A long-running mystery for our tracker group has involved rabbit digestion and pellets. We hear that rabbits employ coprophagy, re-ingesting their scat for a second pass through their digestive system to gain more nutrients out of the tough and fibrous plant materials they eat. Common knowledge from captive rabbits was that the original pellets—the “night droppings,” as they are called—are small, dark, soft, and moist. But the pellets we find in the field are always large, dry, tan-colored, and full of large fibers. They don’t look like they have been reprocessed, but indeed they have.
Thanks to some diligent research by one of our trackers, we have some answers. It turns out rabbits have an ingenious system that allows lagomorphs—such as rabbits, hares and pikas—to make use of plant materials that are fibrous and of low food value. Larger grazers evolved a system of separate stomach compartments that do different jobs, with different stomach acids and enzymes, to fully break down high-cellulose materials into digestible forms. This system starts with the “rumen,” in which grazed materials are first on-boarded in very rough form and brought back into the mouth later, while resting (ruminating), to be more thoroughly chewed before passing back into the stomach and intestinal system. This is called foregut fermentation. Ungulates feed constantly and take many breaks to rest and ruminate.
Rabbits face a similar problem but don’t have the body size to contain multiple stomach pouches. And being such vulnerable plant feeders, they have very large stomachs that allow them to onboard large amounts of food quickly when they are least visible, at dawn and dusk, and then rest and digest later when they are safer.
Over their 60 million to 70 million years of evolution, rabbits have developed a surprisingly different system, known as hindgut fermentation. This refers to their extra organ, the pouch-like cecum, essentially a fermentation chamber located between the small and large intestines. Here is how the process works.
Food passes through the stomach and small intestine first. Stomach acids begin breaking down the food, and the small intestine is where the nutrients are absorbed into the bloodstream through enzymatic breakdown. From there, the food materials enter the colon (large intestine), where they are sorted by particle size. Large, long, indigestible fibers go on through, to be expelled as the large fecal pellets we usually find. Shorter, more digestible fibers are sorted and propelled into the cecum for further processing. In the cecum, the materials are fermented by a mix of bacteria and fungi, a process that releases essential nutrients.
After about eight hours of fermentation, the material is formed into “cecotropes”: small, soft, moist pellets with a high food value and covered with a mucous coating. In domestic rabbit production, these are commonly called night droppings. Rabbits then consume these pellets directly from the anus as they are expelled, and the droppings pass through the digestive system a second time.
In the second digestion, the cecotropes pass through the stomach. The nutrients produced by fermentation are protected from the stomach acids by the cecotropes’ mucous coating. The pellets then move to the small intestine, where the coating breaks down and the nutrients are absorbed. The final bits of indigestible fibers join in the formation of the typical fecal pellets.
Both lagomorphs and ungulates evolved after the asteroid-induced extinction event 65 million years ago. Supercontinent movements also conspired to create a long cooling era during which grasslands became more widespread, leading to rapid adaptations in both diet and locomotion. It is thought that rabbits remained small because of early competition with small ungulate ancestors. Other adaptations to grassy environments included the sharp, strong incisors that lagomorphs use to cut coarse grasses and the sharp-edged molars that both lagomorphs and ungulates use to process the woody materials they consume.
An interesting parallel of coevolution can be found in coyotes and bobcats. While hares such as jackrabbits evolved speed and distance running as a way to take advantage of open-prairie habitat—and coyotes developed speed and distance running as a way to hunt them—cottontails, such as our brush rabbits, evolved to take advantage of the dense grasses that grow close to shrubs and trees. So the bobcat evolved a stalk-and-pounce style of hunting more suited to the cover of thickets.
Thus we find the flattened, M&M-shaped jackrabbit scat widely and sparsely distributed across an open landscape, while the smaller and more spherical brush rabbit scat is often found in large accumulations at the edges of dense thickets. In my own experience, I find that coastal jackrabbits by far prefer to eat the native, wide-leafed dune grasses over the narrow European dune grasses, while brush rabbits seem to enjoy the European grasses equally to the native species, along with the small round rushes growing in wet patches. One can observe their sharp-angled cuts low on stems, at heights a small cottontail can reach.
Richard Vacha is a Point Reyes Station resident and the founder of the Point Reyes Tracking School.