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Chapter 2 provides an overview of the neural and endocrine mechanisms that govern the timing and onset of puberty (reproductive maturation). The cells and hormones that comprise the hypothalamic–pituitary–gonadal (HPG) axis are introduced, followed by an explanation of how both negative and positive neuroendocrine feedback loops regulate circulating levels of gonadal steroid hormones in males and females. The rest of the chapter is devoted to mechanisms that govern the timing of puberty and activation of the HPG axis at the onset of puberty. The role of the metabolic hormone leptin as a permissive signal for the timing of puberty, the role of neural excitation and disinhibition in the awakening of the gonadotropin-releasing hormone (GnRH) neurons at the onset of puberty, and the role of the neuropeptide kisspeptin as a proximal driver of HPG axis activation are highlighted. Finally, recent research on hierarchical gene networks that are ultimately responsible for the developmental unfolding of activation of GnRH neurons at puberty onset is reviewed.

If we are to understand how hormone-mediated traits evolve, we need to examine the mechanisms underlying individual and sex differences in hormones and their effect on physiology, behavior, and ultimately fitness. This chapter begins to unravel this mechanistic black box, focusing on individual and sex variation in production of testosterone (T), sensitivity to T, and the downstream effects of T on organismal biological processes, employing the dark-eyed junco as a model. Correlational and experimental studies at each of these levels of analysis reveal a remarkable degree of independence among the constituent parts of the endocrine system. Further, although the sexes show striking similarities in the abundance of transcript for sex steroid binding and processing molecules at neural targets, the downstream genomic effects of hormones differ between males and females. Thus, while hormonal pleiotropy produces suites of correlated traits, individual variation in circulating T, sensitivity to T, and T-mediated gene expression exists along many different axes within the endocrine system, providing a multitude of different mechanisms on which selection could act. Likewise, the sexes appear to have found partial solutions to sexual conflict over T at each of these parts of the endocrine system, particularly with respect to the downstream genomic effects of T.

Coordinated expression of multiple traits by a hormone can promote an integrated phenotype. Such phenotypic integration can be adaptive if the traits mediated by hormones are most adaptive when co-expressed, but integration may have important consequences for the tempo or ease of evolutionary change. For example, tight integration may hinder the expression of new combinations of traits that may be favored under changing selection pressures. The degree of integration is likely to depend on lability of many links in the complex mechanistic pathway from environmental stimulus to phenotypic response in the face of selection. In this chapter, we explore divergence in testosterone (T)-mediated traits from a mechanistic perspective. We compare two phenotypically distinct dark-eyed junco subspecies, reviewing both upstream variation in T production mechanisms, and downstream variation in sensitivity to T at target tissues. Our analyses suggest that endocrine response mechanisms may be more prone to divergence than circulating hormone levels. An important pattern emerging from these studies is that existing sources of individual variation in endocrine mechanisms (upon which selection may currently act) are not always reflected in differences between populations (that may reflect past selection), posing questions about the evolutionary mechanisms driving diversification.

This chapter considers the physiological basis for migratory activity across organism diversity. One of the keys to timing migration is photoperiod, which allows anticipation of environmental change, and this can interact with an endogenous circannual rhythm to initiate preparation and departure. Preparation includes several known hormonal changes involving the HPG (hypothalamus–pituitary–gonadal) axis and corticosterone in birds and the juvenile hormone/juvenile hormone esterase system in insects. In fish, thyroxin is particularly important. Migration places demands on energetics and requires re-mobilization of resources internally, mediated by hormones, to generate and store enough fuel, especially fat, for the journey. These hormones may be adapted from other functions (hormone capture). There may also be diet shifts to more energy-rich foods prior to departure. During migration guts may be reduced. The physiology of migration varies across the range of species, and it must integrate migration with other functions such as moult in birds, diapause in insects, and reproduction in both groups as well as fish