Every animal on Earth ultimately gets its vitamin B12 from bacteria. No plant or animal can make this vitamin on its own. Only certain species of bacteria and archaea (ancient single-celled organisms) possess the molecular machinery to build B12 from scratch, and every other living thing depends on them, directly or indirectly. How that bacterial B12 actually reaches an animal’s bloodstream, though, varies dramatically depending on the species and where it lives.
Only Microbes Make B12
Vitamin B12 is one of the most structurally complex vitamins in nature, and its production is limited to a small number of bacterial and archaeal species. These microbes build the vitamin through long, multi-step biochemical pathways that come in two versions: one that requires oxygen and one that doesn’t. The oxygen-dependent route is best studied in a soil bacterium called Pseudomonas denitrificans, while the oxygen-free route has been mapped in species like Propionibacterium shermanii and Bacillus megaterium.
These same bacteria are the workhorses behind industrial B12 production for supplements and animal feed. But in nature, they live in soil, water, and the digestive tracts of animals, quietly producing the B12 that entire ecosystems depend on. The critical raw ingredient they need is cobalt, a trace mineral that sits at the center of every B12 molecule.
How Ruminants Grow Their Own Supply
Cows, sheep, goats, and other ruminants have the most elegant B12 arrangement in the animal kingdom. Their multi-chambered stomachs host dense communities of bacteria and yeasts that synthesize B12 during the normal fermentation of food. Because this production happens in the rumen (the first and largest stomach chamber), the vitamin passes through the rest of the digestive tract and gets absorbed in the small intestine, exactly where the body can use it.
The catch is cobalt. Rumen bacteria can produce all the B12 a cow or sheep needs, but only if cobalt levels in the rumen fluid stay above roughly 0.5 micrograms per milliliter. The current dietary recommendation for cattle is 0.2 milligrams of cobalt per kilogram of dry feed. If cobalt drops below that threshold, B12 production falls off within days. Since plants don’t contain B12 themselves, they serve a different role for ruminants: they’re the delivery vehicle for cobalt, which the rumen microbes then convert into the vitamin. A cobalt-poor pasture leads directly to B12 deficiency in cattle and sheep, even though the bacteria are still present and ready to work.
Hindgut Fermenters and Coprophagy
Rabbits, guinea pigs, rats, and many other small mammals face a trickier problem. They also harbor B12-producing bacteria in their guts, but these bacteria live in the large intestine and cecum, which sit downstream from the small intestine where B12 absorption takes place. The vitamin gets made, but it’s in the wrong location to be absorbed.
The workaround is coprophagy: eating their own feces. By consuming droppings that are rich in bacterially produced B12, these animals route the vitamin back through the upper digestive tract, where it can be properly absorbed. This isn’t accidental or occasional behavior. For rabbits and many rodents, coprophagy is a necessary part of their nutritional cycle. Without it, they develop B12 deficiency even though their gut bacteria are producing plenty of the vitamin.
The Ocean Food Chain
In aquatic environments, B12 follows a classic food-chain pattern. Certain bacteria and archaea (a group called Thaumarchaeota appears to be the dominant producer in ocean water) synthesize B12 and release it into the surrounding water. About half of all algae species require B12 to grow, and most phytoplankton acquire it through symbiotic relationships with these bacteria.
From there, the vitamin moves up the food chain in a stepwise concentration process. Zooplankton eat the phytoplankton. Small fish eat the zooplankton. Larger predatory fish eat the smaller fish. At each step, B12 accumulates in animal tissue, which is why top predators like tuna and salmon end up with high concentrations. Bivalves like clams and mussels, which filter massive volumes of water and siphon up phytoplankton directly, also concentrate B12 efficiently. Research shows B12 levels are significantly higher in filter-feeding bivalves than in herbivorous snails that eat seaweed.
Predators, Scavengers, and Insects
Most carnivores and omnivores get their B12 the straightforward way: by eating other animals that already contain it. A wolf eating a deer, a hawk eating a mouse, or a bear eating salmon is consuming B12 that was originally made by bacteria somewhere further down the chain. The vitamin is stored primarily in liver and muscle tissue, so meat-eating animals rarely face B12 shortages as long as prey is available.
Insects are another significant source for many wild animals. Gut bacteria in insects produce B12, and the vitamin content varies enormously between species. House crickets contain around 8.5 to 9.1 micrograms per 100 grams on a dry weight basis. Some beetle species pack far more: one species of longhorn beetle has been measured at over 700 micrograms per 100 grams. For insectivorous birds, lizards, and small mammals, a diet of bugs provides a reliable B12 supply. Seasonal and geographic factors cause wide variation in how much B12 any given insect contains, but as a food group, insects are consistently meaningful sources.
Environmental Exposure: Soil and Water
Wild animals also pick up B12 from their environment in ways that domesticated or urban animals typically don’t. B12-producing bacteria live in soil, and animals that eat unwashed plants, dig for roots, or drink from streams and ponds ingest these bacteria along with any B12 they’ve already produced. Fecal deposits from other animals add to the environmental pool, essentially fertilizing the soil with bacterially derived B12.
This environmental route is difficult to quantify, but it likely matters most for herbivores that aren’t ruminants and don’t practice coprophagy. A wild horse or elephant drinking from a muddy river and grazing on soil-dusted vegetation is picking up trace amounts of B12 that a barn-fed counterpart would miss entirely. Soil invertebrates like earthworms and their remains also contribute, acting as natural fertilizers that redistribute bacterially produced B12 through the topsoil.
Why Absorption Requires a Special System
Getting B12 into the body isn’t as simple as swallowing it. In most mammals, B12 absorption depends on a protein called intrinsic factor, which is produced in the stomach. Intrinsic factor binds to B12 and escorts it to specific receptors in the lower portion of the small intestine (the ileum), where the complex is actively transported into the bloodstream. This process requires calcium and works best at a neutral pH.
Without intrinsic factor, very little B12 makes it across the intestinal wall, no matter how much an animal consumes. This is why the location of bacterial B12 production matters so much. Ruminants produce B12 upstream of the absorption site, so the system works seamlessly. Hindgut fermenters produce it downstream, forcing them to recycle it through coprophagy. And carnivores consume pre-formed B12 in meat, which enters the stomach and binds intrinsic factor in the normal digestive sequence.
The entire global supply of B12, whether it ends up in a cow’s liver, a salmon fillet, or a supplement capsule, traces back to the same handful of microbial species quietly assembling one of nature’s most complex molecules from cobalt and simple building blocks.