How do baby animals that primarily subsist on cellulose get their initial gut flora?

Question

In the case of mammals like giraffes and koalas, is that bacteria common on the plants they eat so when a baby starts to try to stick something besides its mother's milk in its mouth, it can't digest the cellulose at all the first time, but along with the cellulose, into its mouth went some cellulose-digesting flora that then begins a lifelong colony in the animal's gut? Is that about how it happens?

Answer 1

In practice, everything that babies are exposed to from birth forward provides exposure to the microbes that will populate nearly every portion of their bodies, inside and out. While still debated in the scientific literature, it is predicted that the first major exposure to microbes happens during delivery - where the offspring is exposed to microbes from the birth canal and GI tract. In this way there is vertical transmission of microbes from mother to offspring, many of which will likely have cellulolytic activity given their intimate association with the mother. Moving forward, everything the offspring consumes or put in their mouths is a source of microbial exposure and seeds microbes into the GI tract. Mothers milk, the skin baby nurses from, objects from their environment, etc. The duration of this assembly phase and the ultimate composition of microbes varies by species, but over time, via complex communication between host and microbes, a complete microbial community will assemble filling all available niches and host and microbe will provide critical functions for one another. Like how microbes can provide cellulolytic activity to mammals that subsist predominantly on cellulose based diets. What results is a stable collection of microbes in the gut that is resistant to disruption and disturbances over time, much like any other complex ecosystem and mutualistic interactions between microbe and host.