Hatching the cleidoic egg: the role of thyroid hormones

Area of Science:

• Zoology
• Endocrinology
• Developmental Biology

Background:

• Hatching from a cleidoic egg is a critical life stage transition for birds and other oviparous sauropsids.
• Thyroid hormones (THs), often with glucocorticoids, are key regulators of this transition, similar to amphibian metamorphosis.
• This transition involves moving from a protected internal environment to a more hazardous external, terrestrial life.

Purpose of the Study:

• To explore the role of thyroid hormones (THs) in regulating hatching-associated processes in birds.
• To compare the function of THs in precocial versus altricial avian developmental strategies.
• To investigate the hormonal regulation of hatching in altricial birds and its evolutionary implications.

Main Methods:

• Review of existing literature on thyroid hormone function during avian hatching.
• Comparative analysis of TH profiles in precocial (e.g., chicken) and altricial (e.g., passerine) birds.
• Examination of TH-mediated developmental events including respiration, metabolism, and behavior.

Main Results:

• In precocial birds, peak TH levels occur around hatching, influencing lung maturation, respiration, gut development, thermoregulation, and imprinting.
• THs appear to play similar roles in some hatching processes in non-avian sauropsids.
• In altricial birds, TH levels do not rise significantly until post-hatching, coinciding with endothermy development.

Conclusions:

• THs are vital for precocial avian hatching, orchestrating numerous physiological and behavioral adaptations.
• The distinct hormonal profiles suggest different regulatory mechanisms for hatching in precocial and altricial birds.
• Further research is needed to understand TH regulation in altricial hatching and its evolutionary origins.