Cited research: Proc. R. Soc. B doi:10.1098/rspb.2010.1007 (2010)

Vertebrate sexual behaviour and other traits have been correlated with the ratio of the length of the second to the fourth digit, but what underlies the connection between sex hormones and digit ratio remains unclear.

In a study of 1,156 zebra finches, Wolfgang Forstmeier and his colleagues at the Max Planck Institute for Ornithology in Seewiesen, Germany, traced the association to a gene encoding an oestrogen receptor. Different versions of the gene explained about 11% of the variance in digit ratio and 2–5% of the variance in mating behaviour, measured in duration of mating song in males and, in females, number of hops towards males. The researchers think that the receptor may influence digit ratio by activating HOX genes, which regulate morphology. A.K.