Creating the Nation 's first BioPark 
National Zoological Park' Smithsonian Institution ' Washington, D.C. 20008-2598 
Letter from the Desk of David Challinor
 
January 1993
 
The trees in the new Amazonia exhibit were lowered through the 
roof and transplanted about eight months ago; since then the 
canopy under the glass dome has started to close. The exhibit is 
now even beginning to smell like a tropical forest, and its 
success in not only attracting but also educating visitors has 
been a great reward to its planners under the imaginative 
direction of Mike Robinson. The closing canopy of the exhibit 
serves as an artificial microcosm of a real one and helps us to 
understand the complex interactions that occur there. 
The canopy of an unlogged tropical rain forest is closed 
throughout the year, which means that canopy trees must replace 
their foliage faster than their leaves are consumed by folivores 
(leaf-eaters). To protect their foliage for photosynthesis, 
trees have evolved leaves that generally become increasingly 
unpalatable as they age, but this does not prevent hoards of leaf 
eaters, mostly invertebrates (caterpillars, leaf miners, etc.), 
from reaching the canopy to feed. Since most of the nutrients in 
tropical rain forests are found in the fruit and leaves of the 
canopy, some novel arboreal (tree-dwelling) folivores have 
evolved to exploit this rich source of food. There are three 
unusual vertebrate leaf eaters of the New World rain forest: the 
sloth (mammal) and the green iguana (reptile), both of which are 
on display in Amazonia, and the hoatzin (bird). These three 
animals illustrate the point that despite the seeming abundance 
of leaves these vertebrates have to eat, the offsetting cost to 
them to use this food source is great: all three have to 
conserve energy. 
There are two kinds of sloth: the three-toed (Bradypus 
variegatus) and the two-toed (Choloepus hoffmanni), both of which 
live in the canopy and sometimes even share the same tree. The 
three-toed sloth is the one on exhibit in Amazonia, and our 
concern here is the cost to this mammal of maintaining a diet 
almost exclusively of leaves. . 
Mature leaves are tough and can only be digested with the help of 
bacteria that live in the gut of these sloths. Bacteria, 
although efficient decomposers, break down leaf particles slowly, 
requiring the consumed leaves to spend enough time inside the 
sloth for the bacteria to work. The warmer the stomach, the 
faster the bacteria operate, so sloths often stay motionless high 
in the canopy to expose themselves to direct sunlight to raise 
their body temperature. Cold nights followed by many rainy days 
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January 1993 Page 2 
can slow digestion to the point where a sloth can die of 
starvation with a full stomach. By carefully observing radio?
collared sloths, scientists have learned that they descend to the 
ground to defecate only once every six to eight days. For 
reasons still unknown, the three-toed sloth digs a small 
depression with its stumpy tail into which it deposits its dung. 
The rate (about a week) at which food passes through the sloth's 
digestive tract is certainly the slowest for any mammal, and is 
about the same or even slower than for a tortoise. 
The advantage of abundant canopy leaves enjoyed by sloths is not 
as great as first appears because the sloth must choose the 
leaves of those trees that are most digestible and nutritional. 
The trees with highly palatable leaves are generally widely 
scattered, which obliges the slow moving sloth to keep hunting 
for appropriate trees to sustain itself. Thus to survive in the 
apparent midst of plenty, the sloth must move slowly for several 
reasons: to save energy, to slow digestion to give bacteria time 
to break down leaves, and to stay still near the top of the 
canopy to keep its body temperature high enough to insure its 
elaborate digestive mechanism functions properly. 
