Ruminants are walking fermentation vats. They ingest plants and let a host of microbes metabolize the plant material. Ruminants then absorb some of the microbial byproducts and also digest the microbes.
It's been known for awhile that these microbes include bacteria, archaea, and ciliates. The identity of the microbes is still being determined, no less their ubiquity.
Henderson et al. took a huge step towards understanding these patterns. They examined over 700 rumen samples from 32 ruminant species. Samples were collected globally.*
*How this paper ended up in Scientific Reports and not Nature is beyond me. My guess is bias against livestock.
Among their results, the authors show that there is a core set of bacteria and archaea (but not protozoa) in ruminants that they rely on for digestion. The also show clear differences between animals fed browse and concentrate.
This paper is going to take a while to digest (pun intended), but there are some pretty amazing patterns.
**Henderson, G., F. Cox, S. Ganesh, A. Jonker, W. Young, C. Global Rumen Census, and P. H. Janssen. 2015. Rumen microbial community composition varies with diet and host, but a core microbiome is found across a wide geographical range. Sci Rep 5:14567.