Air Pollution Causes Social Instability in Ant Colonies

Press Release

Ozone destroys colony-specific odor signals and leads to attacks within the colony

• Attack: Ants exposed to ozone are attacked by their nestmates, even though they are part of the colony themselves.

• Different smell: Ozone destroys colony-specific scent signals by breaking down alkenes with carbon-carbon double bonds. Even slight changes in the scent profile are enough for ants to recognize them as “strangers” and attack them.

• Loss of biodiversity: The study shows how man-made air pollution endangers social systems in nature and may contribute to global insect decline.

Markus Knaden’s Odor-guided Behavior research group at the Max Planck Institute for Chemical Ecology has been studying the effects of ozone on chemical communication in insects for some time. The research team was able to show that increased ozone levels alter the mating signal in fruit flies because ozone breaks down the carbon-carbon double bonds in the insects’ sex pheromones. After male flies were exposed to ozone, they were no longer able to distinguish females from other males (Air pollution impairs successful mating of flies, March 14, 2023). Ozone also altered mating signals in such a way that the mating barrier between different fly species was removed, leading to the emergence of sterile hybrids (Oxidant pollutant ozone removes mating barriers between fly species, April 11, 2024).

These findings led Markus Knaden and his team to investigate the effects of ozone on social insects such as ants, as nest recognition in the colony is also based on chemical signals.

Recognizing nest mates: a “handshake” of molecules

Ants usually recognize nest mates based on species-specific mixtures of hydrocarbons produced in their glands. These mixtures consist mainly of stable alkanes but also contain alkenes, which are compounds with carbon-carbon double bonds that can be oxidized and degraded. Although alkenes occur in very small quantities, they are crucial for the colony’s unique odor.

Ants learn their colony’s odor immediately after hatching. Later, when they come into contact with other ants, they compare their smell with the familiar odor of their own colony. If they recognize the odor, the other ants are regarded as nest mates and treated friendly. Otherwise, they usually become aggressive toward ants whose odor profiles do not match that of their own colony. “We wanted to know if exposure to increased ozone levels would alter the ants’ odor signature, resulting in aggression upon their return to the colony. The crucial question for us was: Could air pollution disrupt the delicate social structure of ant colonies?” says lead author Nan-Ji Jiang.

Ozone-exposed nest mates are attacked as if they were foreign intruders

For their experiments, the researchers exposed ants from six different species to an ozone concentration of 100 parts per billion (ppb), which is a level often measured in affected areas during the summer. After 20 minutes of ozone exposure, the ants were returned to their colony. Five of the species studied exhibited threats and aggression toward ozone-exposed ants, even though they belonged to the same colony. “We wanted to know if this was accompanied by a change in the amount of alkenes on the ants’ bodies. This was difficult to measure because, since even in uncontaminated ants the amount of alkenes is extremely low. However, we were able to measure these compounds on individual ants using thermodesorptive gas chromatography,” explains Markus Knaden.

The colony’s odor signature consists largely of stable alkanes that are not oxidized by ozone. By comparison, the ozone-sensitive alkenes account for only a small proportion of the compounds that comprise the odor signature of ants. “We had expected that ozone exposure would affect the recognition of nest mates, as we knew that ants carry at least small amounts of easily degradable alkenes on their body. However, we were surprised by the dramatic change in behavior after ants had been exposed to ozone. Apparently, despite their small quantity, alkenes are extremely important for the specificity of the colony odor,” says Markus Knaden.

Clonal raider ants neglect their brood after ozone exposure

Only one species, the clonal raider ant Ooceraea biroi, showed no aggression toward nest mates after ozone exposure. This could be due to the special biology of this species. All individuals reproduce clonally, there is no queen, and the level of aggression between individuals from different colonies is generally low.

Because ozone did not trigger aggression between nest mates in Ooceraea biroi, it presented an opportunity to study other possible harmful long-term effects in collaboration with the Lise Meitner Research Group Social Behavior led by Yuko Ulrich. Behavioral experiments revealed that adult ants in ozone-polluted colonies maintained greater distances from their larvae than those in ozone-free environments. Additionally, many larvae died. Since ozone could largely be ruled out as the direct cause of the larvae’s death, the scientists assume that the neglect of brood care and the larvae’s death are due to disrupted chemical communication between adult ants and larvae.

Environmental toxins threaten the social structures of many insect species

Although the decline of insects worldwide is primarily associated with the use of pesticides and habitat loss, this study discusses air pollutants as a possible cause. The total biomass of the estimated 30,000 ant species is roughly equivalent to the total biomass of all birds and mammals combined. Ants provide important ecosystem services, such as seed dispersal and pest control.

In addition, increased pollutant levels could similarly affect other social insects, such as bees, on whose pollination most of our crops depend. “Oxidizing pollutants such as ozone and nitrogen oxides are often discussed because of their harmful effects on humans. However, we should also be aware that these man-made pollutants can cause also significant damage to our ecosystems,” says Bill Hansson, one of the study’s lead authors.

By Max-Planck-Gesellschaft.