The Inner Workings of Evolution - The Darwinian Tourist - page 61

Figure 26 A hoatzin launches clumsily into the air. Swollen crops make these birds poor
fl yers—they fl ap through the air like galleons under full sail.
Animals that ferment their food, such as hoatzins, sheep, and cows, eat
grass or leaves in enormous amounts. They must then subcontract the job of
digesting this mass of plant tissue to a carousing band of bacteria and other
single-celled organisms that inhabit their stomachs.
Eat, drink, and be merry
The second member of our cast of characters is a collective. It is the thriving
colonies of bacteria that carry out this fermentation and that live in symbio-
sis with their hosts. Some of the bacterial species are so specialized for this
mode of life that they are found nowhere else. Hosts and bacteria absolutely
depend on each other.
Some birds, such as grouse and ostriches, carry out a little fermentation
in various parts of their guts, but no other living birds are able to ferment
the food in their crops as ef ectively as the hoatzins . 6 Hoatzins can retain the