By Scott Solomon, Rice University
An ant can usually tell whether or not another ant belongs to its nest by the way it smells. If you take two ants and put them together in a neutral arena, like a plastic container, you can quickly tell whether the two ants came from the same nest by whether or not they fight. Though sometimes, ant mutations can change this behavior.
Mutation in Red Fire Ants
A fire ant can have a particular mutation that makes it lose the ability to recognize nestmates from non-nestmates.
The mutation affects a part of the genome known as the social chromosome. The social chromosome includes a gene called Gp-9 that makes a protein involved in the ability to detect smells. Ants with the mutated form of Gp-9 treat all other fire ants they meet as if they are nestmates (the alternative would be to treat all other ants as enemies, which would lead to constant fighting even with nestmates).
The mutated Gp-9 gene has been one of the factors that has allowed red imported fire ants to spread across much of the southern United States. This species, whose scientific name is Solenopsis Invicta, is native to Brazil and Argentina but was introduced to North America as accidental stowaways in soil or vegetation.
Red Imported Fire Ants were first detected in the US in 1942 by Ed Wilson, the Harvard biologist who would later help decipher the chemical language of ants.
Successful Ant Empires
The fire ants quickly spread from the point of their first arrival on the Alabama coast. By the 1970s, they had made it west to central Texas and east to northern Florida; by the 1990s, they had made it north to Tennessee and North Carolina.
One of the keys to their success has been the fact that fire ants with the mutated Gp-9 gene don’t fight with their neighbors the way ants normally do. This allows a new nest to become established near existing colonies, so the total number of nests in an area can be higher than it would otherwise be.
Another factor has been that fire ants with the mutated Gp-9 gene tolerate having more than one queen in the colony, a condition known as polygyny. Polygynous nests are harder to destroy with poisons or toxic baits because you have to kill all the queens.
For fire ants, avoiding conflicts between nests has been a successful evolutionary strategy. Yet, pacifists are rare among insects, especially ants.
This article comes directly from content in the video series Why Insects Matter: Earth’s Most Essential Species. Watch it now, on Wondrium.
The Desert Honeypot Ant Tactics
While most ant species act aggressively toward any other ants they encounter, especially those of the same species, the desert honeypot ant, Myrmecocystus mimicus, evolved a way to determine the outcome of such encounters while minimizing casualties.
When workers from rival colonies of desert honeypot ants encounter one another, they release alarm pheromones to call for backup, much like other ants. However, rather than attacking one another, the workers line up and extend their legs, making them look as if they are walking on stilts.
They’re literally sizing up the competition—but rather than fighting one another, they have turned these confrontations into ritualized tournaments. It’s a way of figuring out which colony would win if they did fight by using the number of workers on each side as a proxy for the total size of the colony. The tournaments can go on for days, but eventually, a winner is determined based on which colony recruits more ants to the tournament site.
If the victorious colony is only somewhat larger, then the competition will end there. The victorious colony has won access to the food in that particular territory. But if the colonies are very mismatched—that is, if one colony is much bigger, then the losing colony is in serious trouble.
Workers from the much larger colony will raid the nest of the smaller colony, kill its queen, and carry the workers, larvae, and pupae back to their nest. The worker ants of the raided nest will become slaves—they’ll spend the rest of their lives working inside the nest of the raiders. Even the larvae and pupae, once they emerge as adults, will emerge as slaves, working on behalf of a colony they are not related to.
Size of an Eyestalk Matters
Ritualized combat can be found in other insect species as well. Stalk-eyed flies get their name from the incredibly long stalks that support their eyes and extend outward in opposite directions from the sides of their heads.
While both males and females have long eyestalks, the eyestalks of males are much longer. Why? Because the females like wide-eyed males. The longer a male’s eyestalks, the better his chances of finding mates and passing his genes on to the next generation. As a result of many generations of discerning females, males have evolved highly exaggerated eye stalks.
Males compete with one another for access to the roosting sites where they have the best chances of finding females. But rather than physically fighting, the male with the longest eye stalks wins these competitions, getting access to the mating grounds.
If the difference in eyestalk length is great, the victor is obvious and the territory is ceded without a challenge. But if the males are similarly well endowed, a ritualized tournament ensues in which both flies confront one another head-to-head, turning side to side while keeping their eye stalks parallel. Once they have determined which male has the longer eyestalks, the loser leaves without a physical fight ever having occurred.
Common Questions about Combats in the World of Insects
Fire ants with the mutated Gp-9 gene don’t fight with their neighbors the way ants normally do. This allows a new nest to become established near existing colonies. It also allows having more than one queen in the colony, making it harder to destroy.
Desert honeypot ants have turned confrontations into ritualized tournaments. When workers from rival colonies encounter one another, they release alarm pheromones to call for backup. However, rather than attacking one another, they figure out which colony would win if they did fight by using the number of workers on each side as a proxy for the total size of the colony.
Female stalk-eyed flies like wide-eyed males. The longer a male’s eyestalks, the better his chances of finding mates and passing his genes on to the next generation. As a result of many generations of discerning females, males have evolved highly exaggerated eye stalks.