Component mass as well as calcium and phosphorus content of unbanded and fertile Nile crocodile (Crocodylus niloticus) eggs

Abstract

The Nile crocodile (Crocodylus niloticus) is a predator, scavenger and an economically valuable aquaculture species in sub-Saharan Africa. Nile crocodiles are farmed principally for their skins, which are used to manufacture high-value leather goods. By-products of this process, such as meat and fat, also have economic value. Despite its economic importance, little is known of the Nile crocodile’s reproductive physiology beyond behavioural and anatomical descriptions, and some cursory seasonal endocrine profiles. In this manner, C. niloticus is not unique: such a paucity of knowledge of the reproductive processes exists for all crocodilian species worldwide, though some species are better researched than others. Furthermore, very little is known about factors that affect egg fertility, foetal survival and resulting hatchling survival. If environmental influences during incubation can be controlled for, the effect of maternally- and paternally-associated factors on embryo, foetal or hatchling survival or performance can be investigated. The egg phase is a critical, self-contained period of the crocodile life cycle, and, compared to the juvenile or adult phases, is an accessible, relatively inexpensive specimen type that can be used to investigate these factors. During the first half of embryonic development, the formation of the chorioallantois and its fusion with the overlying shell membrane results in a macroscopically visible, circumferential opaque band immediately beneath the shell around the lesser diameter of the egg. This band grows as incubation progresses. Crocodilian eggs that were not fertilised, or which contain embryos that died early in development, have no such visible band, and are referred to as ‘unbanded’. In research described in this thesis, the grouping effect of clutch on the mass of the various components (shell, shell membrane, yolk and albumen) of unbanded eggs was evaluated. The effect of potential confounding variables on egg mass and the mass of individual egg components was assessed. Clutch was found to have a strong grouping effect on egg mass, as well as on the masses of individual unbanded egg components. The mass of each component of unbanded eggs was strongly positively correlated with the mass of the egg. Fertile eggs had substantially lighter yolks and shells than unbanded eggs of similar mass. Controlling for egg mass and incubation period, foetal mass was inversely associated with the mass of the intra-abdominal yolk. The period within the laying season during which an egg was laid, had no significant effect on its mass, nor on the mass of any of its components. The occurrence of runt hatchlings is a pervasive issue in captive crocodile hatcheries. Such hatchlings are either severely underweight at hatching, or fail to thrive after hatching to the point that they inevitably succumb. Disorders of calcium (Ca) and phosphorus (P) metabolism have been well described in reptiles, and may arise either from insufficient provision, or aberrant metabolism of these elements. Prior researchers found an association between runt Crocodylus porosus hatchlings and low plasma Ca (although based on their findings, this relationship was not manifestly causal). Given the potential role of these elements in foetal and hatchling pathologies, the present research sought to measure and describe Ca and P concentration and content (and variabilities thereof) in the various components of unbanded and fertile C. niloticus eggs. The grouping effect of clutch on the concentration and content of these elements was determined, and the effect of potential confounders was assessed. The Ca and P content of the unbanded egg’s shell and yolk were influenced principally by the mass of the respective component, and to a lesser extent by the concentration of the element in that component. Contrastingly, shell membrane and albumen Ca and P content were influenced primarily by the concentration of the element. Shell Ca concentration was similar to that of pure calcium carbonate. Fertile egg yolk Ca content was significantly lower than that of size-matched unbanded eggs, suggesting a net depletion of Ca, however yolk Ca concentration of fertile eggs was found to exceed that of unbanded eggs in some cases, which could suggest temporary storage of shell-derived Ca. Yolk P concentration and content of fertile eggs was found to be consistently lower than that of unbanded eggs. It was concluded that the yolk is the primary source of P for the developing foetus, while Ca is derived principally from shell and yolk. Research reflected in this thesis will hopefully be of value in preparation for future research, and in the planning of clinical diagnostic and therapeutic interventions on crocodile farms.