Evolutionary patterns of adaptive acrobatics and physical performance predict expression profiles of androgen receptor – but not oestrogen receptor – in the forelimb musculature

Abstract

1. Superior physical competence is vital to the adaptive behavioral routines of many animals, particularly those that engage in elaborate socio-sexual displays. How such traits evolve across species remains unclear. 2. Recent work suggests that activation of sex steroid receptors in neuromuscular systems is necessary for the fine motor skills needed to execute physically elaborate displays. Thus, using passerine birds as models, we test whether interspecific variation in display complexity predicts species differences in the abundance of androgen and estrogen receptors (<i>AR</i> and <i>ER</i>α) expressed in the forelimb musculature and spinal cord. 3. We find that small-scale evolutionary patterns in physical display complexity positively predict expression of the <i>AR</i> in the main muscles that lift and retract the wings. No such relationship is detected in the spinal cord, and we do not find a correlation between display behavior and neuromuscular expression of <i>ER</i>α. Also, we find that <i>AR</i> expression levels in different androgen targets throughout the body - namely the wing muscles, spinal cord, and testes - are not necessarily correlated, providing evidence that evolutionary forces may drive <i>AR</i> expression in a tissue-specific manner. 4. These results suggest co-evolution between the physical prowess necessary for display performance and levels of <i>AR</i> expression in avian forelimb muscles. Moreover, this relationship appears to be specific to muscle and <i>AR</i>-mediated, but not <i>ER</i>α-mediated, signaling. 5. Given that prior work suggests that activation of muscular <i>AR</i> is a necessary component of physical display performance, our current data support the hypothesis that sexual selection shapes levels of <i>AR</i> expressed in the forelimb skeletal muscles to help drive the evolution of adaptive motor abilities.