The Hen's egg: Variation in tensile strength of shell material and its relationship with shearing strength

Synopsis

The tensile strength of egg‐shell material is believed to play an important role in determining whether or not a shell will crack when exposed to an environmental insult. Experiments are described in which shell tensile strength was measured. It is concluded that:

1. the force required to produce tensile failure did not vary linearly with the width of the piece of shell under test, but with its two‐thirds power;

2. it varied linearly with the thickness of the shell;

3. the line relating force at tensile failure to shell thickness did not pass through the origin but intercepted the thickness axis at between about 90 and 130 μm, indicating that the inner layers of a shell, up to about a third of its thickness, contributed little or nothing to its tensile strength;

4. the slope of the line did not vary with hen or strain of hen, indicating that the shells did not differ in the tensile strength of the material constituting their outer layers (i.e. about the outer two‐thirds);

5. there were differences between hens and strains in the magnitude of the intercept: shells of the same thickness but from different hens or strains differed in force at tensile failure;

6. the existence of between‐strain differences in intercept constitutes prima facie evidence that the variation in this trait is under genetic control;

7. there was a fairly large within‐egg residual component of the variance of force at tensile failure; it represents real variation in tensile strength between apparently similar pieces of the same shell and is believed to be an effect of chance in the distribution of flaws in the shell material;

8. for any one hen the intercept is the same as that found when shearing strength is measured, so shearing strength (which can be determined easily, quickly and precisely) can be used as a predictor of tensile strength.

Implications for poultry breeders are discussed; it is suggested that they should select for shell strength instead of shell thickness, measuring strength as shearing strength.