Tapinoma sessile is a species of small ant that goes by the common names odorous house ant, sugar ant, stink ant, and coconut ant.
Etymology
The binomial name Tapinoma sessile was assigned by Thomas Say in 1836. Sessile translates to "sitting" which probably refers to the gaster sitting directly on top of the petiole in the abdomen of the species.
Description
T. sessile is a small ant that ranges in color from brown to black, and varies in length from to inches (1.5–3.2 mm). When crushed, these ants leave a smell which leads to their nickname "stink ant".
A comparison of the side view of T. sessile (below) and a diagram of the a typical ant body (below) shows how T. sessile’s gaster sits atop its petiole. This leads to a very small petiole and to the gaster being pointed downward. The anal pore then opens ventrally (toward the abdomen) instead of distally.
Behavior
Colonies vary in size from a few hundred to tens of thousands of individuals. Big colonies usually have multiple queens.
When offered a choice of food sources, the ants preferred sugar and protein over lipids, and this preference persisted in all seasons. When specific sugar sources were studied the ants preferred sucrose over other sugars, such as fructose or glucose.
Food allocation
Foragers collect food that is around the nest area and bring it back to the colony to share with the other ants. T. sessile has polydomous colonies, meaning that one colony has multiple nests. Because of this, T. sessile is very good at foraging for food when there is great variance in the distribution of resources. Instead of going back to a faraway nest to deliver food, they move workers, queens, and the brood to be closer to the food, so that they can reduce the cost in effort of food transport. This is called 'dispersed central-place foraging'. It was also found that the half-life of the stay at any one nest was about 12.9 days.
Buczkowski and Bennett also studied the pattern of food movement within a nest. They labeled sucrose with Immunoglobin G (IgG) proteins, and then identified them using an enzyme-linked immunosorbent assay (ELISA) to track the movement of food. They found that food was spread through trophallaxis (one animal regurgitating food for another). Despite this trophallactic spread of food, the workers kept most of the sucrose. They also found that some queens received more food than others, suggesting a dominance hierarchy even between queens. They also found that the nests were located in a system of trails, and that their distribution depended on where food was found and the distance between these patches of food.
When searching for food, primary orientation is when ants are exploring a new terrain without the guidance of odor trails. Secondary orientation is when terrain has been explored, and there are pre-existing odor trails which ants use to orient themselves. When T. sessile ants are orienting themselves for the first time they often rely on topography. The major types of elements they rely on are bilaterally elevated, bilaterally depressed, unilaterally elevated, and unilaterally depressed. They use these types of surfaces to orient along, and lay the first odor trails, which can then be followed in the future, to the food source, by other ants.
Seasonal behaviors
It was also found that this ant species practices seasonal polydomy (having multiple colony sites) to have access to multiple food sources. The colony will overwinter in a single nest, and then during spring and summer when resources are more abundant they will form multiple nests. This allows them to better use food sources, that might be spread out. During the winter they will return again to the same nest location. Seasonal polydomy is rather rare, and only found in 10% of all polydomous species. Seasonal polydomy is not found in many ant species, but there are many ant species, including T. sessile, which move within a season: Migration to better forage sites is common.
Competition with other ants
Competition between species is often classified as exploitation or interference. Exploitation involves finding and using limited resources before they can be used by other species, while interference is the act of preventing others from getting resources by more direct force or aggression. When it comes to these behaviors, a species is considered dominant if it initiates an attack and subordinate if it avoids other species. In comparison with eight other ant species, T. sessile was more subordinate on the dominant to subordinate scale. The ant does not show a large propensity for attack, preferring to use chemical secretions instead of biting.
When T. sessile, a subordinate species, was in the presence of dominant ant species such as C. ferrugineus, P. imparis, Lasius alienus, and F. subsericea, they reduced the amount of time spent foraging. This was tested with the use of bait, and when the subordinate species, such as T. sessile, encountered a dominant species they would leave the bait. It would then make sense that the subordinate species would forage at a different time than dominant species, so that they could avoid confrontation, but there is sizable overlap in foraging period on a daily and seasonal basis. Because T. sessile forages at the same time as dominant species, but avoids other foraging ants, they must have excellent exploitative abilities to survive.
Other habits
This species is a scavenger / predator ant that will eat most household foods, especially those that contain sugar, as well as other insects. Indoors they will colonize near heat sources or in insulation. In hot and dry situations, nests have been found in house plants and even in the lids of toilets. Outdoors they tend to colonize under rocks and exposed soil. They appear, however, to form colonies virtually anywhere, in a variety of conditions.
In experiments where T. sessile workers were confined in an area without a queen, egg-laying (by the workers) was observed, though the workers destroyed any prepupa that emerged from the eggs.