If leaf-eating mammals are relatively uncommon, foliverous birds 
are rare. In the canopies of mangrove swamps and low lying 
riparian trees of the Orinoco and Amazon basins lives the 
hoatzin, a crow-sized bird of the cuckoo family (Opisthocomus 
hoazin). This ungainly bird flies poorly and seems to use its 
wings primarily to balance itself as it flops from branch to 
branch. It gorges itself on leaves early in the morning and, 
like sloths, spends mid-day resting and keeping warm to enable 
the bacteria in its enlarged muscular crop and lower esophagus to 
break down its leafy diet. Birds cannot chew, so the crops of 
seed-eating birds often contain small pebbles to help break down 
hard-coated seeds before they enter their stomachs. The 
hoatzin's crop, however, has evolved into a large organ in which 
freshly eaten leaves are stored. Horny ridges inside the crop, 
instead of pebbles, are contracted muscularly to help shred 
leaves, preparing them for bacterial fermentation. As in the 
sloth, particles being digested must be retained in the crop and 
foregut long enough for the bacteria there to ferment the leaf 
particles. Laboratory tests have shown gut retention times of 
over 40 hours, which is astonishingly long for a bird, but 
necessary to sustain a stable popUlation of bacteria in the gut. 
Too much or too little leaf quantity could upset the amount of 
bacteria to the detriment of the bird. 
The cost to the hoatzin of this abundantly available diet is thus 
relatively high. The modification of the crop into an efficient 
fermentation chamber results in its taking up so much space 
inside the bird that there is not enough room at the sternum 
(wishbone) for big flight muscles to attach. This explains the 
January 1993 Page 3 
hoatzin's poor flight ability; it takes two or more months for 
the chicks to fly. The chicks make up for their slow flight 
development by having wing claws that allow them to crawl nimbly 
around trees. When a predator threatens, they merely drop into 
the water under the riparian trees, sink to the bottom and slowly 
claw their way to the surface on tree roots. The extra weight of 
the crop at the base of the neck seems to throw the bird off 
balance, and it appears to hop awkwardly about the mangroves. 
Thus in return for its ability to eat leaves, the hoatzin 
sacrifices flying agility in the adult and late flying 
development in the young. It can fly, however poorly, which 
means that it can exploit a habitat whose food source is often 
patchy. Clearly the continued presence of such odd looking birds 
indicates that their leafy diet is more advantageous to them than 
disadvantageous. A final added benefit is indirect but 
important. When skinned, the fermenting leaves in the hoatzin's 
foregut smell so strongly that the bird is known locally as the 
stinkbird. It is thus seldom hunted even though its flesh is 
perfectly palatable. 
The green iguana (Iguana iguana) is a third arboreal folivore 
that exploits gut bacteria. This large lizard (2 m from nose to 
tail tip) spends about 90% of its time lying prone on a branch, 
digesting its diet of leaves. Like the sloth and the hoatzin, it 
is dependent on its gut bacteria for digestion, which in turn 
requires long food retention time and warm body temperature. 
Iguanas, however, cannot chew as a sloth does nor have they a 
bird1s crop. Therefore iguanas are the most dependent on gut 
bacteria for digestion. Their wide presence in the canopy of 
tropical New World forests is evidence of their success in 
exploiting the readily available and abundant variety of leaves 
that grow there. Problems can arise, especially for females, 
just before egg laying. Their body cavity is then so filled with 
eggs (from 40 to 60 for a big female) that there is not room 
enough for both food and eggs. Food takes second place, and 
gravid females can be severely stressed until they lay. 
Life as a leaf-eater in the canopy is not easy despite what seems 
to be an inexhaustible food supply. Although leaves are indeed 
abundant, folivores must be selective and eat those leaves 
easiest to digest. These canopy dwellers must make endless 
decisions on which leaf to eat and where to go to find it, yet 
they must not move too fast or become too cold to upset the 
cooperating bacteria in their gut. Even a decision on how much 
sun to get on the tree top carries the risk of being targeted by 
large hawks and eagles. Clearly such decisions are not made 
rationally as humans might, but rather on experience and what 
used to be called instinct, but today is often labeled genetic 
January 1993 Page 4 
coding. Thus life for all beings is a trade off, and these three 
animals are a good example of how benefits have a compensating 
loss. 
With this first letter of 1993, I wish you all a Happy New Year 
and urge you to visit Amazonia whenever you can. I will be happy 
to give you a tour. 
David Challinor 
202/673-4705