Because of their ability to regulate the expression of many traits simultaneously, hormones are a major physiological factor underlying the integration of the phenotype. In songbirds, testosterone is particularly important for the expression of several traits related to obtaining mates. In collaboration with Ellen Ketterson, we have explored individual variation in testosterone-mediated phenotypes in dark-eyed juncos.
We found that many traits are associated with short-term fluctuations in testosterone levels mediated by the release of gonadotropin-releasing hormone (GnRH) in the brain. Such short-term testosterone elevations were associated with variation in territorial behavior, parental behavior, and plumage and seem to underlie the fundamental trade-off between mating effort and parental effort experienced by male juncos.
Testosterone is also intimately related to fitness. We found strong stabilizing selection on testosterone production via both survival and reproduction. This suggests that selection favors a stable combination of testosterone-mediated traits, which should maintain the current pattern of physiological integration.
In collaboration with Bob Cox at the University of Virginia, we are combining endocrinological and quantitative genetic approaches to understand how hormones shape underlying genetic architecture in Anolis lizards, which will provide further insight into the role of hormones in the evolutionary process.
Cox, R. M., McGlothlin, J. W., and Bonier, F. In review. Hormones as mediators of phenotypic and genetic integration: an evolutionary genetics approach.
McGlothlin, J.W. and E. D. Ketterson. 2016. Hormonal pleiotropy and the evolution of correlated traits. In E. D. Ketterson and J. W. Atwell, eds., Snowbird: Integrative Biology and Evolutionary Diversity in the Junco. Univ. Chicago Press, Chicago.
Cain, K., J. M. Jawor, and J. W. McGlothlin. 2016. Individual variation and selection on hormone-mediated phenotypes in male and female dark-eyed juncos. In E. D. Ketterson and J. W. Atwell, eds., Snowbird: Integrative Biology and Evolutionary Diversity in the Junco. Univ. Chicago Press, Chicago.
Ketterson, E. D., J. W. Atwell, and J. W. McGlothlin. 2014. Evolution of hormones and behavior. pp. 616-623 in J. Losos, ed. Princeton Guide to Evolution. Princeton Univ. Press, Princeton.
McGlothlin, J. W., D. J. Whittaker, S. E. Schrock, N. M. Gerlach, J. M. Jawor, E. A. Snajdr, and E. D. Ketterson. 2010. Natural selection on testosterone production in a wild songbird population. American Naturalist 175: 687-701. (Online appendix)
Ketterson, E. D., J. W. Atwell, and J. W. McGlothlin. 2009. Phenotypic integration and independence: hormones, performance, and response to environmental change. Integrative and Comparative Biology 49: 365-379.
McGlothlin, J. W. and E. D. Ketterson. 2008. Hormone-mediated suites as adaptations and evolutionary constraints. Philosophical Transactions of the Royal Society B 363: 1161-1620.
McGlothlin, J. W., J. M. Jawor, T. J. Greives, J. M. Casto, J. L. Phillips, and E. D. Ketterson. 2008. Hormones and honest signals: males with larger ornaments elevate testosterone more when challenged. Journal of Evolutionary Biology 21: 39-48.
McGlothlin, J. W., J. M. Jawor, and E. D. Ketterson. 2007. Natural variation in a testosterone-mediated trade-off between mating effort and parental effort. American Naturalist 170: 864-875.
Jawor, J. M., J. W. McGlothlin, J. M. Casto, T. J. Greives, E. A. Snajdr, G. E. Bentley, and E. D. Ketterson. 2007. Testosterone response to GnRH in a female songbird varies with stage of reproduction: implications for adult behaviour and maternal effects. Functional Ecology 21: 767-775.
Jawor, J. M., J. W. McGlothlin, J. M. Casto, T. J. Greives, E. A. Snajdr, G. E. Bentley, and E. D. Ketterson. 2006. Seasonal and individual variation in response to GnRH challenge in male dark-eyed juncos (Junco hyemalis). General and Comparative Endocrinology 149: 182-189.
Greives, T. J., J. W. McGlothlin, J. M. Jawor, G. E. Demas, and E. D. Ketterson. 2006. Testosterone and immune function inversely co-vary in a wild population of breeding Dark-eyed Juncos (Junco hyemalis). Functional Ecology 20: 812-818.
McGlothlin, J. W., D. L. H. Neudorf, J. M. Casto, V. Nolan Jr., and E. D. Ketterson. 2004. Elevated testosterone reduces choosiness in female dark-eyed juncos (Junco hyemalis): evidence for a hormonal constraint on sexual selection? Proceedings of the Royal Society of London B 271: 1377-1384.