Before picking up the book Nature's Compass about animal navigation, I had attended a talk at ASU by Dr. Harold Wolf of University of Ulm, Germany, a few months ago. Dr. Wolf and his research team have been studying navigation by a special group of ants in the genus Cataglyphis.
Any animal that has a nest must find its way back, but how do ants do it? After all, they are small, close to the ground and they travel great distances. Ants that live in tropical or temperate areas may lay chemical trails (pheromones) or use landmarks to find their way around.
Cataglyphis ants, however, live in a particularly difficult environment for navigation, the deserts of Tunisia. Any chemical trails they lay swiftly evaporate or are disrupted by the wind. The desert surface has few visual landmarks and those landmarks that are available are constantly changing. Without the features that allow many ants to navigate, how do foraging Cataglyphis ants find their way back to the nest once they find food?
The desert ants forage singly. An individual ant meanders around outside the nest until it locates a food item. Once it does find something, the ant picks it up and then travels in a straight line right back to its nest. Somehow, via a process called path integration, or dead reckoning, the forager can calculate both the direction and distance it needs to travel back separate from the route it took out, without having sidewalks or roads to guide it like we humans have.
Dr. Wolf explained that the ants are able to use the sun as a compass. To use the sun (that is constantly moving through the sky) as a reliable compass, ants must also have an internal clock that gives them some sense of what time of day they are viewing the sun. Researchers have shown that ants that are left in the dark for a couple of hours are still able to use the sun accurately to get home when they are released, which suggests they have the ability to account for the passage of time.
What if the sun is under a cloud? Desert ants can also orient themselves to patterns of polarized light. Polarized light is the light that vibrates in a definite pattern in one direction, rather than in all directions. Sunlight is not polarized until it hits the atmosphere, where polarization occurs as it bumps into molecules in the air. The scattered light produces a glare in the skies. Light can also become polarized when it reflects off of water.
Sunlight and polarized light are actually picked up by two separate parts of the ants’ compound eyes and form two separate systems of navigation.
These ants also apparently have an odometer that helps them calculate how far they have traveled. One type of odometer is a stride odometer, the presence of which was shown by the now famous experiments of "ant surgeon" Mattias Wittlinger. Wittlinger cut the legs off certain worker ants and glued bristles to the legs of other ants to lengthen them. When the ants were allowed to run home, the ants with short legs undershot the nest and those with too long legs overshot the nest. Apparently the ants were using the number of strides they had taken to estimate the distance back to the nest.
Dr. Wolf indicated there is evidence for an optic flow odometer as well, which is an odometer that keeps track of the amount movement in the environment the retina senses during the course of a journey. As he points out, navigation is an important ability and it makes sense that there would be multiple systems to be used as back-ups if one or more fails.
Dr. Wolf's presentation was very intriguing. It is easy to wonder, however, how much we are learning about these unique ants will apply to ant navigation in general. It does seem that because the system is so much more open to manipulation --than studying navigation by ants in areas with dense vegetation-- that it makes sense to study what ants can do there and then look for similar abilities in other species.
What do you think?
For more information:
Wolf, Harald. 2011. Odometry and insect navigation. The Journal of Experimental Biology. 214: 1629-1641. (Available free online)
Desert Ants Are Better Than Most High School Students At Trigonometry at Scientific American.
Muller, M., & Wehner, R. (1988). Path Integration in Desert Ants, Cataglyphis fortis. Proceedings of the National Academy of Sciences, 85 (14), 5287-5290.(PNAS)
Wittlinger M, Wehner R, & Wolf H. (2006). The ant odometer: stepping on stilts and stumps. Science. 312 (5782): 1965-7.
Grah G, Wehner R, & Ronacher B (2005). Path integration in a three-dimensional maze: ground distance estimation keeps desert ants Cataglyphis fortis on course. The Journal of Experimental Biology, 208 (Pt 21), 4005-11.
Wolf H, Wehner R. (2000). Pinpointing food sources: olfactory and anemotactic orientation in desert ants, Cataglyphis fortis. J Exp Biol. 203:857-868. (link downloads free pdf)
Kathrin Steck, Bill S Hansson and Markus Knaden. 2009. Smells like home: Desert ants, Cataglyphis fortis, use olfactory landmarks to pinpoint the nest. Frontiers in Zoology. 6:5 (available online)