Vachellia mayana (Lundell) Seigler & Ebinger

Acacia mayana Lundell

Acacia mayana Lundell, 1937:210.

Acacia mayana (Figure 17) is the most easily recognized of the swollen-thorn acacias, owing to the pair of bladelike longitudinal flanges extending from the thorn base to tip. The type specimen is representative of the other specimens examined, though it should be noted that there are only 6 specimens at hand of this exceptionally rare swollen-thorn acacia. The leafless isotype of the U.S. National Herbarium was collected from the upper part of the canopy of a mature plant that has been reproducing at least 2 years, since it bears both flowers and fruit. The fruits and flowers of A. mayana are similar to those of A. cornigera and A. sphaerocephala (and therefore very different from the other 9 species of swollen-thorn acacia), but the fruits differ from those of these two species by being longer, thinner, and having a long thin base or stalk before swelling out around the seeds. The inflorescences differ from those of A. cornigera and A. sphaerocephala in that they taper to a point, rather than having a blunt end. More specimens of A. mayana are needed for study.

KEY COLLECTION LOCALITIES

1. Type: Rio Pasion, San Diego, Peten, Guatemala. VI–10–1935, M. Aguilar, no. 495 (US 1688106), mp.

2. Forest S of Finca Yalpemech, near Alta Verapaz-Peten boundary, Depto. Alta Verapaz, Guatemala (100–150 m). III–23–1942, J. A. Steyermark, no. 45184 (F 1188721), mp.

3. 3 mi W Cardenas, Tabasco, Mexico (20 m). VI–18–1966, DHJ, no. 458.

4. Fortuño, Coatzacoalcos River, Veracruz, Mexico (30–50 m). III–1937, L. Williams, no 8425 (F 896540).

5. Retiro, Tenosique, Tabasco, Mexico. VI–19–25–1935, E. Matuda, no. 3410 (F 1027159) (“shrub in virgin forest”).

6. Rio Icuolay, NNW Finca Cubilquitz to Quebrada Diablo, Depto. Alta Verapaz, Guatemala (300–350 m); III–6–1942, J. A. Steyermark, no. 44726 (F 1148694).

NATURAL HISTORY

Acacia mayana is by far the rarest of the swollen-thorn acacias. If the small number of collections represents the true density and spatial distribution of the species, it may well be self-pollinated in contrast to the other swollen-thorn acacias. The only specimen that I have been able to find (site 3) was growing in the same type of old second-growth cornfield regeneration that commonly harbors A. cornigera, but with one exception. The forest was about 15 m tall and the single A. mayana was heavily shaded, yet growing very well; an A. cornigera in the same shade regime would have been spindly with undersized thorns. It is possible that A. mayana is a species of primary forest, such as A. melanoceras and A. allenii; Matuda recorded it at site 5 as a “shrub in virgin forest.” If this is the case, then its rarity in areas under heavy agricultural pressure probably means that it will become extinct in the lowlands.

If the above hypothesis is correct, then A. mayana is a mature and wet-forest edition of A. cornigera. It is interesting in this context that A. cornigera has perhaps produced both a beach-edge species (A. sphaerocephala) and a wet mature forest species.

It is tempting to view the oddly shaped thorns of A. mayana as the result of a long past hybridization with a sword-thorn acacia such as A. macracantha. When A. macracantha hybridizes with A. chiapensis, the result is often a “roundish” thorn in cross section, with longtitudinal shallow-sided ridges down each side of the thorn. On the other hand, A. mayana displays no other traits suggesting a history of introgression with other acacias.

Whether the ridges are the result of introgresion, or a novel mutation, they have a possible adaptive value. A. cornigera thorns on heavily shaded shoots have weak walls and are easily opened by birds for the ant brood inside; apparently the starving shoot does not have the energy to make the thick-walled thorns that effectively protect the ant colony in more insolated microhabitats. The longitudinal ridges on A. mayana thorns may be a way to solve the strengthening problem without

putting a lot of bulk into the thorn wall (this bulk is also adaptive as insulation in heavy insolated sites) in a heavily shaded habitat. First, it is important to note that the ant colony grows slowly at a comparatively large cost to the acacia in the shaded habitat, and the loss of thorns with brood will be proportionately larger to the acacia than in an insolated site. Second, the slow replacement rate of thorns in shaded sites, observed on all forest species of swollen-thorn acacias, means that the birds have a longer time per thorn to locate the tree and go after its ants. Figure 82 shows that birds can be a threat to the ant colony in A. mayana, and it is important to note that they opened all the thorns on the shoot except the few largest ones.

All specimens of A. mayana examined have been occupied by mature colonies of P. ferruginea. In view of the small sample size, however, this should not be taken to indicate that it is a suitable host for only this species of obligate acacia-ant. The colony in the specimen at site 3 was as normal as can be expected of a colony that has just lost about three-fourths of its brood and workers to a bird. While the petiolar nectaries of A. mayana arenearly twice as long as those of A. cornigera and A. sphaerocephala, it should not be assumed that the ant colony gets twice as much nectar per leaf from the shaded plant. There are no data to show a correlation between nectary size and nectar production.

A total of 12 pods from herbarium sheets was examined and there was no attack by bruchids. Owing to the small sample size, and the small sizes of the crops from which the pods were taken, this should not be taken as an indication that A. mayana is free from predispersal seed predation. On the other hand, if in fact it does occur as widely scattered individuals in mature forest, it is possible that the bruchids that commonly infest A. cornigera and A. sphaerocephala cannot cope with the very low prey density that such a rare population of swollen-thorn acacias presents (as with A. melanoceras and A. allenii).