Common House Ants Form Supercolonies, Prosper in Urban Settings

(Mar. 31, 2010) — One of the most common house ant species might have been built for living in some of the smallest spaces in a forest, but the ants have found ways to take advantage of the comforts of city living.

Grzegorz Buczkowski discovered odorous house ants living in supercolonies, creating complex networks entomologists have never seen with the species before now. (Credit: Purdue Agricultural Communication photo/Tom Campbell)

Grzegorz Buczkowski, a Purdue University research assistant professor of entomology, found that odorous house ant colonies become larger and more complex as they move from forest to city and act somewhat like an invasive species. The ants live about 50 to a colony with one queen in forest settings but explode into supercolonies with more than 6 million workers and 50,000 queens in urban areas.

"This is a native species that's doing this," said Buczkowski, whose results are published in the early online version of the journal Biological Invasions. "Native ants are not supposed to become invasive. We don't know of any other native ants that are outcompeting other species of native ants like these."

Odorous house ants live in hollow acorn shells in the forest. They're called odorous because they have a coconut- or rum-like smell when crushed. They're considered one of the most common house ants.

In semi-natural areas that are a cross of forest and urban areas, such as a park, Buczkowski said he observed colonies of about 500 workers with a single queen. He said it's possible that as the ants get closer to urban areas they have easier access to food, shelter and other resources.

"In the forest, they have to compete for food and nesting sites," Buczkowski said. "In the cities, they don't have that competition. People give them a place to nest, a place to eat."

Buczkowski observed the ants in three different settings on and around the Purdue campus. He said it might be expected that if the odorous house ants were able to multiply into complex colonies, other ants would do the same.

But Buczkowski found no evidence that other ants had adapted to new environments and evolved into larger groups as the odorous house ants have. He said it's possible that odorous house ants are better adapted to city environments than other ant species or that they had somehow outcompeted or dominated other species.

"This raises a lot of questions we'd like to answer," he said.

Buczkowski said understanding why the supercolonies form could lead to better control of the pests in homes, as well as ensuring that they don't outcompete beneficial species.

Future studies on odorous house ants will include studying the ant's genetics and trying to understand the effects of urbanization of odorous house ants.

Plotting and Treachery in Ant Royal Families

(Mar. 11, 2010) — Social insects -- ants in particular -- are usually thought of as selfless entities willing to sacrifice everything for their comrades. However, new research suggests that ant queens are also prepared to compromise the welfare of the entire colony in order to retain the throne.

Queen ant and her retinue. (Credit: University of Copenhagen)

A team from the University of Copenhagen, led by postdoc Luke Holman of the Center for Social Evolution, describes in Proceedings of the Royal Society B, published on the 24 February 2010, that ant queens are much more devious than previously thought.

Often, an ant colony has more than one queen. Multiple queens can produce a larger initial workforce in incipient colonies, increasing the chance the colony will survive the hazardous first year. But queens do not happily cohabit forever; soon after the young workers hatch, they begin to slaughter surplus queens until only one remains.

Ant queens were found to cleverly adjust how many new workers they produce for the colony. Queens produce fewer workers when sharing the colony with other queens, especially if the colony already has many developing workers. Queens therefore seem to know when they can expect a showdown for the throne, and conserve energy accordingly.

Such strategic investment in worker production is complemented by sophisticated chemical communication by queens. Ants have been called "walking chemical factories," because they produce many different odours for tasks such as recognising friends and enemies and signalling their status and role within the colony. Olfactory cues also indicate whether a queen is healthy and fertile. The Copenhagen team found that queens which were fertile had stronger chemical signals, and were also more likely to be spared execution by workers. Workers therefore appear to be selecting the fertile queen as their ruler based on smell.

"Execution of the most selfish ant queens by workers would increase the incentive for queens to be team-players that work hard to help the colony. This rudimentary "legal system" could have helped ants to evolve their highly advanced societies, just as in humans ," says lead author Luke Holman. Co-authors of the study were Patrizia d'Ettorre (now a professor at Paris University) and Stephanie Dreier, who have studied the evolution of social insect behaviour, communication and survival strategies for several years.