How do you find your way to a bakery in a new area? You would probably plug it into your GPS. How would you tell your friend how to find the best bakery in town? You would probably text them directions. If you were an ant, you would sweat out some “let’s get this bread” juice. Bert Hölldobler and E. O. Wilson, two world experts on ants, wrote about this in their book, The Superorganism: The Beauty, Elegance, and Strangeness of Insect Societies. In describing ant communication, they explained how ants orient themselves and others. Ants excrete pheromones, chemicals that when smelled trigger a response. Across all the known species of ants, more than 40 pheromone-producing glands have been discovered. More specifically, ants use exocrine glands, that is, glands that not only make a substance but also excrete it through ducts.
A diagram of a weaver ant, showing different glands that can release pheromones.1
Pheromones have a lot of meanings, but they are mostly divided into two subcategories: recruitment and orientation. Recruitment is for gathering ants together for some purpose, such as showing nestmates a food source or assembling nestmates for a job that needs multiple ants, like lifting something heavy. Ants also use chemical signals for orientation, that is, creating trails to show other ants how to get to a destination, such as a new nest or food source. Ants rely on and trust chemical signals so much that they will follow a trail made by anything that smells like their nest. When wandering, ants leave two different trails: one to orient themselves and find their way back to their nest, and one to recruit other ants if the mission is a success. There are multiple methods to this two-trail approach. Pachycondyla marginata ants leave a dotted trail as they walk, dropping their gaster, or abdomen, every few steps to release pheromones. However, once ants have found something interesting, such as a food source, they will drag their gaster back to the nest, leaving a continuous trail. Other ants will follow the trail the original ant made. The better the food, the more ants will go retrieve it, and the stronger the trail gets. The strength of the trail is then directly related to the deliciousness of the food.
A Pachycondyla marginata worker bends forward her gaster to excrete pheromones and make a trail.2
In contrast, Onychomyrmex scouts actually lay two trails simultaneously. As she wanders, a scout will lower her abdomen irregularly, excreting a recruitment pheromone from her sternal gland. This pheromone is very volatile, meaning it evaporates quickly, so the trail only lasts for a few minutes. To lay a longer-lasting trail so she can find her way home, an ant will drag her back feet to activate a different gland. Foragers attracted by the recruitment trail will also drag their back feet to strengthen the homing trail, but will not lay an additional recruitment trail.
An Onychomyrmex scout lays a recruitment trail from her sternal gland and a homing trail from her back foot.3
The strength of a chemical signal depends not only on the amount of ants that have laid it, but also on distance. The place where the pheromones are released is called the active site, and as you can imagine, the farther you get from the active site, the weaker the smell. Everything is about concentration. Ants don’t actually follow a one-dimensional spot of scent on the ground; they react to the concentration of vapor particles in the air. In this way, the active site is actually a three dimensional hemisphere, where the scent is strongest in the middle and weakest at the edges. Ants travel along the gradient, or the path along which the concentration increases most quickly. In the case of the hemisphere, that would just be in a straight line to the center. As time goes on, the scent disperses, so the concentration goes down. The active site shrinks, because the radius is wherever the threshold concentration that ants can detect is. The speed at which the active site disappears depends on the volatility of the pheromone, which is why, for example, a homing trail might last longer than a recruitment trail. Because the active site of a point is a sphere, the active site of a continuous line, or trail, is a hemispherical “tunnel”.
An active site released by an ant recruiting help in the middle. Ants within the active site travel along the gradient to find the ant at the center.4
So now that we know how ants create and find trails, how do they know which direction to go? Ants use several kinds of context clues. They look for landmarks, such as a pebble or plants near their nest. Ants also use the angle of the sun to orient themselves. They don’t actually look up at the sky, but instead look at the polarization of the sunlight, which will shift as the sun moves in the sky. Ants even account for the passage of time using the arc of the sun. This way, if they are out of the nest for hours at a time, they can still find their way home. Before leaving the nest, ants figure out the angle of the sun based on the polarized light, and then reverse that to find the way home. Hölldobler and Wilson performed an experiment in which ants were put in an arena with two lamps, one on either side. Only one lamp was turned on. When that lamp was turned off and the other turned on, all of the ants turned around and began heading the opposite direction. This experiment replicates how ants use light to orient themselves in the natural world. Outside, ants check the angle of the sun before they leave the nest. To return home, they turn 180˚ so that the angle is reflected in the other side of the sky. By switching which side the light was coming from, the scientists made the ants think that they had gotten turned around, so they changed direction. Maybe they should have just used a GPS.
Further Reading:
Bert Hölldobler and Edward O. Wilson. The Superorganism: the Beauty, Elegance, and Strangeness of Insect Societies. W.W. Norton, 2009.
Media Credits:
[1] Nelson, Margaret C. 1990, The Superorganism: The Beauty, Elegance, and Strangeness of Insect Societies. By Bert Hölldobler and Edward O. Wilson. W.W. Norton, 2009. 201. Web.
[2] Nelson, Margaret C. 1996, The Superorganism: The Beauty, Elegance, and Strangeness of Insect Societies. By Bert Hölldobler and Edward O. Wilson. W.W. Norton, 2009. 192. Web.
[3] Nelson, Margaret C. 2009, The Superorganism: The Beauty, Elegance, and Strangeness of Insect Societies. By Bert Hölldobler and Edward O. Wilson. W.W. Norton, 2009. 186. Print.
[4] The Superorganism: The Beauty, Elegance, and Strangeness of Insect Societies. By Bert Hölldobler and Edward O. Wilson. W.W. Norton, 2009. 208. Web.
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