A model by which to estimate the volume of Nile crocodile eggs after they have hatched

Abstract

The value of their leather stimulates commercial farming with several crocodilian species. The survival and growth of crocodile hatchlings depends on their birth mass. There exists a positive relationship between the hatchling mass and egg size of several crocodilian species. The size of crocodilian eggs is often not measured before hatching. The aim of this study was to create a model whereby the volume of individual Nile crocodile eggs can be estimated after they have hatched and the shell been broken, so that the relationship between egg volume and hatchling mass can be studied on an individual basis without the size of the eggs having been measured prior to hatching. Infertile eggs were photographed in side view, with one pole towards the bottom and the other towards the top and a scale in the focal plane. Their volumes were measured by water displacement (displacement volume). A custom-written computer program was used to measure the photo images. The program turned the image of the egg upright and the position of the polar axis was determined without considering the position of the upper pole. Various transverse diameters (perpendicular to the polar axis) were measured, and the volume of the egg calculated by integrating the function describing the horizon of the egg on the polar axis (calculated volume). The best models for estimating the calculated volume were determined by using information limited to various distances from the bottom pole of the egg’s image towards its upper pole, and for eggs of different polarity. The models were then used to estimate the volumes of 138 eggs of which 15%, 20%, 25% and 30% of the image of each egg were masked from the upper pole towards the bottom pole. Volumes of 60–135 ml derived from analysis of the masked photographic images of eggs permitted estimation of the eggs’ volumes to within 4.57 ml of their actual volumes with 95% confidence. This study makes it possible to estimate the volume of eggs from the shells of hatched eggs and identify a variation larger than 7.72% in the birth mass of Nile crocodile hatchlings from eggs with an estimated volume of 60 ml, with the precision increasing to identify a variation larger than 3.40% for hatchlings from eggs with an estimated volume of 135 ml with 95% confidence.

Die waarde van hul leer stimuleer kommersiële boerdery met verskeie spesies van die Crocodilia. Oorlewings- en groeivermoë van krokodilbroeilinge hou verband met hul geboortemassa. Daar bestaan ’n positiewe verband tussen broeilinggrootte en eiergrootte van verskeie spesies van die Crocodilia. Krokodileiers se grootte word dikwels nie gemeet voordat hulle uitbroei nie. Die doel van hierdie studie was om ’n model te skep waarmee die volume van individuele Nylkrokodileiers geskat kan word nadat hulle uitgebroei het en die dop gebreek is, sodat die verband tussen eiervolume en broeilingmassa op ’n individuele basis bestudeer kan word sonder dat die grootte van eiers gemeet is voordat hulle uitbroei. Onvrugbare eiers is in sy-aansig gefotografeer, met een pool na onder en die ander na bo en ’n skaal op die vlak van fokus. Hul volume is met waterverplasing gemeet (verplasingsvolume). ’n Rekenaarprogram wat vir die doel geskryf is, is gebruik om die fotobeelde te meet. Die program het die eierbeeld regop gedraai en die posisie van die poolas bepaal sonder om die boonste pool se posisie in ag te neem. Die lengte van verskeie dwarsdeursnitte (loodreg op die poolas) is gemeet, en die volume van die eier bereken deur die funksie wat die horison van die eier op die poolas beskryf te integreer (berekende volume). Die beste modelle is bepaal om die berekende volume te skat deur gebruik te maak van inligting beperk tot verskillende afstande van die onderste pool in die rigting van die boonste te gebruik, en vir eiers met verskillende polariteit. Die modelle is daarna gebruik om die volumes van 138 eiers waarvan 15%, 20%, 25% en 30% van elke eierbeeld, van die boonste pool in die rigting van die onderste, gemaskeer is. Deur analise van die gemaskeerde fotobeelde kan die volume van eiers met volumes van 60 ml tot 135 ml met 95% vertroue binne 4.57 ml van die werklike voorspel word. Hierdie studie skep die moontlikheid om die volume van uitgebroeide Nylkrokodileiers te skat. Variasie in Nylkrokodille se geboortemassa van groter as 7.72% kan met 95% vertroue geïdentifiseer word vir broeilinge uit eiers met ’n geskatte volume van 60 ml, terwyl die presisie verbeter tot ’n variasie groter as 3.40% vir broeilinge uit eiers met ’n geskatte volume van 135 ml.