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The Strange and Dangerous World of Folivory

The special features of folivorous existence

The Strange, Dangerous World of Folivory”    by Fiona Sunquist in the International Wildlife magazine of January-February, 1991, is summarized here:

  • The leaf eaters of the world, folivores (the name comes from the Latin folium, for leaf and vorare, for eat) including such far-out creatures as the giraffelike okapi, Australia’s eucalyptus-eating koala , the moss-green owl parrot of New Zealand, the indri lemur of Madagascar, and the slow-moving sloth all must be highly specialized to survive.
  • To eat leaves, which are abundant yet all-but-void of nutrition, most folivores have specialized stomachs, many are slow moving and sleep a lot, and all excel at making a little energy go a long way.
  • It is these energy-saving adaptations, however, which also lock folivores into their own strange world and make them vulnerable to change.
  • For folivores, the good news is that leaves are almost everywhere. There are billions of them, just sitting there, rustling in the breeze. As well as being easy to find, they don’t run away nor do they hide.
  • The bad news is that some of the leaves fight back. The war between folivores and trees has become a complicated arms race, with trees trying to defend their leaves and the eaters attempting to overcome the defenses.
  • One line of leaf defense is physical; such as, an array of spines, hairs, wax, and a cellulose-like lignin that make certain leaves difficult to bite or chew. Other plants are so rich in silica that they can wear a chewer’s teeth to stubs in no time.
  • Many leaves use chemicals for the second line of defense. These chemicals are aimed mainly at insects that consume most of the world’s foliage.
  • Inside the leaves lurks a natural pharmacopeia of toxins, insecticides, and antifungal compounds.
  • Plants produce substances like nicotine, pyrethrin, strychnine, and phenols that provide them with such effective protection that scientists see some of these compounds as alternatives to synthetic insecticides.
  • Folivoric mammals have two major strategies for dealing with plant toxins.
  • Some, like koalas and sloths, have very narrow diets and tend to specialize in detoxifying a few kinds of leaves.
  • Others, such as howler monkeys, often deal with the chemical poisons in their food by eating just a few samples from manydifferent trees.
  • The simplest way to extract energy form leaves is to chew, squash, or grind the leaf to release the digestible cell contents. The process involves considerable chewing and there is little energy reward, but, apart from grinding teeth, no special equipment is needed.
  • To summon any more energy from a diet of leaves requires a special enzyme that can digest the cell walls themselves.
  • Mammals and birds cannot produce this useful digestive juice, but bacteria can.
  • Serious leaf-eaters like sloths, koalas and just about all folivores (except pandas), keep leaf-digesting bacteria in ther guts.
  • Most folivores also have specialized digestive tracts.
  • Sloths and tree kangaroos have large complex stomachs that, like a cow’s belly, function as fermentation chambers.
  • Bacteria break down the leaves, but food can take a week or more to pass through the digestive tract.
  • Other folivores such as howler monkeys and indri lemurs have a different strategy: simple stomachs which rely on absorbing nutrients through an extra-long intestine.
  • Sloths, colobine monkeys and some types of antelope have gut bacteria that are finely tuned to digest only selective kinds of leaves.
  • The demands of living on low-energy and often poisonous food means that most folivores live close to the limit of their energy supply.
  • They must conserve energy wherever possible, and this often translates into being very slow.
  • It is no coincidence that the sloth, the world’s slowest mammal, is a folivore.
  • Besides being slow, folivores also spend much of their time resting.
  • Koalas spend 18 to 22 hours a day sleeping, and even when they are awake, they move slowly and look tired.
  • Their lethargy reflects an extremely slow metabolism, which allows them to survive on an energy-poor diet.
  • Sloths, iguanas and possibley other folifores also use solar heat to compensate for low energy levels.
  • For folivorous birds, life is even more difficult.
  • Flying requires an enormous amount of energy, so most birds that feed on leaves have either turned to gliding or abandoned flight completely.
  • Like the flightless owl parrot of New Zealand, folivorous birds tend to be found on islands where there are few natural predators.
  • Because rearing the young is one of life’s most energy expensive activities, some leaf-eaters may give birth only once every three or four years.
  • Typically they produce just one small offspring, which grows very slowly and depends on its mother for a long time.
  • The giant panda can ill afford to waste limited energy on its young either.
  • They do not conceive until they are five or six years old and then they only have one or two tiny young.
  • Like the panda, many other folivores find themselves backed into a corner by their highly specialized ways.

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