Why do turtles die if you only feed them lettuce?

Because they are not herbivores, and thus cannot survive on a vegetarian diet. All turtles are either carnivores or omnivores.

Tortoises are herbivores. But even they cannot survive on just lettuce, as it is almost completely devoid of vitamins and minerals.

See the Related Links below for more information on the nutritional requirements of turtles and tortoises.