Temperature-dependent sex determination in the Nile crocodile, Crocodylus niloticus, in the Okavango River, Botswana, and the effect of global climate change

Maciejewski, Kristine (2006-12)

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

Thesis

ENGLISH ABSTRACT: The Nile crocodile, Crocodylus niloticus exhibits temperature-dependent sex determination, where sex is determined by the incubation temperature of the egg prior to hatching. Laboratory incubation of the eggs indicated that the lower and upper pivotal temperature for the Nile crocodile in the Okavango River, Botswana was 31.4°C and 33.4°C respectively. Exclusively females were produced at a constant temperature of 30.0°C, 30.5°C and 34.0°C, 71% females were produced at 31.0°C and the majority males were produced at 31.5; 32.0; 32.5 and 33.0°C. Crocodylus niloticus in the Okavango Region therefore has a female-male-female pattern of temperaturedependent sex determination where females are produced at lower and higher incubation temperatures. Embryonic development, incubation period, hatching success and development rates were strongly temperature dependent. The location of nests plays an important role in determination of incubation temperature. At a distance of 6m from the river, soil temperature was at a maximum at a depth of 25cm. Breeding females choose nesting sites based on optimal soil temperatures. Along the Okavango River the average nest sites were 5.6m from the river, and the eggs were at an average depth of 24.5cm. Calculation of mean nest temperature during the thermosensitive period (sex determining period) of incubation for ten wild Nile crocodile nests indicated that the nests along the Okavango River are primarily female-biased. An increase in average air temperature due to Global Climate Change could possibly shift the population to a malebiased sex ratio, leading to eventual extinction of the Nile crocodile in the Okavango River. Stellenbosch University https://scholar.sun.ac.za

AFRIKAANSE OPSOMMING: Die Nylkrokodil, Crocody/us niloticus, vertoon temperatuur-afhanklike geslagsdeterminasie waar die geslag afhanklik is van die inkubasie-temperatuur van die eiers net voor hulle uitbroei. Laboratorium-inkubasie van die eiers het aangedui dat 31.4°C en 33.4°C, respektiewelik die beslissende lae en hoe temperatuur vir die Nylkrokodil in die Okavango Rivier, Botswana is. Uitsluitlik wyfies is geproduseer by 30.0°C, 30.5°C en 34.5°C, 71 % wyfies is geproduseer by 31.0°C en 'n meerderheid mannetjies is geproduseer by 31.5°C, 32.0°C, 32.5°C en 33.0°C. Crocodylus niloticus in die Okavango gebied het dus n wyfie-mannetjie-wyfie-patroon van temperatuurafhanklike geslagsdeterminasie waar wyfies geproduseer is by lae en hoe inkubasietemperature. Embrionale ontwikkeling, inkubasie-tydperk, uitbroei-sukses en ontwikkelingstempos was sterk afhanklik van temperatuur. Die ligging van die nes speel 'n belangrike rol in die inkubasie-temperatuur van die eiers. Die temperatuur van die sand was die warmste by 'n afstand van 6m vanaf die rivier en by 'n diepte van 25cm. Telende wyfies kies nesgebiede gebaseerd op optimale sandtemperature. Langs die Okavango Rivier was die gemiddelde nes 5.6m vanaf die rivier en die eiers was 'n diepte van 24.5cm onder die grand. Berekening van gemiddelde nestemperature gedurende die termosensitieve tydperk van broei vir tien Nylkrokodilneste langs die Okavango Rivier, het 'n wyfie-gunstige neiging getoon. Die geslagsverhouding van die krokodilkleintjies is afhanklik van die inkubasie-temperatuur van die nes.; 'n verhooging in temperatuur, a.g.v. globale klimaatsveranderinge, sal die bevolking na 'n mannetjiegunstige geslagsverhouding toe skuif en uiteindelik lei tot die uitsterwing van die Nylkrokodil in die Okavango Rivier.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/50648
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