You are aware, of course, that no mammals are capable of manufacturing Vitamin B-12 in their bodies.
Any B-12 contained in a mammal (or fish, reptile, bird - or indeed any multicellular organism) was obtained by the animal by ingesting a source of B-12.
B-12 is created only by certain bacteria and archeae.
Regards,
“Any B-12 contained in a mammal (or fish, reptile, bird - or indeed any multicellular organism) was obtained by the animal by ingesting a source of B-12”
that source being exclusively the ingestion of bacteria capable of fixing cobalt in the form of methylcobalamin, something that ruminants and some other animals are excellent at, but NOT something humans are capable of ...
humans will die if they don’t consume animal products containing B-12 or synthetic B-12 in the form of a pill of injection ... any diet that requires synthetic supplementation to avoid certain death is clearly not a diet that nature intended ...