The acclimatization process takes several weeks and can involve behavoiral as well as molecular changes. For instance, migrating animals may express several forms of metabolic enzymes. One form may have optimal activity at a relatively high temperature and the other at relatively low temperatures. This expression of enzyme isoforms occurs during hibernation as well. Heat stress reactions .
Cells can react to rapid increases in temperature to protect proteins from denaturating.
Nitrogenous waste products. Ammonia Ammonia is very toxic and only aquatic animals excrete their nitrogen waste as ammonia. In fishes, the NH4 is exchanged across the gills with Na. Urea Urea is quite soluble and it can be excreted without the loss of large volumes of water. This water conserving feature offers a major advantage for terrestrial animals.