Hello from Brookfield, Wisconsin.
It's great to see this service online--yet another wonder of the Web.
My question involves small (not much over a millimeter long), red ants that nest around our home and seem particularly to like areas near our front walk.
More than once I have noticed these guys flowing onto the concrete and forming a large gathering in the open air--see attached photos.
Why do they do this?
These are a species of pavement ant, Tetramorium caespitum. They are common in urban areas, hence their common name. The event pictured is a large territorial battle between colonies. Territoriality is common in ants as a whole, varying by species and colony age. Ants typically protect their territories for access to food or nesting space.
The closely related Japanese pavement ant, Tetramorium tsushimae, is similarly territorial to the pavement ants that you see in Wisconsin. In this species, colonies with larger territories containing larger numbers of seeds and other food resources are able to raise larger numbers of reproductive individuals. However, food is not the only factor determining colony success. The ideal temperatures for raising queens and males are between 27.5 and 30 degrees Celcius (81.5 to 86 degrees Fahrenheit) and colonies also engage in territorial battles to gain access to nesting sites with these temperatures. Therefore, the pavement ants on the sidewalks outside your house are probably fighting for access to both food and optimal nesting sites.
While fights among pavement ants often lead to the deaths of large numbers of workers, this is not a requirement for ants to maintain territories. A species of honeypot ant, Myrmecocystus mimicus, is also highly territorial, but, rather than risk the lives of workers, engages in ritual displays. Hundreds of ants from each competing colony confront each other and stand as tall as they are able while inflating their gasters to appear larger. Eventually, a winner is decided based exclusively on the differences in workers between colonies and territory is ceded to the apparently stronger colony. If colonies are drastically different in size then the smaller colony will be destroyed but otherwise, no physical interactions occur. You should take a look at the papers listed at the end of the post if you want to know more details.
Thanks for your question,
Ben Rubin, James Trager, & the AntAsk Team
Holldobler B. (1981) Foraging and spatiotemporal territories in the honey ant Myrmecocystus mimicus Wheeler (Hymenopera: Formicidae). Behavioral Ecology and Sociobiology 9: 301-314.
Sanada-Morimura S, Satoh T, Obara Y. (2006) Territorial behavior and temperature preference for nesting sites in a pavement ant Tetramorium tsushimae. Insectes Sociaux 53: 141-148.