ESTIMATING CUCKOLDRY IN BIRDS - THE HERITABILITY METHOD AND DNA-FINGERPRINTING GIVE DIFFERENT RESULTS

We compared two methods that have been used to assess cuckoldry in bir ds, i.e. DNA fingerprinting and comparison of heritability of morpholo gical traits between the putative parents. DNA fingerprinting confirme d that the putative father had sired all young in 53 broods of great r eed warblers Acrocephalus arundinaceus, from which 72 young became rec ruits in our study area in subsequent years. On the same data set, we found the heritability of both tarsus length and wing length between m ales and their offspring to be lower than between females and their of fspring. In several studies, a lower heritability value between males and their putative offspring as compared to females and offspring has been interpreted as a result of cuckoldry. In the present study, herit ability was estimated between offspring (measured as adults) and their genetic parents, as confirmed by DNA fingerprinting. This suggests th at maternal effects may explain why tarsus length and wing length in t he offspring are more similar to their mother than to their father. Th e present study then violates one of the basic assumptions of the heri tability method, i.e. a similar contribution from the mother and the f ather to the expression of offspring traits. Whenever phenotypic trait s (e.g. tarsus length and wing length) are under maternal influence, t he method of comparing the heritability of such traits between the par ents would over-estimate the frequency of cuckoldry. Because this kind of maternal effects are very difficult to control for, our study ques tions the heritability method as a reliable tool for revealing levels of extra-pair paternity.