Adaptation of gestation or egg-laying in species depends on the amount of internal heat generated in digesting the food

Anatomically and physiologically, the reproductive process of gestation or egg-laying, and dietary habits in vertebrates appear to be distinct processes. An in-depth analysis of the dietary habits of vertebrates reveals that the gestation or egg-laying characteristic in these species is tightly coupled with the digestive process. Once the food has been ingested, it is then broken down to the molecular level to be absorbed into the body. The amount of energy required to digest the food depends upon the amount and composition of the food material that was ingested. The denser (ex. bones and muscle) and bigger the size of the food bits ingested, the higher the amount of energy required to break down the material - that in turn requires higher amount of gastrointestinal acids. Where there is higher amount of energy is consumed, there will be an excess amount of heat gets generated. Because of the proximity of uterus to digestive system, a layer develops around the embryo to protect it from this heat. Therefore, it appears that the higher amount of heat generated in digesting the food results in egg-laying characteristic in species such as birds and reptiles, which ingest large chunks of raw meat. Rest of the vertebrates adapted to gestation due to chewing the food into small pieces before ingesting which generates less internal heat in digestion. Species, which generate higher amount of internal heat in digestion, adapted to ectothermic nature such as reptiles. And the species, which generate less amount of internal heat, adapted to endothermic nature such as mammals. Birds are endothermic but they would have been ectothermic if there were no insulating feathers.