Adaptations in Herbivore Nutrition

Introduction

Cows and colobus monkey, horses and rhinos, rabbits and squirrels —from a nutritional standpoint, what do these animals have in common? They’re all herbivores, but are they really all the same? Herbivore nutrition separates animals into two main categories, depending on where food particles are broken down and fermented prior to absorption: foregut fermentation and hindgut fermentation. Hindgut fermenters are further divided into colonic and cecal fermenters.

Herbivores come in many shapes and sizes.

Foregut fermentation

Cows are ruminants, having a multi-chambered stomach, but they are also foregut fermenters, with the rumen acting as the fermentation chamber. Colobus monkeys are the non-ruminant version of a cow in that they have a 3-chambered stomach and are also foregut fermenters. They have a sacculated forestomach that is positioned above the true stomach. Plant matter is fermented and broken down prior to entering the stomach to allow for greater absorption of nutrients. Of the monogastric herbivores, examples of foregut fermenters include some primates (monkeys and prosimians), macropods (kangaroos and wallabies) and sloths.

The physiological process involved in foregut fermentation allows the animals to obtain protein from the synthesis of microbes in the fermentation chamber. The main advantage of foregut fermentation is that animals such as monkeys and kangaroos consume diets fairly poor in quality. The colobus monkey, for example, lives on a diet of leaves and flowers, which tend to be highly lignified. Lignin is a group of complex polymers that bind to cellulose fibers. Lignin hardens and strengthens the plant cell wall, and it serves as a principle component of wood.

Hindgut fermentation

The second group of herbivore digestion is the hindgut fermenter. This adaptation is found in monogastric herbivores or herbivores with a one-chambered stomach. This adaptation is further categorized into two groups: colonic fermentation, which is typical of larger species such as horses and rhinos, and cecal fermenters, such as rabbits and rodents. Colonic fermenters typically have a proportionally longer large intestine than small intestine. For instance, the large intestine comprises 62% of a horse’s digestive tract, whereas cecal fermenters have a considerably enlarged cecum compared to the rest of the digestive tract.

In cecal fermenters most digestion of plant matter occurs after it leaves the stomach and enters the cecum. Food particles move through the stomach, down the long small intestine and enter the proximal colon. From here, digestible matter enters the cecum and indigestible matter passes down to the distal colon and is excreted. Food matter that enters the enlarged cecum then forms into cecotrophs that are re-consumed by the animal so they can absorb important nutrients, like the B vitamins. This action of re-ingesting cecotrophs is the monogastric herbivore’s version of a cow chewing its cud, but their food is re-ingested after it has left the stomach instead of beforehand. Cecotrophs are swallowed whole and digestive enzymes are released that allow for absorption. Additionally, microbes that were breaking down food particles in the cecum now provide important amino acids and B vitamins for the animal to absorb.

What are the advantages of hindgut fermentation? First, it allows the animal to consume a diet high in fiber and moderately low in protein, although not typically as low as the foregut fermenters consume. In the cecum, the digestible plant parts are recycled so the animal may absorb the nutrients that were missed during the first round of digestion. High fiber keeps the cecal factory in good working order. A lack of adequate fiber causes the cecum to become sluggish and prevents it from maximizing the absorption quotient of food that passes through it.

Cecal fermentation is also a natural adaptation for species that consume items containing relatively high levels of secondary plant compounds. Secondary plant compounds such as alkaloids, oxalates, tannins and terpenes bind with protein and various minerals, thus preventing their absorption. All animals require absolute levels of protein and other essential nutrients. Herbivores must adapt to herbivory defense mechanisms of plants so they can utilize these nutrients. Cecal fermentation is one adaptation for extracting amino acids from the microbes that break down cellulose in the cecum. Other adaptations that mitigate the negative effects of secondary plant compounds include tannin-binding salivary proteins, reduced fecal nitrogen excretion, consuming foods high in protein to counteract the negative effect of protein-binding tannins, and caching food, allowing tannins to be leached into the soil before consumption.

In the wild, most lagomorphs and rodents consume diets that are unpalatable, and potentially toxic, to livestock and domestic pets. Jackrabbits reportedly consume diets of shrubs and forbs that are toxic to cattle. Shrubs such as sagebrush contain terpenes, a secondary plant compound toxic to most ruminants because it kills cellulolytic bacteria found in the rumen . Starch-digesting bacteria found in the cecum are able to detoxify much of the compound prior to absorption in lagomorphs, so jackrabbits, cottontails and hares can exist on plants that would kill a cow.

Lagomorphs and many rodents reportedly browse on trees such as Douglas-fir (Pseudotsuga menziesii), ponderosa pine (Pinus ponderosa), lodgepole pine (P. contorta), western hemlock (Tsuga heterophylla) seedlings, as well as oak (Quercus spp.) seedlings and sprouts, particularly in the winter months. All of these plants have relatively high tannin content. During the spring months, these same animals will consume tender shoots, buds, new leaves and clover, all of which are good sources of protein.

Tree squirrels will preferentially consume acorns from white oak trees during the autumn, rather than bury them. When there is an excess of these nuts, squirrels will bury some for later use, but to prevent germination will bite off the endocarp beforehand so they can be eaten during the winter months. Coincidentally, white oak acorns have very low tannin levels in them. Acorns from the black oaks, however, contain three times the amount of tannins as white oaks. The same squirrels will typically cache the black oak acorns immediately and dig up later for food during the winter. Burying the acorns allows the tannins to be leached out into the soil, thus lowering the amount in the nuts when harvested.

Chinchillas have a seasonally varied diet, but fiber comprises approximately 66% of the diet annually, most of which is highly lignified (bark, wood stems, shrubs, and bromeliads), whereas seeds form very little of the natural diet. The highly lignified diet suggests chinchillas have adaptations to absorb proteins from other means than directly from the diet.

Exotic pet diets

So how does all this affect the diet of exotic pets? Since many of the small mammals in the exotic pet trade are herbivores, and cecal fermenters, it is important to remember how these species have evolved to digest food and absorb nutrients.

• Many species require a diet high in fiber to maintain a healthy cecum.
• Diets high in starches and refined sugars (as is commonly given as treats) may be fine in limited supply. When starches, are given on a regular basis or in large quantities this may lead to a disruption of cecal pH, loss of microbes for digestion, and diarrhea.
• Fiber is an important dietary component for most, if not all, cecal-fermenting pets. Gastric disease commonly results from a lack of adequate fiber in the diet.
• Additionally, lagomorphs and some rodents have evolved to consume diets that are either low in protein, or contain secondary plant compounds that interfere with protein absorption; therefore, providing excess levels of protein in a captive diet can be problematic. Feeding clover (high in protein) is associated with bloat in chinchillas. Some types of respiratory disease have been attributed to increased ammonia from excess dietary protein in rabbits (Jenkins, 1997).

Conclusion

When considering appropriate captive diets for small, herbivorous mammals, it is important to understand the evolutionary adaptations they have developed so we can provide them with a diet containing nutrients in the amounts and types they need so they can have a long and healthy life.

References