ReviewHormonal organization and activation: Evolutionary implications and questions
Highlights
► Comparative endocrinology addresses both ultimate and proximate causation. ► Diversity in sexual differentiation processes provides excellent examples. ► Sexual differentiation of behavior in Japanese quail is one such example. ► Diversity in vertebrate sex determining systems is another. ► Phylogenetic views (“tree” thinking) help solve ultimate causation puzzles.
Section snippets
Introduction: asking evolutionary questions about endocrine mechanisms
Biology comes in two flavors. One flavor tackles questions about the internal machinery of life such as physiological and molecular mechanisms. The other addresses questions about ecology and evolution, including the history of life as well as processes resulting in change and adaptation to environments. The scientific goals of these two flavors are often referred to as proximate and ultimate causation [46]. The two sets of questions are known as “how” and “why” questions. Comparative
Sexual differentiation
Animals that come in two sexes undergo a process of sexual differentiation. “Sexual differentiation” refers to the developmental pathways resulting in sex differences in anatomy, physiology, nervous systems, and behavior. Understanding sexual differentiation has long been an important part of comparative endocrinology, and in the case of some animal groups it is one of its many success stories. As is now well known, two key categories of mechanisms are sex genes and gonadal sex steroid
Understanding hormonal organization and activation of behavior in Japanese quail
The role of hormones in sexual differentiation of avian behavior is best understood in the Japanese quail, a galliform species in the same family (Phasianidae) as the chicken (Fig. 2). A summary of this work will reveal additional unanswered “why” questions. Advances in evolutionary developmental biology since that research was carried out offer new ways to think about how to approach answering those questions.
The key behavioral sex differences that have been the focus of the research include
Diversity in sex determining mechanisms
One of the striking features of Table 1 is the dramatic difference between how hormonal organization of behavior operates in quail compared with mammals. Although estrogens are also important in mammalian organization, they act to masculinize females, not demasculinize males [49]. This raises two important unanswered questions. One concerns the relationship between the contrast in sexual differentiation pattern (ovaries organize female quail, testes organize male mammals) and the contrast in
Conclusions
The study of mechanisms producing sex differences in animals continues to be an important part of comparative endocrinology and a rich source of questions about the ultimate causes of the mechanisms themselves. Most of these questions have not yet been satisfactorily answered. Some progress is beginning to be made, however, by incorporating a more explicitly phylogenetic view and the conceptual framework of evolutionary developmental biology. These are essential for understanding what has
Acknowledgments
The author’s research on sexual differentiation in quail and zebra finches was supported by the US National Science Foundation.
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