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Reproduction is an energetically costly phenomenon. Therefore, to optimize reproductive success, male birds invest enough energetic resources for maintaining well-developed testes. The hypothalamic–pituitary–thyroid (HPT) axis in birds can crosstalk with the hypothalamic–pituitary–gonadal (HPG) axis, thus orchestrating both the reproduction and metabolism. However, until now, how the free-living birds timely optimize both the energy metabolism and reproduction via HPT-axis is not understood. To uncover this physiological mechanism, we investigated the relationships among body mass, testis size, plasma hormones including thyroid-stimulating hormone (TSH), thyroxine (T 4 ), triiodothyronine (T 3 ), metabolites including glucose (Glu), triglyceride (TG), total cholesterol (TC), uric acid (UA), diencephalon mRNA expressions of type 2 ( Dio2 ) and 3 ( Dio3 ) iodothyronine deiodinase enzymes, thyrotropin-releasing hormone ( TRH ), thyroid-stimulating hormone ( TSH) , gonadotropin-releasing hormone  I  ( GnRH-I) , and gonadotropin-inhibitory hormone ( GnIH ) in a male Eurasian tree sparrow (ETS,  Passer montanus ). We found significantly larger testis size; elevated diencephalon  Dio2  and  TRH  mRNA expressions, plasma T 3 , and UA levels; and significantly lowered Glu, TG, and TC levels during mating relative to the non-mating stages in male ETSs. However,  Dio3, TSH, GnRH-I , and  GnIH  mRNA expression did not vary with the stage. Furthermore, life-history stage dependent variation in plasma T 3  had both direct effects on the available energy substrates and indirect effects on body mass and testis size, indicating a complex regulation of metabolic pathways through the HPT- and HPG-axes. The identified differences and relationships in mRNA expression, plasma T 3  and metabolites, and testis size in male ETSs contribute to our understanding how free-living birds adjust their molecular, endocrinal, and biochemical features to orchestrate their reproductive physiology and metabolism for the maintenance of well-developed testes.

frontiers in endocrinology

Hypothalamic–Pituitary–Thyroid Axis Crosstalk With the Hypothalamic–Pituitary–Gonadal Axis and Metabolic Regulation in the Eurasian Tree Sparrow During Mating and Non-mating Periods