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 (AR and ERα) 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 AR 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 ERα. Also, we find that AR 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 AR expression in a tissue-specific manner. 4. These results suggest co-evolution between the physical prowess necessary for display performance and levels of AR expression in avian forelimb muscles. Moreover, this relationship appears to be specific to muscle and AR-mediated, but not ERα-mediated, signaling. 5. Given that prior work suggests that activation of muscular AR is a necessary component of physical display performance, our current data support the hypothesis that sexual selection shapes levels of AR expressed in the forelimb skeletal muscles to help drive the evolution of adaptive motor abilities.
Professor Barnett Schlinger, Chair DHEA (dehydroepiandrosterone) can circulate at relatively high levels with a variety of reported actions on the central nervous system (CNS). Some of these actions require the conversion of DHEA into more active steroidal metabolites catalyzed by the enzyme 3β-HSD. Whereas DHEA is often measured in blood, efforts to evaluate neural 3β-HSD are limited. A role for neural 3β-HSD has been demonstrated in some nonbreeding oscine songbirds when DHEA promotes aggressive behavior, most likely after its neural conversion into active androgens and estrogens. Our lab studied courtship behavior of male Golden-Collared Manakins (Manacus vitellinus) and found that low levels of courtship persist when circulating testosterone levels are basal. Therefore, we tested the hypothesis that DHEA might activate behavior in these sub-oscine birds by determining if DHEA is measurable in blood and by evaluating 3β-HSD expression in the brain and spinal cord. For comparison, we examined 3β-HSD expression in similar CNS tissues of zebra finches, an oscine species in which plasma DHEA and neural 3β-HSD expression have been reported previously. DHEA was detected in manakin blood at levels similar to that seen in other species. Although 3β-HSD was present in all finch iii brain regions examined, 3β-HSD was expressed only in the manakin hypothalamus where it was present at relatively high levels. In the spinal cord, 3β-HSD was detected in some but not all regions examined in both species. These data indicate that manakins have the neural machinery to convert circulating DHEA into potentially active androgens and/or estrogens. ivThe thesis of Joy Eaton is approved.
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