Let us begin by saying that the world is a big scary place and it is difficult to know where to go next.
Still, let us assume for now that you have the knowledge of where to go. The great problem that follows is how to get there. The answer, at least for human beings, is “maps.” The maps that we use may be printed on a piece of paper, stored in our smartphone-internet interface, or simply stored in our memories. No matter where they are stored, we can retrieve them in times of need, and we can observe the relationship between points in space that the maps give to us and we usually reach where we want to go.
But animals do not have printed maps, nor do they carry smartphones that use satellite data to give you a slightly unnerving list of places where you have been going out to eat your dinner this month. When an ant finds a piece of its own dinner, we have seen that it follows the pheromone trail to go back to its nest.
But things don’t always go according to plan in the ant world too, and ant-schedules can also get messed up. If an ant following the pheromone trail is blown off its course by a particularly strong wind, for example, and if it manages to get back to the trail, the chemical signal alone does not provide enough information for making a decision on which direction it should follow. Also consider the ants that live in the Sahara desert where a trail on the shifting sands would not be very reliable.
We love the sun!
These ants have come across some honey in this delightful image by Balaram Mahalder.
In order to find their bearings in such map-deprived situations, human beings have used the position of stars and planets for thousands of years, and ants apparently do the same. About one hundred years ago, a Swiss entomologist called Felix Santschi discovered that ants rely on the position of the sun to orient themselves on their way home.
How did he test this?
First, he used a board and blocked the sun from an army of ants that were returning home with their food. The shadowed ants were suddenly disoriented and did not follow the path to their home anymore.
Next, he got a mirror and placed it near the moving ants, still keeping the sun out of their view, in such a way that the ants only saw the reflection of the sun in the mirror. The ants fell for the trick and started to move, but not in the direction of their nest. But when they had walked away from the mirror (and the shadow of the board) and could finally see the real sun, they re-oriented themselves towards their nest. A more recent work involving ants and mirrors is described in this BBC News article, which also talks about the role of ant memory in navigation.
If you would like to test this on your local ants, Jae Choe suggests a little experiment. You can simply pick up an ant going back home in the dark and put it back on its trail but turn around so that it faces the opposite direction. It will not turn back. You just sent a poor ant away from its nest, into the dark dark night. Now if you do the same thing during the day, this (luckier) ant will promptly turn back and reach home safe.
It has now been established that insects acquire this sense of direction by sensing the pattern of polarized light. This is also the mechanism used by bees that dance to convey the direction of the food source relative to the sun’s angle.
There is a problem with all this. There always is.
The problem in this case is that the sun moves. Well, the earth moves, and the sun’s position in the sky changes quite a bit. So what would happen if you picked up an ant going home (again) during the day and kept it in the dark for an hour and finally let it go? Since the ant does not carry around a watch, you would think that it will just assume that nothing has changed. However, the sun's position in the sky has changed by about 15 degrees.
If the poor ant does assume that nothing has changed, it will take a new path which makes an angle of 15 degrees with the original path it had been taking before you decided to experiment with it. But that does not happen. The ant has an internal biological clock that kept ticking during that time when you kidnapped it and kept it in the dark. This internal timekeeping helps the ant to realize that the sun has moved, and it will recalculate the direction it needs to take relative to the sun’s changed position, and it will still follow the right path back home.
Ants are not the only animals to have the internal biological clock, though. Our bodies have a cycle, also called a circadian rhythm, that has a period of about twenty four hours. This is the clock that keeps you awake at night after a long flight to the east or west. And to prove that our clock also goes on working in the dark, people who live in the dark without clocks have been observed to follow the (approximately) twenty-four hour cycle for activity and sleep.
Does this mean that ants can see the sun as well as we do? And can they see other things too? I will consider the vision of ants in my next post. Leave a comment and let me know your thoughts.
Links to references and image sources are provided in the text.