Every year, an alien species raids a quiet planet. The locals stand by helplessly as the aggressive invaders collect prisoners, taking them back to the alien home world for hard labor. It's the plot of more than one sci-fi story, but what if it happened in real life?
Surprise! It's already happening in real life, just not with different planets. It happens between different populations right here on Earth. Scientists label this event social parasitism.
One of the most dramatic examples of social parasitism happens between two ant species in the US. 🐜
Temnothorax longispinosus is a small, dark brown ant that builds modest nests under rocks or inside rotting logs. These ants would live fairly peaceful lives if it weren’t for a much more aggressive neighbor: Temnothorax americanus.
T. americanus is a socially parasitic ant. Instead of gathering food and raising young on their own, these ants raid the nests of T. longispinosus. During an attack, they chase away adult workers and steal eggs, larvae, and pupae, bringing them back to their own nest. When the stolen ants grow into adults, they are immediately put to work for their captors!
This happens enough that T. americanus nests are mostly made up of captured workers. Only the queen and a few workers actually belong to the egg-stealing species!
This sort of nest hijacking isn't limited to ants. Other insect species do it too! 🐝
The bee species Bombus bohemicus invades the nests of other bee species. The invading queen kills or overpowers the original queen and then tricks the workers into raising her young. The workers are fooled because the imposter smells just like one of their own.
The large blue butterfly Phengaris arion will lay its eggs on plants that have ant colonies at the roots. The caterpillar will hatch and drop to the ground, emitting chemicals that smell just like the larvae of the host ant species. The ants collect the caterpillar as one of their own and bring it to the nest, where it quietly feeds on the colony's eggs and young. If worker ants get suspicious, the caterpillar mimics a sound produced by the queen ant. The ants will look the other way as the caterpillar devastates the colony before moving on!
What's interesting about the ant species is that their behaviors are affected by different factors. Weather can influence behavior: the victims, T. longispinosus, are more likely to fight off their attackers when the weather is cold. Genes also make a difference. Both types of ants have about the same number of genes overall, but both host ants and parasite ants have certain genes that turn on and off during normal activities. They also have genes that only turn on during a raid!
In a raid, attack and movement genes turn on in parasitic ants, while defense and stress genes turn on in the defending ants. The pie charts below show the ratio of genes that change in normal versus raiding situations. Raiding ants are found in the left three charts, while host ants are shown in the right three charts.
Here are some questions I think of when I look at this graph:
💡 Compare the total genes that turn on and off in each species. Which species has the most genes that turn on and off? Which species has the fewest? Is there a trend among species?
💡 In each species, look at how each pie chart shows the ratio of genes that turn on in a raiding state to the genes that turn on in normal behavior. Which species has the highest percentage of genes that turn on in a raiding state? Is there a trend among certain species?
💡 Calculate the actual percentage of genes that turn on in raid activity out of the total genes that change state for T. curvispinosus.