Granivory, also known as Seed Predation, is when an animal feeds exclusively or near exclusively on seeds. As seeds are highly nutritious, this behavior is found in a wide variety of species, able to be observed in such creatures as mice, Corn Smut worms, and songbirds. Even the largest seeds, the Coconuts that one would think were impossible to break into, are regularly feasted upon by Coconut Crabs. And humans.
Sometimes the means by which an animal eats a seed causes the more
important part of the seed to pass through the animal's digestive tract,
and so the predatory relationship between creature and seed becomes
vital for the continued distribution of the plant species, as in the
relationship of birds to wild chili peppers.
Other times the seed is consumed and not distributed. So plants evolve physical and chemical defenses to prevent their seeds from being consumed, which leads to corresponding adaptations in their predators, such as can be seen in the Coconut Crab, whose claws are so large and fearsome in order to keep up the crab's ability to break the coconut shell.
One might say that humans living entirely on bread are also granivores, but that is not a natural circumstance by any means. Even the poorest among us will add mustard if they can.