One of the greatest mysteries of nature is the instinct to migrate. Every year millions of creatures feel the need to move for one reason or another. Most of us have seen the arrival or departure of migrating flocks of birds. Migration, however, is not confined to birds, but can be seen in reptiles (for example, turtles, frogs), insects (butterflies, locusts), fish (eels, salmon, tuna) and mammals (reindeer, seals, lemmings, whales, bats). Many of these creatures succeed in navigating over long distances. Just how they manage to do this still remains a mystery. There are several possibilities. They may navigate by using one or more of the following:
1. The sun;
2. The stars;
3. The Earth’s magnetic field. (When a small bar magnet is attached to a pigeon, it is unable to navigate.);
4. A sense of smell;
5. Geographical features. (Birds flying from South America to Canada seem to follow coastlines and valleys);
6. Changes in temperature. (Salmon can detect a change in water temperature as small as 05 of a degree F.)
7. Sound (Whales and bats seem to use sonar).
У чому сенс життя чоловіка.