Below are some short descriptions of especially fascinating life histories of ants that were found on campus. For a list of all of the ants we found, go here
Photographer: April Nobile, www.antweb.org
T. sessile is a native ant that is a major pest in urban environments thoughout the holarctic region. Colonies range in sizes from less than 100 workers to upwards of 100000. This extreme variation of T. sessile's colony size is a tale of rural country bumpkins moving into fast paced urban environments.
In its natural, woodland environment, colonies tend to be tiny - oftentimes comprised of less than 100 ants. In urban environments, however, colonies are huge (containing as many as 100000). This change in colony size is typically coupled with a change in colony nest structure. Colony structure in ants can be either monodomous (meaning each colony consists of one nest) or polydomous (more than one nest). T. sessile's nest structure changes from monodomous in woodland environments to extremely polydomous in urban environments (upwards of 10 nests for one colony!). Research indicates that multiple factors cause this extreme natural history change.
The increase in food availability in cities allows T. sessile's population levels to erupt. As the number of workers in a nest increases, the colony must increase its foraging area to maintain colony growth. This results in workers having to travel farther to search and collect food. At some distance, the energy and time cost that goes into collecting a resource outweighs the benefit of the resource itself - this is the way in which ant colony growth is limited. How can a colony continue to grow once their foraging range reaches this point?
To get around these central place foraging constraints, some ant colonies build a new nest apart from the old one. Workers distribute themselves amongst the two nests in a way that decreases the foraging pressure in the area immediately surrounding each one. Oftentimes, intricate networks develop between multiple nests of polydomous colonies - with workers shuttling brood, food, and queens between them depending on that nest's particular needs.
Despite the apparent benefit of being polydomous, many successful species of ants are monodomous - even when they are big. Literature suggests that the ability of an ant colony to perform colony defense, communicate information, thermoregulate, and maintain colony cohesion affects whether or not a colony will actually do better in the polydomous state. Studies indicate that many factors may influence the optimal colony structure at a particular point in time.
Many mathematical models of central place foraging in ants have been developed to determine why central place foragers search the way they do. However, few models have incorporated polydomy into the mix. I study the ways in which polydomy affects communication, defense, and foraging in ants.
For more information, see Debout, Gabriel, et al. "Polydomy in ants: what we know, what we think we know, and what remains to be done." Biological Journal of the Linnean Society 90.2 (2007): 319-348.
Photographer: Jen Fogarty, www.antweb.org
Temnothorax ants demonstrate the ability to enact "intelligent" colony wide decisions despite the fact that they are leaderless (despite the name, queens do not control or command worker behavior). This rational behavior is demonstrated when a Temnothorax colony's nest is destroyed. Temnothorax nests are susceptible to destruction since they typically are constructed inside decomposing twigs and other leaf litter. When this occurs, the workers set off to find new potential nest sites. This process becomes interesting when multiple "good" potential nest sites are found - laboratory studies have shown that when a Temnothorax colony is given multiple nests of varying quality, they almost always choose the best. How does the colony choose the best one?
Ideally, each ant from the colony would investigate each potential and cast a vote of her favorite. The colony's decision would then be based on complete information, meaning the colony as a whole would know each of its options and be able to compare and choose a best. However, this type of decision making never happens in Temnothorax. Each ant typically only finds and visits one potential nest site at most.
A key feature in the "decision making" process is this: when a new potential nest is found, the amount of time that the worker spends investigating the site before returning home is inversely proportional to the site's quality - so the better a site is, the faster a worker runs back to its own nest. To communicate the presence of the new site, the worker gets another workers attention, and then leads her to the site (this is called tandem running). The new worker then repeats the same process.
Consider a situation where different workers know about different potential nests. Workers that are led to the best potential nest site spend less time investigating the site before running back to the colony and leading fellow workers. When enough ants have reached the new site, the workers begin carrying their nestmates to the new site! The number of workers at a good site grows more quickly than other sites as a result of this positive feedback.
This process is incredible because it allows many ants to come to a collective decision despite incomplete information. Ants arriving at good and bad sites behave the same - except that an ant that comes to a suboptimal site spends more time investigating it before deeming it worthy of being a new nest site location. In this manner, an ant colony behaves as an analog computer to arrive at an optimal decision while requiring minimal cognitive abilities of any particular ant. Mathematical models show that this positive feedback allows a collective decision to be made.
For more information, see Franks, Nigel R., et al. "Information flow, opinion polling and collective intelligence in house–hunting social insects." Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 357.1427 (2002): 1567-1583.
Photographer: Joe Eason, www.antweb.org
Tetramorium ant battles can last for days, and involve thousands of individual ants. They occur throughout summer, and are abundant on campus. Interestingly, some battles have little to no causalties, and may be a form of signaling rather than fighting. In these signal battles, ants from opposing colonies pair up (pairs are known as dyads) and lock on to each other.
As ants arrive to the battle, they keep track of the number of self-ants (ants from their colony) and nonself-ants (ants from the opposing colony) that they bump into. If an ant doesn't bump into any free ants from their colony, it gets the impression that the opposing colony has all of their ants locked up in the signal battle. Upon sensing this, an ant will return to its colony and recruit more of its ants by laying pheromone. The opposing colony will now face the same scenario.
As the signalling game continues, the battle-front moves back and forth in the area between the two colonies. An outnumbered colony's ants will return home many times to recruit more ants. Since these returning/recruiting ants are not engaging the opposing ants in battle, the battle-front will be pushed towards their colony. The smaller the colony, the fewer ants that are available to fight - and the closer the battle-front will be to the numerically outnumbered colony at equilibrium. Thus fights act as a form of neighbor colony assessment and to determine territory size.
Students at the University of Utah study these signal fights with mathematical models. Results from such models are compared to observations of actual fights to determine which model assumptions are valid. This back and forth between mathematical modelling and fieldwork generates new questions and guides future research.
For more information, see Plowes, Nicola Joy Raine. Self organized conflicts in territorial ants. ProQuest, 2008.
Photographer: April Nobile, www.antweb.org
The thief ant is named after habit of stealing brood (larvae) from nearby colonies. Thief ants purposely build their nest close to an established nest with this in mind. Next, tunnel systems connecting the thief ants' nest to the brood chambers of the soon-to-be victim's nest are made. Once completed, the robbers emerge into their neighbor's brood chamber and begin stealing larvae!
Of course, the ants belonging to the victim's nest are not so keen on having their larvae used as chow by a neighboring colony. Any sort of defense of the larvae is quickly thwarted, however, as the thief ants lay down a strong repellant (think ant-mace)! The secretion does not bother the thief ants, and allows them to calmly load up on brood that is then carried back to their colony.
For more information, see Blum, M. S., et al. "Alkaloidal venom mace: offensive use by a thief ant." Naturwissenschaften 67.3 (1980): 144-145.