Neuroendocrine Systems and Avian Sexual Differentiation

Sexual differentiation of the endocrine and behavioral componentsof the avian reproductive system occurs throughout embryonicdevelopment, initiating some of the early cellular events thatform the central nervous system (CNS) and culminating in theorganization of male or female neuroendocrine responses. Earlycellular events have been studied intensively in recent yearsand these developmental processes appear to involve specificgrowth factors in the development of certain tissues. Subsequentto cellular differentiation, primordial germ cells migrate tothe appropriate anatomical location and contribute to the developmentof the single ovary or testes; steroidogenesis begins soon thereafter.Other portions of the hypothalamo-pituitary-gonad (HPG) axisalso appear during early embryonic development with migrationof the gonadotropin-releasing hormone (GnRH) neurons from theolfactory region of the CNS to the midbrain and separate formationof the pituitary gland. The gonadal steroid hormones affectdevelopment of accessory sex structures as well as the laterorganization of neuroendocrine regulatory systems and secondarysex characteristics. Manipulation of steroids during embryonicand early posthatch periods results in altered endocrine andbehavioral responses in adult birds. There are marked speciesdifferences in the timing of these events, especially when precocialand altricial species are compared. Altricial species hatchin a less developed state and as such are more dependent onparental care. Of necessity, the precocial species must be capableof feeding and other motor capabilities at hatch and coincidentallytheir other physiological systems also appear to be more matureat this time. Finally, there is the separate issue of song birdsversus those avian species that do not have elaborate learnedsongs. It appears that most of the species with elaborate neuralcircuitry responsible for song are altricial. This may benefitthem from the standpoint of gaining more time and contact withthe parents to enable them to learn the appropriate song. Thereare also hormonal and neuroendocrine components critical inthe process of song development. Finally, the effects of environmentalfactors, such as endocrine disrupting chemicals take on addedsignificance when viewed in the context of exerting permanentorganizational effects which are likely to alter endocrine andbehavioral components of reproduction in the adult     

Aging and Reproduction: Comparative Endocrinology of the Common Tern and Japanese Quail

SYNOPSIS. The endocrinology of reproductive aging in avian specieshas been described primarily in captive domestic birds, suchas the Japanese quail (Coturnix japonica), which show majorchanges in hormones and reproductive performance. To explorereproductive aging in longer-lived avian species, longitudinalstudies are being conducted on the Common Tern (Sterna hirundo)in which age-related changes in nesting patterns and clutchsize have been monitored. However, little information is availablerelevant to endocrine status of breeding pairs of Common Terns.This review summarizes information pertinent to the breedingbiology and endocrine status of the Common Tern during reproductiveaging and compares these findings to data available from theJapanese quail. Fundamental mechanisms in the process of agingin avian species may become apparent in the comparison of thesedata and data from field species.