Testosterone production, sexually dimorphic morphology, and digit ratio in the dark-eyed junco

The hormonal environment an individual experiences during development can have lasting effects on behavior, morphology, and physiology. However, measuring endogenous hormone exposure during early embryonic development and relating it to adult phenotype has proved to be challenging. The relative length of digit 2 to digit 4 (2D:4D) in adults is thought to reflect the relative concentration of androgens and estrogens during development and has been used as a morphological proxy of developmental hormone exposure, enabling the exploration of the relationships between individual differences in hormone exposure and phenotype. Here, we use this approach and ask whether 2D:4D relates to the responsiveness of the adult hypothalamic-pituitary-gonadal axis and to sexually dimorphic structures (wing, tail, and tarsus length) in a common songbird, the dark-eyed junco (Junco hyemalis). Among males, we found a negative relationship between 2D:4D and the ability to elevate testosterone in response to a physiological challenge, an injection of gonadotropin-releasing hormone. We also found age- and sex-specific relationships between 2D:4D and wing, tail, and tarsus length. We found a positive relationship between 2D:4D and body size measures in females, and second-year and older males, but a negative relationship in first-year males. We conclude that individual variation in exposure to developmental hormones, as reflected by 2D:4D, is correlated with adult hormone production ability and sexually dimorphic morphology in adulthood, suggesting that endogenous variation in steroid hormone exposure may have long-term consequences similar to those seen in experimental manipulations.