- Animals that others prey on are alert and always attentive in case they can see, hear or smell a predator. Those that don't get preyed on.
- Hungry predators are always alert to find prey, those that don't starve.
Some animals do not behave in ways that help them survive and are killed off because only the fittest survive. Others don't behave in ways that help them reproduce and leave no offspring.
it's not quite as straight forward as that, animals and humans (Remember humans are a type of animal) sometimes risk injury or death to help relatives, that way the Genes of the unselfish individual are more likely to survive in the body of the relative.