Hormonal control of behavior: novel mechanisms and model organisms
metabolism of hormones, provided significant control over cellular processes, including in the central nervous system (Callard et al. 1978; McEwen 1981). Foundational concepts like these have been refined and reshaped over the subsequent years to sharpen our understanding of the vast and diverse reach of hormones in the control of animal behavior. With the availability now of many animal genomes and sophisticated bioinformatics tools, we are experiencing a new wave of fundamental research into the ways in which hormones influence the cellular transcriptome to define those genes and gene networks on which hormones act to modify cellular function and behavior. Where these approaches have become especially exciting is that we have come more and more to appreciate just how many individual tissues within a single organism can be targets of a particular hormone to influence the animal’s behavioral phenotype. We also now appreciate that some signals that were once considered just hormones are seen to be essential neural-to-neural signals and thus not really hormones at all (Saldanha et al. 2011; Mitre et al. 2017). Some hormones seem to act on virtually every significant tissue in the body and perform a myriad of functions by a multiplicity of mechanisms and control of innumerable tissue-specific genes and gene networks. Altogether, these various hormonal actions serve to control many facets of behavior. While a focus on how and when hormones are released into the bloodstream remains a popular area of research, it is these latter mechanistic functions that are of current greatest interest, including on free-living animals. While we are generally familiar with the clinical implications of advances in endocrine physiology, they are also of great interest to biologists asking diverse questions about the anatomical, physiological and behavioral capabilities of wild animals. Evolutionary biologists consider the ways in which endocrine systems participate in Endocrinology is traditionally defined as the study of chemical signals present in blood that act on target tissues at some distance from the tissue source of the signal. These chemical signals, or hormones, can produce innumerable effects on those target tissues, from effects that occur locally and almost instantly, to some that impact tissues massively and last a lifetime. That degree of influence is dictated by numerous factors starting with the time and amount of hormone released into the circulatory system and ending with the diverse cellular mechanisms through which the hormones influence cell and tissue function (Nelson 2015). The early years of behavioral endocrine research placed a significant focus on the steroid hormones and on determining the amounts of hormone in blood. In some cases, wild vertebrates were the subjects, illustrated by classic summary works such as that of Farner and Wingfield (1980). Alongside these kinds of studies were others aimed at elucidating hormone-dependent intracellular signaling pathways, work largely utilizing laboratory or domesticated animal models (e.g., O’Malley 1984). Such studies also revealed the significant role for enzyme-catalyzed reactions that, by their