Teeming ants act like both a liquid and a solid

Ants are actually liquid-like and solid-like at the same time, according to new research.

These properties could explain the remarkable ability of collections of ants to change shapes and tasks based on the demands of their environment. When floodwaters hit, they self-assemble and form rafts to stay alive. They can also use their bodies to build bridges and span gaps.

A group of researchers from Georgia Tech probed the mechanical properties of fire ant aggregations by putting thousands of ants into a rheometer, a machine used to test the solid-like and liquid-like response of materials such as food, hand cream, or melted plastic.

The ants were sheared at constant speeds from about 0.0001 rpm up to about 100 rpm. The researchers found that the behavior of live ants was similar to that of dead ants: when the aggregation is forced to flow, live ants let go and play dead. In this case, the viscosity dramatically decreased as the speed increased.

“It’s not unlike ketchup,” says Alberto Fernandez-Nieves, an associate professor in the School of Physics. “The harder you squeeze, the easier it flows. But with ants, this happens much more dramatically than with ketchup.”

“Ants seem to have an on/off switch in that they let go for sufficiently large applied forces,” says David Hu, an associate professor in the George W. Woodruff School of Mechanical Engineering. “Despite wanting to be together, they let go and behave like a fluid to prevent getting injured or killed.”

Dropping a penny through an ant aggregation creates this same behavior. Ants will flow around the coin as it sinks through the aggregation. This flow takes a relatively long time to happen. However, when the aggregation is poked quickly, it responds like a spring and returns to its original shape.

“This is the hallmark of viscoelastic behavior,” says Fernandez-Nieves. “The ants exhibit a springy-response when probed at short times, but behave fluid-like at longer times.”

Dead or alive

The group quantified this by looking at the ants’ response to tiny wiggles of the rheometer. They found that the ants are equally liquid-like and solid-like. They did the same experiment with dead ants and saw that they are also solid-like. This showed that live ants are liquid-like and solid-like because of their activity.

“Remarkably, the observed behavior is similar to what is seen in materials that are not alive, like polymer gels right at the point when they become a gel,” says Fernandez-Nieves.

“This is quite puzzling, and we are now performing many more experiments to try and understand where these similarities arise from and how much they can be pushed. Doing this will hopefully extend our current way of thinking about materials, that like the ants, are active and thus out-of-equilibrium. There is much more interesting work we plan on doing with ants.”

Like liquidy jello

Michael Tennenbaum, a graduate research assistant who participated in the study, also compared the behavior of the ant aggregation to jello.

“Imagine if you wanted to make the most jello possible out of a packet of gelatin. It would be solid, but also very liquidy,” he says. “That’s because there would be just barely enough gelatin to make it solid-like but not enough to make it completely solid. The jello would be both solid-like and liquid-like.”

Hu has also used the liquid-like nature of the ants to study self-healing materials.

“If you cut a dinner roll with a knife, you’re going to end up with two pieces of bread,” says Hu. “But if you cut through a pile of ants, they’ll simply let the knife go through, then reform on the other side. They’re like liquid metal—just like that scene in the Terminator movie.”

Hu says it’s this flexibility that allows ants to enjoy the best of both worlds. They’re able to become solids to make things and liquids to avoid breaking into “smithereens.”

The US Army Research Laboratory and the US Army Research Office Mechanical Sciences Division, Complex Dynamics and Systems Program, supported the work. Any conclusions expressed are those of the principal investigator and may not necessarily represent the official views of the funding organization.


What do hens do?

A hen does not know if her eggs are fertilized or not. In fact (much like a human) a rooster can be infertile, so a hen’s eggs might not be fertilized even if she is in a flock with a rooster. Even more than that, a hen doesn’t know the difference between her eggs and eggs laid by another hen, even if the eggs look totally different and are a completely different color than her eggs. I routinely use one hen to hatch other hens’ eggs. I will even give a hen golf balls to sit on as placeholders if she’s ready to brood and I don’t have all the eggs I want to hatch gathered yet. Because a hen can’t tell if her eggs are fertile or not, I’ll tell you what a hen will do with any eggs she lays, fertile or not.

Many modern breeds and commercial hybrid hens will do nothing with their eggs other than lay them and walk away. A commercial hybrid isn’t anything scary, by the way, it’s simply a specialized cross-bred chicken, much like a Labradoodle or Puggle dog. Modern commercial breeds and hybrids have had the instinct to brood bred out of them over generations. In a modern egg production facility, you do not want a hen to “go broody.” When hens are ready to raise chicks, they will stop laying eggs for that period and it’s very hard to convince them to give up the idea and start laying eggs again. A broody hen sits on a nest (even if it’s empty) and doesn’t take good care of herself; she won’t eat much and doesn’t drink much, and can lose quite a bit of body weight and get in bad condition. She may not snap out of this trance if she doesn’t get triggered by feeling chicks hatching beneath her; chickens have no concept of how long it takes chicks to develop (21 days, by the way). Even before the advent of modern “factory” farming, farmers were trying to breed away the instinct for brooding chicks. This breeding has largely succeeded for many breeds.

This does not mean that no hens will brood eggs; many breeds still retain their instincts to mother. Silkies, for instance, are renown for their desire to sit on eggs. Other breeds such as Orpingtons, Brahmas, Cochins, Marans, Cornish, and others go broody quite regularly. I myself have a little Marans hen that was broody a month ago and hatched a clutch of chicks. When a hen that has broody instincts lays an egg, she is forming a “clutch” of eggs. She does nothing to care for these eggs other than hide them in a secure place until she is ready to sit on them. She will continue to lay eggs in this clutch until she has “enough,” which is a number anywhere from seven to as high as 20+. Once there are “enough” eggs, a hormonal switch will occur that will put her into what’s best described as a broody trance. She will stop laying eggs and begin to sit on them instead.

There are very good reasons that she does not sit on the eggs from the beginning. Firstly, she needs to continue to eat and drink so that she doesn’t lose body condition and can continue to produce eggs for her clutch. Secondly, all the eggs need to begin developing on the same day. An egg does not start forming a chick as soon as it’s laid. Instead, the eggs are kind of in a state of suspended animation. You can’t even tell the difference between a fertilized and non-fertilized egg without cracking it and looking very closely. Once an egg is above about 98 degrees F for approximately 24 hours, however, it will begin to develop. This way, all the chicks start developing when the hen settles down to sit on them and are all developing at the same time, and none of them have a head start. Then all the chicks will hatch over a short period (usually less than 24 hours) and are all ready to venture out for food at about the same time. If some eggs started developing a week before the others (for example if the hen started sitting on them while she was still adding eggs to the clutch) then some chicks would be ready to hatch a full week before the ones that were in the last eggs to be laid. If this happens, the hen will abandon the not-fully-developed eggs and care for her already hatched chicks, and the not-ready eggs will die.

Two to three days after the first chick has hatched, the mama hen will come out of her broody trance and start to care for the chicks. In the meantime, the chicks will all stay under Mama and require no food or water; they are fed from the remnants of the yolk that is in their body for this purpose. Mama will care for them for a while–the exact time is different for each mother hen. Some care for them only until they are 12 weeks old, some will care for them longer.