According to Medawar's concept on the accumulation of harmful mutations, bat longevity should be explained by the fact, that they have no enemies: since the organisms were not being exterminated at the young age, the selection towards longevity took place at their ancestors; the specific mutations causing senescence have not been accumulating. However bats have natural enemies, and in natural environment bats usually die long before maximum lifespan. This is confirmed by direct observations and, especially, by the fact, that the bat populations increase slowly. In 1950-60s the bat populations declined over the territory around Saint-Petersburg because of inappropriate scientific research (mass sampling, unsuccessful banding and experiments), and they still cannot reach the past condition. The bat longevity is caused by long periods of the inactive condition, slow reproduction rate, little light exposure. Concept of mutations accumulations have to be rejected. The following concept is proposed instead: senescence results from any change in organism, i. e. the changes of an organism are inevitable, are not always healthy, and are possible in definite directions, that is why they become unhealthy in a moment.