Background: Several species of Pogonomyrmex ants will alter the exterior nest area (nest yard) or move their nests if the area becomes shaded. Shading may alter the internal nest temperature and thereby effect both the metabolism of workers (how quickly they can become active) and the development of the young (eggs, larvae, and pupae).
The Comanche harvester ant (Pogonomyrmex comanche) nests in prairie characterized by oak motts of various sizes and surrounding oak forest. Comanche colonies do not nest within the oak motts nor the forest and are rarely within 5 meters of the oak forest border. I have observed these ants moving to new nest sites several meters from the original nest site. Carlson and Gentry (1973) found that Pogonomyrmex badius moved their nests in response to shading. I wondered if shading might be a factor determining nest movement for the Comanche harvester ant as well. And thereby, impacting the spatial pattern of colonies within the prairie.
Methods: This experiment was carried out in two areas of Todd Island at the Fort Worth Nature Center and Wildlife Refuge in Fort Worth, Texas, from June through November 2012.
I made canopies of canvas using PVC tubing as a framework and placed these over nests chosen to be shaded. I chose 8 nests to be shaded and 8 nests to be controls, unshaded. I monitored the nests for temperature, movement of the entire colony to a new nesting site, and movement of the entrance.
I measured temperatures 3 times a day at the beginning of each month using a Magellan 600 Explorer GPS unit and a soil thermometer probe. The temperatures measured were: air, surface, 10 cm depth, 20 cm depth, and 30 cm depth. Activity level of the colonies was also observed when the temperature measurements were made.
The canopies measured 40 cm X 40 cm and were placed 20 cm above and centered over the nest entrance. The canvas was attached to the framework using twine. To secure the canopy in place over the nests, wooden rods were placed in the soil and the legs of the framework were placed over these. Canopies were left out for the duration of the experiment.
The data were analyzed with Fisher’s Exact Probability Test and Chi Square Test.
Results: There was no significant effect of the canopies on the frequency of colony nor entrance movement.
|Chi Square Results
|Fisher’s Exact Probability Results
However, there was an impact on colony activity due to the shading during midday and later. The canopies kept the surface temperature low enough for nest maintenance activities to continue through the hotter part of the day. Colonies that were shaded remained active during the hot midday when other colonies were inactive (with no ants on the exterior nest yard). The shaded colonies continued with nest maintenance activities only — mostly bringing soil to the surface. No foraging was taking place for any colony from midday on.
Conclusion: The impact of shading on the internal nest temperatures does not appear to be a significant factor for Pogonomyrmex comanche colony movement. Despite what has been observed for other species in this genus, this is not entirely surprising since 1) Texas is hot and in summer, the soil surface is too hot for the ants to walk on much of the day and 2) these ants nest in prairie associated with oak forests, so most nests are shaded at least part of the day. Field observations indicate that colonies are likely to have different activity periods based on when that colony is shaded.
Literature: Carlson, D. M. and Gentry, J. B. 1973. Effect of shading on the migratory behavior of the Florida harvester ant, Pogonomyrmex badius. Ecology 54: 452-453.