Domatia range from hollow stems to more
sophisticated shelters such as specialized pouches or
swollen thorns. Extrafloral nectaries are present on
leaf blades, leaf petioles, stems, or other locations on
the plant. These glands manufacture various energy-
rich sugary compounds as well as certain amino acids.
In addition, some plants have bead bodies, which are
modified hairs rich in oil.
Extrafloral nectaries were initially puzzling (why do
plants have them?), but it was quickly learned that plants
with such bodies are populated by various aggressive
ant species. This observation led to the protectionist
hypothesis, which asserts that the relationship between
plants and ants is fundamentally mutualistic. The
alternative idea, called the exploitationist hypothesis,
argued that the ants fed on the sugary nectaries but
provided no actual service to the plants. That seemed
to suggest an odd evolutionary blind alley, at least as
far as the plants were concerned. Extensive research
has strongly supported the protectionist hypothesis.
Here are some examples.
Cecropia trees (chapter 7) have glycogen- rich
structures called Müllerian bodies (a form of extrafloral
nectary) located at the base of the leaf petiole, where the
large leaf attaches to the stem. Ants of the genus Azteca
live in domatia within modified hollow pith of the stem
and feed on the Müllerian bodies (plate 11-37). I have
frequently encountered the ants of a cecropia, and they
are pugnacious and appear to behave protectively of
their tree. The underside of the wide, palmate cecropia
leaf is velvet- like, with a carpet of tiny hairs and hooks
that allow ants to gain purchase and move easily across
the leaf. Cecropia species that normally lack ants have
leaves with smooth undersides.
In a now- classic study Daniel Janzen documented the
protectionist activities of Pseudomyrmex ferruginea,
an ant species found on five species of Acacia tree.
Commonly called the bull’s horn or swollen- thorn
acacias, these trees have pairs of large hollow thorns on
the stem that shelter the ants (plates 11- 38– 39).
A single queen ant burrows into a thorn of a sapling
acacia to begin a colony that may grow to 12,000 ants
by the time the tree matures. By the time the tree is
seven months old, 150 worker ants are patrolling the
stem. The acacia ants attack other insects that land or
climb on the tree, including beetles, hemipterans (true
bugs), caterpillars, and other ants. Ants also clip plants
that begin to grow nearby or that overtop and shade the
acacia (thus taking its sunlight), and attack mammals,Plate 11- 37. Azteca ants on the bole of a cecropia. The brown
areas beneath the leaf axils periodically produce food for the
ants. Photo by Scott Shumway.Plate 11- 38. The branches of this understory acacia look
innocent enough, until you get close and meet the ants that
guard the plant. Photo by John Kricher.Plate 11- 39. The swollen, paired, hollow thorns of this acacia
tree house small but aggressive Pseudomyrmex ants, seen
here swarming over the branch. Photo by John Kricher.chapter 11 evolutionary arms races: more coevolution, more complexity 193