Why do fleshy fruits often have seeds with very tough seed coats?

The fleshy fruits are the fruit which contains soft and fleshy ripened ovary which is usually sweet in taste like the berries and the mangoes.  

The fleshy fruits become fleshy as an adaptive mechanism of seed dispersal. The fleshy part encloses the seed with a hard coat as an adaptive mechanism.  

When the fleshy part is eaten by the animals, the smaller seeds can be passed down the digestive tract of the animals unharmed and undigested.

The toughness also protects the seeds from the mechanical pressure is applied to the seeds.

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