When you think of ants, you likely think of a big group, whether they are all intertwined on a raft or creating a trail from that piece of food you left out. Many ants will recruit other ants by laying down a trail of pheromones on the way back to their nest to share the location and help defend the food.
But what if you were Pseudomyrmex gracilis, the graceful twig ant? These ants don’t work together in groups to find food. What would you do instead?
I interviewed one of the Harvey Mudd Bee lab’s collaborators, Dr. Scott Powell from George Washington University to find out more about P. gracilis behavior. Dr. Powell and the head of the Bee Lab, Dr. Donaldson-Matasci, did their postdocs together and collaborate on ant behavioral research together. In one of Dr. Powell's research projects they observed the foraging behaviors of various arboreal ants including P. gracilis.
P. gracilis are an invasive species to Florida and live in trees often occupied by as many as 5 other species of ants, so competition for resources can get pretty fierce. Competition can be even more dangerous to them.
For such a small (to us) creature, that must be pretty hard, but as the Harvey Mudd Bee lab and Dr. Powell’s lab has observed, they have a funny trick up their sleeve: one that we have already recorded: throwing other ants.
Dr. Powell has spent years watching arboreal ants interact with each other, in Brazil and Florida.
For example, he often uses bait to observe recruitment and to trace the ants back to their nests. He noticed when the bait had a small number of other ants on it, P. gracilis would sprint up and throw the enemy ants off the bait, and they would then grab pieces of food to run back to their nests.
This throwing behavior was not something Dr. Powell had observed before in any other arboreal ants (out of the many he has watched), and it was unique to this specific species in the Pseudomyrmex genus. Dr. Powell suggested to me this could be for a few different reasons.
Size: Twig ants are very large compared to other arboreal ants in Florida where this behavior was observed, with a length of about 8 to 10 mm! For comparison Turtle ants, another arboreal species that is native to Florida and often competes with P. gracilis for resources, are around 4 mm. This makes them very easy for Twig ants to pick up.
Hunting Style: Rather than the usual ant strategy of relying on numbers, P. gracilis need a way to disrupt that strategy as solitary foragers. What better way then to remove other ants from the fight?
Mandibles: P. gracilis have blunt mandibles focused on gripping onto things, not slicing. This both helps them run food back to their nest and throw other ants.
Ant mandibles get their versatility from the closer muscle, which is more powerful than the opener muscle and directly impacts the mandible’s power and how fast it is able to close. Ants with larger heads are able to have a higher amount of muscle fibers, giving them more power in their bite. Ants with narrower heads are able to stretch their muscle fibers leading to a quicker bite.
From the picture above, you can see P. gracilis’s wider head, indicating the higher amount of force they can develop from their mandibles.
In short, P. gracilis have short, blunt, and strong mandibles that allow them to counter the foraging strategies of ants that rely on numbers, at least in situations where there are not a lot of enemy ants on the food.
When ants form a large group against an enemy ant what often ends up happening is a couple of ants will hold the enemy down as other ants tear off their limbs.
[5] Image of Oecophylla Longinoda pinning down Polyrhachis taken by Alex Wild
This wouldn’t be as effective for P. gracilis, who hunt alone and would likely want to avoid scenarios like this. Hence, the throwing which is backed up by their strong and blunt mandibles.
In the lab this semester my partner, Lexie Stinson and I are studying turtle ant armor.
[6] Image of Cephalotes varians taken by Alex Wild
We conduct fights between P. gracilis and the turtle ants so we can take a closer look at the damage the turtle ants have sustained. Understanding the fighting style of P. gracilis will help us gain a better understanding of what damage the turtle ants can take, and how well their armor is able to protect them.
Further Reading:
Gronenberg, W., Paul, J., Just, S., & Hölldobler, B. (1997). Mandible muscle fibers in ants: fast or powerful? Cell and Tissue Research, 289(2), 347–361. https://doi.org/10.1007/s004410050882
Powell, S., Costa, A. N., Lopes, C. T., & Vasconcelos, H. L. (2010). Canopy connectivity and the availability of diverse nesting resources affect species coexistence in arboreal ants. Journal of Animal Ecology, 80(2), 352–360. https://doi.org/10.1111/j.1365-2656.2010.01779.x
Media Credits:
[1] Video by Lexie Stinson and Phoebe Harrison.
[2] Video from Ant Lab
[3] Image by Alex Wild. https://www.alexanderwild.com/Ants/Taxonomic-List-of-Ant-Genera/Pseudomyrmex/i-W4xHkdW/A
[4] Image by Alex Wild. https://www.alexanderwild.com/Ants/Taxonomic-List-of-Ant-Genera/Pseudomyrmex/i-BWr6dzt/A
[5] Image by Alex Wild. https://www.alexanderwild.com/Ants/Natural-History/Ants-fighting/i-rWVCfpC/A
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