Trees present a difficult nutritional challenge because their structural integrity comes from compounds largely indigestible to most animals. Plant cell walls are reinforced with cellulose and lignin, complex organic polymers that create a rigid, woody structure. Lignin is particularly resistant to digestive enzymes, requiring specialized adaptations to access stored energy. Trees also deploy chemical defenses, such as bitter-tasting tannins, to discourage herbivores. Animals that successfully incorporate wood, bark, leaves, or sap into their diets have evolved specialized tools and internal systems to overcome these barriers.
Consumers of Bark and Woody Tissue
Animals consuming dense, woody parts must first access the nutrient-rich inner layers beneath the tough outer bark. Beavers use powerful, ever-growing incisors to fell saplings and strip the bark and cambium layer from branches. The cambium, a layer of living cells just inside the bark, is sought after because it contains concentrated sugars and less lignin than the outer wood. Beavers possess a specialized hindgut fermenter digestive tract, which houses microorganisms that produce the necessary enzymes to break down complex carbohydrates.
Porcupines also rely on inner bark, especially in winter when other vegetation is scarce. They use sharp claws to climb and robust teeth to peel away the outer layer to reach the phloem and cambium. Like beavers, porcupines utilize an enlarged large intestine and cecum filled with microbes to ferment the high-fiber diet. Larger herbivores, such as elephants, consume bark and woody branches using powerful grinding molars. These animals often strip large sheets of bark, exposing the wood and accessing carbohydrates. Wood-boring insects, like beetles and termites, live directly within their food source, tunneling through the wood. Termites host symbiotic protozoa and bacteria in their guts that actively digest cellulose and hemicellulose, efficiently breaking down the tree’s core structure.
Animals That Rely on Foliage and Twigs
While some animals target wood, others focus on green, photosynthetic material, requiring different digestive strategies. Browsers, such as deer and moose, primarily consume leaves, buds, and fine terminal twigs, seeking material with lower fiber content and higher nutritional density. These large herbivores are ruminants, possessing a four-chambered stomach system that functions as a continuous microbial fermentation vat. The first chamber, the rumen, holds bacteria and protozoa that break down cellulose and neutralize plant toxins.
The partially digested material, or cud, is regurgitated and re-chewed, fracturing cell walls to give microbes better access to nutrients. This complex process allows ruminants to extract sustenance from tough foliage that simpler systems cannot process. Giraffes are specialized browsers, using their height and prehensile tongues to selectively pluck leaves and soft shoots from tall trees. Specialized folivores, such as koalas, have adapted to a diet of highly toxic eucalyptus leaves using an extremely long cecum where detoxification and fermentation occur. Their diet is highly selective, focusing on specific leaf species and ages to minimize the intake of potent secondary metabolites.
Specialized Sap and Fluid Feeders
A different strategy involves bypassing solid material by extracting the tree’s internal fluids. This liquid diet provides a concentrated source of sugar and water without extensive structural breakdown. Sapsucker birds, a type of woodpecker, drill distinct, organized rows of small holes, called sap wells, into the bark to access the flowing sap. They use a specialized, brush-tipped tongue to lap up the sugary fluid as it pools.
Sapsuckers create two types of wells: deeper holes for dilute, water-transporting xylem sap, and shallower holes to collect nutrient-rich phloem sap. Phloem sap, which transports sugars produced during photosynthesis, is a valuable energy source. The sap also attracts insects, which become trapped, providing the sapsuckers with protein. Insects like aphids, cicadas, and scale insects also feed exclusively on fluids, using sharp, needle-like mouthparts called stylets to puncture the bark and tap directly into the tree’s vascular system.
The Ecological Role of Tree Eaters
The activities of tree-eating animals profoundly influence the structure and function of the ecosystem. As ecosystem engineers, beavers dramatically alter landscapes by felling trees and building dams, creating flooded wetlands. These impoundments introduce new habitats for species like waterfowl, amphibians, and fish. Increased light penetration from tree removal promotes the growth of varied understory vegetation. Dead wood, or snags, resulting from beaver activity and the cavity-excavating behavior of woodpeckers, creates sheltered nesting sites for secondary cavity nesters, such as owls, ducks, and squirrels, that cannot excavate their own homes.
The consumption of tree parts is integral to nutrient cycling, the process by which elements are returned to the soil. When herbivores consume leaves, twigs, and bark, they partially process these materials. Their waste products, along with uneaten portions and felled wood, decompose more quickly than intact trees. This decomposition by fungi and bacteria releases nutrients back into the forest floor, making them available for new plant growth. Animals that eat fruits also serve as seed dispersers, carrying seeds far from the parent plant in their digestive tracts. This movement allows tree species to colonize new areas, maintain genetic diversity, and drive forest succession.