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Excerpts from Jim Conrad's
Naturalist Newsletter

Bull-Horn Acacia, ACACIA COLLINSII

from the March 28,  2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
BULL-HORN ACACIA & ITS ANTS

On a forest trail I almost stepped on something that would have lamed me if I'd been barefooted. It was a twig section from the tree above me, the tree shown above. This is one of a couple of Bull-Horn Acacias we have here, probably ACACIA COLLINSII, Subín in Maya, now in the dry season nearly completely leafless. A close-up of some thorns shows what's bull-hornish about them below:

Bull-Horn Acacia, ACACIA COLLINSII, thorns with ant holes

In that picture notice that two of the 2-¾ inch (4 cm) spines bear holes near their tips. Ants chew these holes, enter the hollow thorns and live inside. A single ant colony may span several A. collinsii trees. If a herbivore comes along and touches the tree, the ants rush onto the animal and bite. Thus it's a mutualistic relationship, with both tree and ant benefiting.

The tree not only provides handy shelters for the ants but also feeds them. Take a look at the expanding leaf below:

Bull-Horn Acacia, ACACIA COLLINSII, Beltian bodies & glands for ants

Acacia leaves are bipinnate -- twice compound -- so the entire feathery, purplish, ant-mounted structure in the picture's lower right corner is a leaf about to expand. At the top, left of the leaf the shoehorn-like thing with two green-doughnut-like items in the horn is the leaf's stem, or petiole, and the green doughnuts within petiole's concavity are glands producing sweet, energy-rich nectar the ants feed on. Notice that many but not all the leaves' ultimate leaflets bear teardrop-shaped, dark purple, shiny things. Those are Beltian bodies, which are protein- rich structures eaten by the ants as well. Once the leaves are fully expanded, the Beltian bodies will have been eaten and there won't be a sign left of them.

The Bull's-Horn Acacia above me that day was practically leafless, but nearby grew a shoulder-high sapling. Saplings often bear leaves even when larger trees of their species don't. You can see a couple of the Bull-Horn Acacia's feathery, bipinnate leaves below:

Bull-Horn Acacia, ACACIA COLLINSII, leaves


from the April 3, 2011 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
BULLHORN ACACIA FLOWERING

Now the Bullhorns are producing very distinctive spikes of tiny, densely packed, yellow flowers, shown below:

Bull-Horn Acacia, ACACIA COLLINSII, flowers

At least two closely related species of Bullhorn Acacia occur in the central Yucatán. Earlier I decided that our local species was probably was Acacia collinsii, but there was always a little doubt about whether it might be Acacia cornigera. The only difference I can find mention of is that the fruits, or legumes, of A collinsii split open when mature, while those of A cornigera don't.

While photographing the above flowers it occurred to me to look for old fruits in the leaf litter below the trees. After a lengthy search I found what's shown below:

Bull-Horn Acacia, ACACIA COLLINSII, legume husk

That's a woody, split-open legume with its seeds, or beans, long gone. Therefore, now it's almost certain that our local plants really are Acacia collinsii.

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