plasma-monsters, versamel van krokodille met 'n to tale lengte (TL) groter as 110 cm, is geanaliseer vir: (i) estradiol-17P (E2), testosteroon (T), progesteroon (P) in wyfies en testosteroon in mannetjies m.b.v. ensiem-immuno-essaiering (EIE); (ii) vitellogenien (Vtg) m.b.v. eendimensionele sodiumdodesielsulfaatpoli-akrielamied-gel-elektroforese en (iii) triasielgliserol (TAG), totale cholesterol (CHO) en fosfolipiede (FL) m.b.v. kolorimetriese ensiem-essais. Sperm van mannetjies, groter as 180 cm TL, is ook versamel en onder die mi.kroskoop bestudeer vir die teenwoordigheid of afuresigheid van spermatosoe. Wyfies bereik seksuele volwassenheid by 'n TL van 230 cm. Slegs 60 % van volwasse wyfies het voortgeplant elke jaar. Seksueel-aktiewe wyfies produseer plasma-vitellogenien gedurende die winter en vroee somer (Junie-September), wanneer verhoogde plasma-stero'iede (E2) waargeneem is. Geslagssterol"ed-honnoonkonsentrasies is vergelyk tussen seksueel-aktiewe en seksueel-onaktiewe wyfies, maar hulle het nie dieselfde siklus getoon nie. Seksueelonaktiewe wyfies het verhoogde hormoonkonsentrasies getoon gedurende Augustus i.p.v. Junie, en het nie beduidende vlakke van plasma-vitellogenien geproduseer nie. v Een mannetjie met 'n TL van 192 cm het spem1atosoe geproduseer. Dit is dus moontlik dat C. niloticus-mannetjies seksuele volwassenheid by hierdie grootte bereik. Hierdie grootteklas mag dalk egter nie in staat wees om suksesvol voort te plant nie, weens kompetisie met groter mannetjies. Spermatosoe was teenwoordig in die semenmonsters geneem in Junie, wanneer die plasma-testosteroon begin styg het tot regdeur Oktober. Plasma-testosteroon het 'n piek bereik in Augustus gedurende die paarseisoen. Geen seisoenale variasie is opgemerk in plasma TAG, CHO en FL, asook geen korrelasie met seisoenale variasie in enige van die plasma-stero'iedhom10011konsentrasies of voortplantingsverskynsels in wyfies of mannetjies nie. Variasie in die konsentrasie van plasma-geslagsstero'iedhonnone en plasma-lipiede is ook ondersoek in die verskillende grootte-klasse. Vier liggaams- en kopmates is ook geanaliseer vir geslagsdimorfisme. Die vonn van die kop in volwasse mannetjies verskil aansienlik van die van volwasse wyfies. Mannetjies het 'n breer en kotter kop as die van wyfies. Basiese inligting oor die groeivonn en vergelykings wat die snoet-kloaaklengte of TL bereken vanaf enige tipiese mate, word ook verskaf.

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