Honey Bee NavigationPosted at: July 3, 2002 09:40 AM | Comments (0) | Edit
The average worker bee flies more than 900km during a lifetime of foraging. For such a small insect this is a very big distance. The majority of food is found within 2.5km of the hive, but a bee will also visit familiar flowers up to 10km away. Given that honeybees have low resolution vision and only large, or close landmarks are of any use to them, the navigation skills required for traveling such distances are awesome. This amazing navigational ability is one reason why bees are so interesting and are one of the most studied insects on the planet. In the text below we will describe how each of the honeybee's senses is used for navigation. All of these senses are redundant. That is to say, remove any one of them and the bee will probably still be able to navigate without problems. The bees themselves, however, probably prefer to use all their senses in combination and simultaneously.
The Sun Compass
Honeybees use the sun as their primary navigation compass. Their vision is of such low resolution that they are unable to resolve the sun's disk. They can, however, see a bright area in the sky and this is sufficient for them to orientate themselves. They are also able to account for the movement of the sun by using their internal clocks. This means that on returning from a foraging run the angle of the return flight path relative to the sun will be different to the angle of the outward flight path. The amount by which the flight angle is adjusted depends on the time spent foraging between the outward and return flights. The flight path adjustment calculation is not hard coded into the bee's brain. Instead, the direction and speed of the sun's movement is learnt during a young bee's early training flights. This means that bees from the same hive can be reared in different hemispheres and each will learn to navigate accurately, despite the fact that the sun moves in different directions in different hemispheres.
The amount of nectar released by flowers changes during the course of a day. Again, honeybees use their internal clocks to remember at which time of day particular food sources are at their best. The internal clock is accurate to within 15 minutes and bees will regularly turn up at the same location at exactly the right time every day. A bee can remember and separate up to nine feeding appointments per day.
Polarised UV Light
When the sun is obscured by clouds or trees, but patches of blue sky are still visible, the honey bee is able to use polarized light as a backup navigation system. The light coming directly from the sun is unpolarised. Some of this sunlight, however, is scattered by air molecules and a pattern of polarized light is set up in the sky. This pattern consists of a roughly circular set of gradients centered around the sun. The polarization is at its most intense at a 90' angle from the sun. By detecting the polarization angle bees are able to infer the location of the sun. Exactly how they manage to do this is still unknown.
[polarized light in sky diagram]
When the sky is overcast neither the sun nor patterns of polarized light are visible. In this case honeybees navigate using landmarks such as the shoreline of a lake or the edge of a forest. Prominent landmarks like these can even take precedence over the position of the sun. Experiments have shown that if a hive is moved during the night from one side of an elongated lake to the other, in the morning the bees will fly off in the wrong direction along the shore, even if the sun is shining.
When there is a prominent and unambiguous landmark near to the hive, a lone tree for example, bees are able to learn the position and movement of the sun relative to the landmark. This way they are able to correctly orient themselves from memory even when the sky is overcast.
The Earth's magnetic field changes on a daily cycle. It is suspected that this cycle is used by bees to maintain their internal clock. Sensitivity to the magnetic cycle would be especially useful to bees who remain inside the hive and are unable to detect sunrise and sunset. It has been experimentally shown that subtle magnetic disturbances can disrupt the bee's time-keeping abilities.
An important tool for navigation is the ability to measure the distance traveled. The honeybee has a couple of techniques for measuring how far it has flown. One technique is to judge how much effort was expended during the flight. The other technique, which is probably more important, makes use of what is called "Optic Flow". Optic flow is a measure of the movement of images across the visual field. The further a bee flies the greater the number of images that will pass through the visual field. A bee is able to remember how much optic flow there was during a flight and to use this to infer the distance. It seems that the outward journey is the most important for judging the distance of trips. This is in contrast to the direction which is averaged from both the outward and inward journeys.
An elegant experiment has been performed to demonstrate the use of optic flow in distance measurement. In the experiment worker bees were forced to fly down a narrow tube as they emerged from the hive. The inside of this tube was painted with black and white stripes. These stripes, together with their proximity to the flying bees, increases the perceived optic flow. This confused the bees into thinking that they had flown further than they really had. When the tube was removed they would try to return to the remembered locations of food but would always end up flying too far.
Optic flow is not used to calculate absolute distances. As the experiment above demonstrated, the amount of optic flow can vary according to the appearance of the environment. Flight paths in different directions from the hive can have significantly different amounts of optic flow depending on whether the path is across an open field or through an orchard. Absolute distance, however, is not important to the bee. All that matters is that she can remember the perceived distance in a certain direction.
The advantage of measuring optic flow over effort expended is that it is robust to changes in wind. Optic flow is also used for judging the distance to landing. If the bee keeps its perceived image velocity constant then the actual velocity will decrease to zero as it approaches touchdown.