More than a simple egg: Underlying mechanisms of cold tolerance in avian embryos

Avian embryos, which develop within eggs, exhibit remarkable tolerance to extremely low temperatures. Despite being a common trait among all birds, the mechanisms underlying this cold tolerance in avian embryos remain largely unknown. To gain a better understanding of this phenomenon and the coping mechanisms involved, we reviewed the literature on severe cold tolerance in embryos of both wild and domestic birds. We found that embryos of different bird orders exhibit tolerance to severe cold during their development. In response to cold stress, embryos slow down their heartbeat rates and metabolism. In severe cold temperatures, embryos can suspend these processes, entering a torpid-like state of cardiac arrest. To compensate for these developmental delays, embryos extend their regular incubation periods. Depending on their embryonic age, embryos of all bird species can tolerate acute severe cold regimes; only a few tolerate chronic severe cold regimes. We also discussed various extrinsic and intrinsic factors that affect the tolerance of bird embryos to low temperatures before and after incubation. Cold tolerance appears to be a heritable trait shared by wild and domestic embryos of all bird classes, regardless of egg size or development (altricial/precocial). Driven by environmental variability, cold tolerance in avian embryos is an optimal physiological and ecological strategy to mitigate the adverse effects of cold conditions on their development in response to fluctuating environmental temperatures.