Yes, lions eat grass. You do not have to go far on the Internet to see videos of lions eating grass so that conclusively answers the question. It is worth noting, however, that there are no Google Scholar listed articles on WHY lions eat grass. And my excellent textbook (the best) on the Wild Cats of the World does not mention lions eating grass under “feeding ecology”. The point is worth making because we have to rely on anecdotal evidence on the internet as to why they do it.
My mind immediately turns to domestic cats because they, too, eat grass – long grass. The reason why they eat grass and the reason why lions eat grass is probably the same and the best reason that I have as to why they do it comes from Dr. Desmond Morris, the respected zoologist and author, who decided many years ago that the variety of usual explanations such as to vomit up hair balls or to use grass as a laxative or to add roughage to the diet didn’t make much sense.
He argues that the amount of grass eaten is very small and when you watch a cat, they invariably eat long grass and “one gets the impression that they are merely taking a little juice from the leaves and stems, rather than adding any appreciable solid bulk to their diet”. The description fits the way lions and domestic cats eat grass and because of it he argues that the most likely explanation is that the grass contains minute quantities of a chemical substance which they can’t get from any other foods and the substance in question is folic acid. It plays an important role in the production of haemoglobin. Haemoglobin is a protein in red blood cells which carries oxygen to the body’s organs. If the body is deficient in folic acid the animal may become seriously anaemic.
Dr. Desmond Morris came to these conclusions in 1986 in his book Cat Watching. No one’s come to a better conclusion since. There may be other reasons why lions eat grass: to add fibre, minerals and vitamins to the diet. Grass may help to regulate a lion’s digestive system, prevent indigestion and facilitate defecation. It may help to expel indigestible items and cure stomach upsets. This is a very expansive set of reasons but, as mentioned in the first paragraph, I don’t see any scholarly articles by scientists on the topic which supports them.
Apparently, there was a lioness born in captivity in the 1940s whose name was Little Tyke and who was rejected by her mother. The lioness became a rancher’s house pet and apparently was treated as a pet perhaps being fed incorrect food. The lioness rejected meat and foods with blood. I’m not sure how this lioness was able to maintain a balanced diet and therefore health. But she was a vegetarian lioness.
Apparently, at one point her owners, George and Margaret Westbeau, offered anyone $1000 to get her to eat a carnivorous diet but nothing worked. Blood appeared to revolt her. We are told that she ended up big and healthy “on a diet of grain, milk and eggs, with rubber boots to chew (with perfume sprinkled on them!). She refused bones. We don’t know anything about her health and lifespan.
However, in the wild, like most of the big cats lions feed “on almost every imaginable land mammal and even a few aquatic species” according to Mel and Fiona Sunquist in their book which I mentioned in the first paragraph. Ungulates are the principal prey because there a large and therefore the food goes a long way but they eat birds and ostrich eggs at the other end of the spectrum.