Everybody needs a hug from time to time. This simple gesture of affection can brighten a day. Even ants hug each other! Except, when they hug, it is in preparation for a wartime event. It marks harsh environments and times of hardship. The ants known for this peculiar behavior are Solenopsis invicta, commonly known as fire ants, though I just call them "The reason I don't go camping."
[1] Fire ant foraging on leaf.
If you've ever been part of the classic man vs. ant competition for kitchen counter supremacy, I'm sure water has been your weapon of choice. Water, or any liquid for that matter, is a short-term effective deterrent for ants. It separates the little critters from their colonies. Outside of the kitchen counter battlefield, this happens in many other environments. Events such as flash floods may wash away ants faster than you can say "Where's my bug spray?"
But here's where things get interesting. In order to stay together during these floods and similar conditions, the ants hug. Yes, you heard that right. They hug to form a raft for two reasons: 1) to transport the entire colony together, and 2) to increase the survivability of the colony. Once the ants organize into a raft-like structure, they are able to breathe and stay afloat. Even the ants on the bottom trap air bubbles between themselves to survive. The raft is very light yet has a wide surface area, increasing buoyancy. It's like a tiny, living, ant-made cruise ship.
[2] Ants linking arms to create a raft in a flooded region.
If you're saying, "So ants hug each other, what about it?", then I implore you to keep reading. Let's run a small experiment. Take your index fingers and link them together as a chain. Now pull your hands apart, using only your index fingers to bind your hands together. Eventually, your hands separate as your fingers fail to combat the force. If you do this enough, you'll start to notice that the bond between your fingers gets weaker as you pull your hands apart harder.
And this is why ants hugging are so cool. When ants make a raft and some force, like a wave, acts to pull them apart, the ants' bonds actually strengthen. Wait, what?! Yeah, you read that right. The bonds between the ants actually strengthen as they're under more stress. Boom. Mic drop.
[3] Common toy, Chinese finger trap
This type of bond is known as a Catch Bond: “a bond that increased in lifetime when the bond was stretched by mechanical force.” It's similar to the forces felt when using a Chinese Finger Trap. As you pull, the trap gets stronger. By now, you've audibly said "wow" at least three times and fallen in love with Solenopsis invicta. Well, save your breath, because the best is yet to come: data. (Cue the dramatic music!)
Robert Wagner, at Cornell University, devised an experimental setup to measure the bonds between ants. The setup consists of an ant raft suspended between columns. Wagner applied a variable force to one of the columns, stretching the ant raft a measurable distance.
[4] Experimental set up of ants in laboratory, undergoing stress testing.
As he did this, he also measured the forces between the ants, with strings in tension, to draw a correlation between the deformation of the raft and the force expressed. The resulting graph shows the catch-bond relationship in action. As the raft is stretched, the force increases. There is, however, a limit to the bond. Eventually, the ants express a maximum amount of force, and the bonds weaken thereafter.
Wagner observed this and decided to measure it further. He noticed a difference in the strength of the bond as the force was applied at different speeds. He plotted three different speeds on the same graph, and the results are fascinating.
[4] Graph relating the strain applied at various speeds on the ant rafts to the stress exserted by the ants.
Though initially intimidating (like a swarm of fire ants), this graph gives more insight into how the ants bond. As the strain is applied quicker (the red data line), the ants act as a more brittle material. They have a very strong initial response to the quickly applied force but then peak in their strength at a raft deformation of ~0.4 mm/mm. However, when the force is applied slowly (the teal data line), the maximum force the ants exert is lower, but they don't start to decrease in strength until later, around 0.5 mm/mm.
To gain a better understanding of this data, think back to the 2016 internet sensation that swept the country: slime. With many slime mixtures, if you pulled very fast, it would break, acting very brittle. But if you pulled on the slime very slowly, it would string apart and create a seemingly infinitesimally thin bridge that would inevitably stick to your brand new cardigan and never wash out. I loved that cardigan. Anyway, back to the ants.
Solenopsis invicta, or fire ants, have shown two very unique traits in their bonds:
- Their bonds mimic catch bonds, which strengthen as stress is applied to them.
- Their bonds can withstand more stress as the stress is applied slower.
So, the next time you find yourself face to face, or foot to face, with these ants, take a moment to appreciate their incredible skills. These tiny engineers organize into rafts during floods to stay together and thrive in the face of adversity. They may be a scare (did I forget to mention they sting?) but are fascinating to the core.
Further Reading:
Abdelrahman, M.K., Wagner, R.J., Kalairaj, M.S. et al. Material assembly from collective action of shape-changing polymers. Nat. Mater. 23, 281–289 (2024). https://doi.org/10.1038/s41563-023-01761-4
Haight, K.L. Defensiveness of the fire ant, Solenopsis invicta, is increased during colony rafting. Insect. Soc. 53, 32–36 (2006). https://doi.org/10.1007/s00040-005-0832-y
N.J. Mlot, C.A. Tovey, D.L. Hu, Fire ants self-assemble into waterproof rafts to survive floods, Proc. Natl. Acad. Sci. U.S.A. 108 (19) 7669-7673, https://doi.org/10.1073/pnas.1016658108 (2011).
Pereverzev YV, Prezhdo OV, Forero M, Sokurenko EV, Thomas WE. The two-pathway model for the catch-slip transition in biological adhesion. Biophys J. 2005 Sep;89(3):1446-54. doi: 10.1529/biophysj.105.062158. Epub 2005 Jun 10. PMID: 15951391; PMCID: PMC1366651.
Media credits:
[1]: Photo by Alex Wild. https://www.alexanderwild.com/Ants/Making-a-Living/Tramps-and-Invaders/i-GnTQTZw/A
[2]: Public domain image. https://commons.wikimedia.org/wiki/File:RIFA_Raft_After_Heavy_Rain.jpg
[3]: Public domain image. https://commons.wikimedia.org/wiki/File:Finger_trap_toys.jpg
[4]: Figure 1 from R.J. Wagner, S.C. Lamont, Z.T. White, F.J. Vernerey, Catch bond kinetics are instrumental to cohesion of fire ant rafts under load, Proc. Natl. Acad. Sci. U.S.A. 121 (17) e2314772121, https://doi.org/10.1073/pnas.2314772121 (2024).
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