Dogs seem to eat feces for many reasons, such as poor digestion, hunger, boredom, stress, curiosity, seeing mom do it when cleaning up after her litter, getting attention, or avoiding punishment by hiding “accidents”.

I'm not sure how eating one's own feces would affect the microbiome but Coprophagia is not unusual for many different species. Without a ruminant's multi-chambered digestive system (see: cattle, deer), animals like rabbits actually have to eat once digested food in order to extract the full nutrients from hard to digest grass and plant matter.

Suposedly they have already been exposed to bacteria in their own feces. Other dogs' feces however...

Traversing the gut backwards is quite a feat for a bacterium...

Consuming ones own faeces could be an effective way to travel from one end to the other...

Lots of parasites [1] use that method too.

[1]: https://en.wikipedia.org/wiki/Pinworm_(parasite)

Evolution only optimizes for having offspring in the future. Trying to find a rational reason for some evolved behavior is not necessarily a fruitful way to think about it.

No, dogs only eat their own feces when they lack nutrients on their diet.

How can you tell that they lack nutrients? I have a basset hound and despite his supposedly excellent sense of smell he still enjoys "chocolate" snacks on a regular basis. He's an adult dog and getting Kirkland food. His weight is consistent and the vet is happy about what he weighs. I would love to be proven wrong (please!) but I doubt he's "snacking" because he lacks nutrients. I think he might be a little slow mentally and maybe he's _that_ hungry and doesn't have access to anything else to eat.

and why infants eat their nose pickings. nail biting might be related, too.

Their shit is a good source of vitamin B12, which is presumably part of the reason why they eat it.

Why did the B12 pass through their system in the first place? Or is the B12 somehow generated on the way out?

It's produced by bacteria in the intestines, but not absorbed during the first pass through.

Why would it be absorbed any better the second time around?

(This thread is making me feel queasy.)

Like others have said, it takes time for B12 to be produced in significant quantities, and by that time it has passed the small intestine where it can be absorbed.

I'm not sure, but dogs have significantly shorter (looking quickly, 1/4 the length?) digestive tracts than we do. Less surface to absorb things.

probably produced too late in the process...

Rabbits for example re-eat every piece of faeces because the digestion of grasses take so long it takes two passes to get the energy out...