Hormones, life-history, and phenotypic variation: opportunities in evolutionary avian endocrinology.

Life-histories provide a powerful, conceptual framework for integration of endocrinology, evolutionary biology and ecology. This has been a commonly articulated statement but here I show, in the context of avian reproduction, that true integration of ultimate and proximate approaches has been slow. We have only a rudimentary understanding of the physiological and hormonal basis of phenotypic variation in (a) reproductive traits that contribute most to individual variation in lifetime fitness in birds (e.g. laying date, clutch size, parental effort) and (b) trade-offs that link these traits or that link reproduction to other life stages (e.g. migration, molt). I suggest that some reasons for this relative lack of progress include (a) an increasingly reductionist and centralist (upstream) focus which is more and more removed from ecological/evolutionary context, and from peripheral (downstream) mechanisms that actually determine how phenotypes work (b) a long-standing male-bias in experimental studies, even though the key reproductive traits which contribute most to variation in fitness are female-specific traits (e.g. onset of vitellogenesis, egg size or number). Endocrine systems provide strong candidate mechanisms for regulation of phenotypic variation in single traits, and two endocrine concepts capture the essence of life-history trade-offs: (a) hormonal 'pleiotropy', when single hormones have both positive and negative effects on multiple physiological systems and (b) hormonal conflict between regulatory systems required for different but over-lapping or linked life-history stages. I illustrate these ideas with examples of reproductive anemia, migration-reproduction overlap, and molt-breeding overlap, to highlight some of the tremendous opportunities that exist for comparative endocrinologists to contribute to mechanistic studies of avian reproduction in an evolutionary context.