Does p,p′-DDE thin eggshells?

Abstract

It is generally accepted that p,p′-DDE causes many species of bird to lay eggs with thinner shells, but the published evidence for this proposition is ambiguous. Data on egg size (measured by maximum length), shape (measured by the ratio of maximum breadth to maximum length), shell thickness (measured indirectly by Ratcliffe's index, I) and p,p′-DDE content (log μg g ⁻¹ fresh weight) were therefore examined in samples of eggs taken from three species: two samples from the gannet Sula bassana L., three samples from the shag Phalacrocorax aristotelis L. and six samples from the heron Ardea cinerea L. The value of I is virtually unaffected by changes of egg size but is affected by egg shape, and this variable bias in Ratcliffe's index did in some instances materially affect the conclusions to be drawn from the data. A revised index (J), derived from the prolate spheroid, was therefore developed. The value of J is virtually unaffected by changes of egg size and shape, and yields a much more accurate estimate of the product of the mean shell density and thickness.

The conventional negative linear regression of Ratcliffe's index on p,p′-DDE content occurred in many, but not all, of the egg samples that had at least a 10-fold range of p,p′-DDE concentrations. Two other samples both contained two eggs with p,p′-DDE concentrations of not more than 0·1 μg g⁻¹ fresh weight. The shell index I increased, or at least did not decrease, until the p,p′-DDE content exceeded about 0·1–0·2 μg g⁻¹. We suggest that a curvilinear relationship with a maximum turning point is probably a common physiological response to pollutants.