system
Given the following post, respond as if you're commenting on Reddit:
human
How can some animals eat just one thing and get all nutritions and vitamins and all that?

We need to eat a balanced diet to get everything we need. Some our bodies can just create (classic vitamin D using our skin), but others we just have to consume.

How can certain animals (how many are there? What is the percentage) just eat one thing. Like blue whales only eat krill. Does Krill just happen to contain everything they need? Or Pandas and eucalyptus leaves.

I know cows occasional lick salt. Do those animals that we (mainly I ;) perceive to be single item eaters also snack on other items? But how does a blue whale, with very specialized eating habits, eat anything else?

Side question: Does every animal need vitamin C? Scurvy must be horrible for an animal...
gpt
Humans can't make their own vitamin C. Apes can't either. Other animals produce vitamin C in their liver. So, fresh liver is a good source of vitamin C. 

Worth remembering if you are ever decide to leave civilization and hunt for food.