The North Ronaldsay Neolithic Sheep, live almost entirely on Seaweed. Restricted to the coast, to preserve the agricultural pastures inland, they have adapted physiologically to the lack of copper in seaweed. So much so, that lush green grass might end up killing them with Copper poisoning! They sleep when the tide goes out, suggesting that all the lazy ones washed away.

More information:

bbc.com/north-ronaldsay-sheep-eat-seaweed-and-little-else