Digit ratio unaffected by estradiol treatment of zebra finch nestlings

There is rapidly growing evidence that the relative length of the second to fourth digit (digit ratio) in human hands is correlated with a large number of physiological and behavioral traits that are influenced by sex hormones. However, it is still unclear why these correlations exist. Very recently, similar correlations with relative toe lengths have been found in other vertebrate species including the zebra finch (Taeniopygia guttata). These non-human study organisms allow us to test experimentally various hypotheses regarding the mechanisms responsible for these patterns. In the present paper we first attempt to clarify the various hypotheses that have been proposed regarding the pleiotropic factor that is thought to affect both, the digits and the sex-hormone dependent physiology and behavior. We then present an experimental test of one specific hypothesis, namely that high levels of estradiol during the main growth period affect the relative size of the second vs. fourth toe. Twenty-three zebra finch nestlings from nine different families were treated daily (days 5–11 posthatch) with a high oral dose of estradiol benzoate administered by gavage. While the treatment had clear effects on the birds’ reproductive physiology in adulthood (shown elsewhere), we found no significant effect on their digit ratio compared to control birds. At best, the effect could have been very small or restricted to certain genotypes. Hence, it seems unlikely that the estradiol level during the main growth phase is the pleiotropic agent that causes digit ratio to correlate with sex-hormone dependent behavioral and physiological traits.