A plaster cast of a large Pogonomyrmex badius nest. This nest consisted of 135 chambers and 12 meters of vertical shafts. The top-heavy distribution of chamber area and spacing is typical for the species, as are the helical shafts and the decrease of chamber size with depth.
Superficial chambers from Pogonomyrmex badius nests of increasing size, shown in side and top views. These “chambers” are probably produced by a modification of the behavioral program producing shafts throughout the deeper portions of the nest. Note the increasingly interconnected and loop-like morphology as the shafts become extended and enlarged.
A. The descending shafts of Pogonomyrmex badius nests form a helix. B–D. Chambers are constructed outward from the outside of this helix, so that the intersection between chambers and the helical shaft forms an angle between 25 and 70°. The angle may be either right or left of vertical (compare A and B with C and D). E–F. Chambers are initiated at the outside of the helix as small, flat-bottomed niches.
Chambers of Pogonomyrmex badius nests become more lobed in outline as workers enlarge them. Chambers begin as small indentations in the wall of the shaft (left). Large chambers often appear to wrap around the shaft (right).
The ratio of the actual perimeter of Pogonomyrmex badius nest chambers to the perimeter of a circle of equal area estimates the complexity of the chamber outlines. Most of this increase results from an increase in the number of lobes. Because the largest chambers are located in the upper third of the nest, the greatest deviations from circularity occur in this part of the nest.
Casts of Pogonomyrmex badius nests of increasing size: (A) a very small nest with a single vertical chamber series and shaft; (B) a medium-small nest with the beginning of a second vertical series and shaft; (C) a large-mature nest with 4 vertical series and shafts. The right nest (C) cast is of dental plaster, the middle (B) of aluminum, and the left (A) of zinc. The middle cast is incomplete, the lowest chambers having failed to fill with metal. Its maximum depth was probably intermediate between the large and small casts.
The total chamber area of Pogonomyrmex badius nests grows more slowly than does the population of workers that excavate the nest. For every 10-fold increase in the number of mature workers, the area increases 7.5-fold. The space per workers therefore decreases as nests grow. However, workers are very unevenly distributed within the nest.
The uppermost portions of a medium-small Pogonomyrmex badius nest (top) and a small one (bottom). Note the ribbon-like descending shaft in the larger nest, but not the smaller.
Pogonomyrmex badius nests grow through the addition of chambers, chamber enlargement and nest deepening. Patterns are shown by decile (increments of 10% of the maximum nest depth) and colony size class. A. More chambers are added in the upper regions of the nest than the lower. B. The mean size of chambers at each depth increases with nest size. C. The combination of increased numbers and mean size of chambers greatly increases the total area in each decile, much more in the upper regions than the lower. D. The increase of total area and maximum depth are proportional, so that the distribution of the percent of the total area in relation to the percent of the maximum depth does not change much as colonies grow. In other words, the size-free shape of the depth-area distribution does not change with colony size.
The vertical spacing between Pogonomyrmex badius nest chambers increases to a maximum in the 7th or 8th deciles then decreases again. This pattern does not change much with nest size because nest deepening and chamber addition occur simultaneously, preserving spacing. One nest in size class 2 contained outliers in the 5th and 6th deciles. None of the other size classes were different in how spacing varied with depth.
The proportional decrease in chamber area with depth of Pogonomyrmex badius nests is similar no matter what the colony size. Every 10-fold increase in depth is associated with a 75% decrease in summed chamber area in the decile (note log-log scale). Depths are the mean depths for each decile and size class. None of the slopes are significantly different from each other: slope of the log-log regression was - 0.59. Intercepts were: size class 0 = 1.69; size class 1 = 2.40; size class 2 = 2.72; size class 3 = 3.09; size class 4 = 3.35. These estimate the area at 1 cm depth, and have only mathematical reality. Actual mean areas in the uppermost deciles were 38, 190, 516, 1700 and 2670 cm2 for size classes 0 to 4, respectively.
The depth of shaft branch points is deeper in shallower nests of Pogonomyrmex badius nests than in deep ones (A), but is independent of total nest area (B).
Each additional vertical series of chambers of Pogonomyrmex badius nests contributes less to the total chamber area because the mean size of its chambers is smaller than higher series.
Plaster casts of Pogonomyrmex badius nests excavated by 250 workers taken from the uppermost chambers, the middle-depth and the bottom of a mature nest. Each group was penned in an escape-proof enclosure for 4 days, after which workers coming to the surface were recaptured, and a plaster cast of their nest was made. The figure shows one of 4 similar replicate sets.
All measures of nest excavation were highest for the older workers taken from the top of a mature Pogonomyrmex badius nest, intermediate for the middle group and lowest for the youngest group from the nest bottom. A: rate of excavation, as grams of plaster per worker per day; B: cm of shaft per worker per day; C: number of chambers in nest.
Most of the differences in the size of the excavated Pogonomyrmex badius nests were associated with the number of workers actively digging. Only actively digging workers could be recaptured at the surface as they brought up soil to discard. Younger workers from deeper in the nest were rarely recaptured. Because replicates began with varying numbers of workers (125 to 250), digging rate is plotted against fraction recaptured.
The carbon dioxide concentration in air taken from Pogonomyrmex badius nest chambers of increasing depth. The concentration in the deepest chambers is about 5 times that at the surface (2 replicates). B. The distribution of nest area by decile is roughly the mirror image of the carbon dioxide concentration, suggesting that the carbon dioxide concentration gradient might serve as a template for nest excavation and worker assortment. The steepest gradient is in the top half-meter or so, and this is also the zone with the most rapid decrease in chamber area.
A. The “ant hotel” used to test the depth preference of young and old workers. B. Initially, each “hotel room” contained 25 young workers taken from the bottom of a mature Pogonomyrmex badius nest, and 25 old workers from the top (each also contained 10 larvae and pupae). Each chamber thus contained 17% of each age group. After allowing 4 days for reassortment, old workers moved upward so that a large proportion of them were recaptured in the upper chambers. The downward movement of young workers was less dramatic.
