The majority of plants with a very sour fruits are distributed by birds. Getting them into the digestive tract, the seeds are subjected to harsh chemical processing. First they must pass through the stomach with hydrochloric acid, and then through the intestine alkaline reaction medium. And all this at temperatures above 40 degrees Celsius!
Sour pulp, once in the stomach, reduces the production of stomach acid, because the nerves that are responsible for its secretion, responsive to the total acidity in the stomach. And the acid of the fruit for his seed is not dangerous.
Acid protects the seeds and transfer to the intestine of the bird: it will neutralize the lye.
Since acid increases for seeds able to successfully avoid digestion and passing the body of a bird, come up.
Exactly the same applies digestive system of a mammal, but with the difference that the acid and alkali there is less temperature, usually below. Therefore, chances for seeds to remain intact longer. Then it becomes clear and the increase in the number of acidic fruits to the North of the large mammals there is less and birds more. Plants with small seeds must adapt to eating it by the birds.
You can, of course, to have a very hard shell, not amenable to digestion. But then the seeds will be a large number of them in the small fetus, respectively, decrease and the ability of a species to spread will decrease. In the North of Russia hard seeds have only large plants with relatively large fruits, mainly trees and shrubs - hawthorn, raspberry, BlackBerry. But such seeds for plants is not quite convenient: no processing in the avian intestine dense shell does not give the seeds to grow. This has been proven experimentally: hawthorn seeds buried in soil, protecting it from birds and rodents, and they almost never come up. And next together ascended the sowing, the last bird "processing"!