Travel Reference
In-Depth Information
fermenting mix in their crops for long periods, an unusual ability for such
small-bodied animals, and as a result they are just as ei cient as sheep in
gaining energy from the fermentation process. When did they acquire this
fermentative expertise? We do not know, just as we do not know when and
how their babies acquired their claws, because all the close relatives of the
hoatzins have disappeared.
As a result of their symbiosis with digestive microbes, the crops of hoat-
zins are greatly swollen. This gives them a comic embonpoint and makes
them awkward fl iers. And this in turn is why they build nests on riverbanks
rather than in the depths of forests—they need a clear line of fl ight to land
on their nests.
The energy that the hoatzins depend on comes from an apparently unprom-
ising food source: plant fi bers. Woody plant tissue is chiefl y made of cellulose.
The fi bers of cellulose in turn consist of long chains of the sugar glucose. In
theory, therefore, cellulose should be eminently digestible. Unfortunately, its
glucose molecules are bound together by chemical linkages that need special
hydrolytic enzymes to unlock them. The plants' woody fi bers have been made
even less palatable through a wide variety of chemical modifi cations, includ-
ing tight chemical linkages that bind the cellulose to other carbohydrate chains
called lignins and hemicelluloses. The result is a thicket of interlocking mol-
ecules. These strong woody fi bers are superb at supporting the plants so that
they can thrust their leaves into the air and sunlight. At the same time the fi bers
are totally resistant to the digestive systems of insects, birds, and mammals.
Enter these new members of our cast of characters, the fermentative bac-
teria, which are able to break down these fi bers. The animal hosts of these
bacteria have evolved many ways to help them in their task.
The most sophisticated ruminant digestive systems, such as those of cat-
tle and sheep, have stomachs that are divided up into a series of four cham-
bers. Recently swallowed food is easily regurgitated back into their mouths
from the fi rst two chambers. The animals can masticate their food and mix
it with saliva whenever they have the leisure to safely chew their cud. The
softened and broken fi bers are then swallowed again, so that they are more
easily fermented by the huge populations of microbes that live in these fi rst
two fermentative chambers.
