Tuesday, January 11, 2005

Latest news from the Florida Harvester Ant

Tschinkel, W., 2004. The nest architecture of the Florida harvester ant, Pogonomyrmex badius. Journal of Insect Science, 4:21

The architecture of the subterranean nests of the Florida harvester ant, Pogonomyrmex badius, was studied through excavation and casting. Nests are composed of two basic units: descending shafts and horizontal chambers. Shafts form helices with diameters of 4 to 6 cm, and descend at an angle of about 15-20o near the surface, increasing to about 70o below about 50 cm in depth. Superficial chambers (< 15 cm deep) appear to be modified shafts with low angles of descent, and are distinct from deeper chambers. In larger nests, they have a looping, connected morphology. Chambers begin on the outside of the helix as horizontal-floored, circular indentations, becoming multi-lobed as they are enlarged. Chamber height is about 1 cm, and does not change with area. Chamber area is greatest in the upper reaches of the nest, and decreases with depth. Vertical spacing between chambers is least in the upper reaches and increases to a maximum at about 70 to 80% of the maximum depth of the nest. The distribution of chamber area is top-heavy, with about half the total area occurring in the top quarter of the nest. Each 10% depth increment of the nest contains 25 to 40% less area than the decile above it, no matter what the size of the nest.

Nests grow by simultaneous deepening, addition of new chambers and/or shafts and enlargement of existing chambers. As a result, the vertical spacing between chambers is similar at all nest sizes, and the relative distribution of chamber area with relative nest depth did not change during colony growth (that is, the size-free nest shape was the same at all colony sizes). Total chamber area increased somewhat more slowly than the population of workers excavating the nest. The branching of shafts was consistently shallow (< 40 cm), somewhat more so in large nests than small. Large colonies rarely had more than 4 shaft/chamber series. Each new series contributed less to the total chamber area because its chambers were smaller. Incipient colonies were usually 40 to 50 cm deep while mature colonies were commonly 2.5 to 3.0 m deep.

Workers captured near the top of a mature nest (and therefore older) and penned in escape proof enclosures, excavated larger nests than did young workers captured from the bottom of the nest. Most of this difference was due to a larger fraction of older workers engaging in digging, rather than an increase in their rate of work. All ages of workers produced similar top-heavy nests. When different ages of workers from different levels of a mature colony were allowed to re-assort themselves in a vertical test apparatus buried in the soil, older workers moved upward to assume positions in the upper parts of the nest, much as in the colonies from which they were taken. The vertical organization of workers based on age is therefore the product of active movement and choice. A possible template imparting information on depth is a carbon dioxide gradient. Carbon dioxide concentrations increased 5-fold between the surface and the depths of the nest. A preference of young workers for high carbon dioxide concentrations, and a tendency for workers to dig more under low carbon dioxide concentrations could explain both the vertical age-distribution of workers, and the top-heaviness of the nest’s architecture.

!!! Worth to take a look at the amazing pictures of "plaster-nests" :) !!!

Strehl, C.-P. and Gadau, J., 2004. Cladistic Analysis of Paleo-Island Populations of the Florida Harvester Ant (Hymenoptera: Formicidae) based upon divergence of Mitochondrial DNA Sequences, Florida Entomologist 87(4)

To examine the relationships of geographically isolated paleo-island populations of Pogonomyrmex badius (Latreille 1802) in Florida we generated a phylogeographic hypothesis based on mitochondrial DNA (mtDNA) sequences. We found at least three distinct mtDNA lineages and a positive correlation between genetic and geographic distances. The relationships between nowadays isolated P. badius populations might resemble a long lasting separation due to either restricted gene flow caused by inbreeding, paleo-climatic events or the impact of novel invasive species. The current depletion of the only representative of the ant genus Pogonomyrmex in the south-eastern USA makes a more fine-scaled mapping of the remaining, small P. badius populations necessary to identify evolutionary distinct units for conservation purposes.


At 1:54 PM, Blogger Alex Wild said...

Hey, interesting work on the Florida Pogo phylogeography. Any plans to look at nuclear markers as well?

At 2:25 AM, Blogger cpkhs said...

Hey Alex,
this phylogeography was a minor - but interesting - side-aspect of the "big" picture of Pogo-phylogeny, and for this no nuclear markers were planned yet (with the exception of the microsatellite-work to demonstrate polyandry: here we also found geographical substructuring...).


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