The relationship between hatchling mass and egg volume in the Nile crocodile (Crocodylus niloticus) : the productivity of eggs varies

Abstract

The survival and vitality of Nile crocodile (Crocodylus niloticus) hatchlings is important for the success of commercial crocodile farming and the maintenance of wild populations. In this study we describe the relationship between the mass of Nile crocodile hatchlings and the estimated volume of the eggs from which they hatched and, following that, we describe the variation in the ratio between the mass of individual hatchlings and the egg volume from which each hatched (the productivity of the egg) within and among clutches. The volumes of 316 Nile crocodile eggs from 51 clutches were estimated by means of a purpose-made algorithm that uses key dimensions of eggs obtained from photos of the hatched shells. The hatchling from each egg was weighed immediately after hatching. Neither the duration of incubation prior to hatching (P = 0.88) nor the year in which clutches were laid (P = 0.35), nor the number of eggs per clutch (P = 0.57) affected hatchling mass. There exists a strong, linear, positive relationship between the mass of Nile crocodile hatchlings and the estimated volume of the eggs from which they hatched (r = 0.88, P < 0.001, n = 316). Without considering estimated egg volume, 86.9% of the variation in hatchling mass occurs among clutches and 13.1% within clutches. Taking estimated egg volume into account, 59% of the variation in hatchling mass occurs among clutches and 41% within clutches. Egg productivity varied from 0.57 g/ml to 0.81 g/ml, with 45.4% of the variation occurring among clutches and 54.6% within clutches. Further research is required to investigate the relationship between hatchling mass and the growth of hatchlings, as well as between egg productivity and the growth of hatchlings.

Die oorlewing en lewenskragtigheid van Nylkrokodil (Crocodylus niloticus) broeilinge is belangrik vir die sukses van kommersiële krokodilboerdery en die instandhouding van populasies in die natuur. In hierdie studie beskryf ons eers die verband tussen die massa van Nylkrokodilbroeilinge en die geskatte volume van die eier waaruit elkeen gebroei het. Daarna beskryf ons die variasie in die verhouding tussen die massa van individuele broeilinge en die volume van die eier waaruit elk gebroei het (die produktiwiteit van die eier) binne en tussen broeisels. Die volumes van 316 Nylkrokodileiers uit 51 broeisels is met ’n doelgemaakte algoritme geskat op grond van sleutelafmetings van die eiers wat op foto’s van die uitgebroeide eierdoppe gemaak is. Die broeiling wat uit elke eier gebroei het is direk na uitbroei geweeg. Nóg die duur van inkubering tot uitbroei (P = 0.88) nóg die jaar waarin broeisels gelê is (P = 0.35) nóg die aantal eiers in die broeisel (P = 0.57) het broeilingmassa beïnvloed. Daar bestaan ’n sterk, lineêre positiewe verband tussen die uitbroeimassa van Nylkrokodilbroeilinge en die geskatte volume van die eiers waaruit hulle gebroei het r = 0.88, P < 0.001, n =316). Sonder inagneming van geskatte eiervolume is 86.9% van die variasie in broeilingmassa die gevolg van variasie tussen broeisels en 13.1% die gevolg van variasie binne broeisels. Indien geskatte eiervolume in berekening gebring word is 59% van die variasie in broeilingmassa vir ’n bepaalde geskatte eiervolume die gevolg van variasie tussen broeisels en 41% die gevolg van variasie binne broeisels. Die produktiwiteit van eiers het van 0.57 g/ml tot 0.81 g/ml gevarieer, met 45.4% van die variasie wat tussen broeisels voorgekom het en 54.6% binne broeisels. Verdere navorsing is nodig om die verband tussen broeilingmassa en die groei van broeilinge, sowel as die verband tussen eierproduktiwiteit en die groei van broeilinge te ondersoek.