A captive deer, pampered and protected, may live for 18 years or so before its biological clock runs down. However, in the wild it is a rare animal that surives to a patriarchal old age. Animals generally die young, well before the time allotted them by their biological clocks. In the woods most deer are dead before their third birthday and 50 percent of the new quail population are dead by the time they are 15 weeks old. A couple of months is about all a wild mouse can count on.
Animals die young in the wild as they are subject to natural controls that are missing in captivity.
A good example is the weather. This can occur directly sometimes as when a spring flood drives the muskrats out of their lodges and burrows and drowns many of their young. Other times, conditions work indirectly as when a cold spring kills oak blossoms, cutting the acorn crop in the fall and causing starvation for squirrels. Disease is another cause of mortality in the wildlife populations. Since many of the world's most virulent diseases have either been eradicated or are under control, we tend to think good health is the natural state and bad health is a departure from the norm.