Salmon eggs hatch in freshwater streams and, during their first year of life, the young salmon migrate distances up to 1,000 km in order to reach the ocean. Here they spend up to 5 years where they feed and grow, acquiring more than 95% of their biomass. During the summer of their maturing year, they begin the long journey back to their home streams to spawn. Although it is still uncertain how salmon navigate back to their spawning grounds, current hypotheses suggest that they have a highly developed sense of smell that allows them to remember odors they encountered on their migration to the ocean. They then use these odors to help them navigate back to the streams where they were born. At the spawning grounds, females use their tails to form a hollow cavity in the stream gravel where they lay up to 8,000 eggs. The males fertilize the eggs, and both adults typically die soon thereafter. The physiological response that allows salmon to survive in fresh water, then in salt water, and then fresh water again is an example of ________.