Animal Navigation

 

ID Hair Design is the only hairdressing salon in Newcastle where you can get really good honey. Ian, the proprietor, is proud of his bees and particularly of their uncanny ability to find their way back to the hive. Even when they go on holiday in the wilds of Northumberland, the bees have no difficulty navigating a course back to the hive after a foraging excursion through countryside that they have never seen before.

Research on animal navigation covers species from the ant to the whale. Techniques have to be adjusted to the size and the habits of the subject. Some scientists follow individual Sahara desert ants, plotting their course in a notebook. Others use satellites to track whales through the ocean. Substantial funding for work in this area comes from the need to understand the movemements of commercially important species, particularly fish.

Individual scientists are driven by the urge to puzzle out how animals’ navigation equipment works. “We don’t know for sure how animals navigate” says Julian Metcalfe, a Ministry of Agriculture, Fisheries and Food scientist who tags plaice with data-logging equipment to follow their annual migration up and down the North Sea. “There’s a lot of debate about whether they use vision, smell or a magnetic compass. As soon as you knock out one of these mechanisms others may take over” he says.

In fact the bee’s navigational abilities are unremarkable by animal standards. The trick it uses to return to its hive in an unfamiliar location is called path integration. Like many species, the bee has a compass and an odometer so she can measure the length and the direction of each leg of her outward flight. By keeping a running tally she always knows the direct route back to the hive.

Path integration is difficult to study directly in the bee, says Tom Collett of the University of Sussex. They are too fast to follow and they are too small to carry remote tracking equipment. Saharan desert ants, which make long meandering foraging expeditions on foot and then return directly to the nest are much more amenable.

Proof that ants use path integration to calculate their return route comes when a sneaky experimenter moves the homeward bound ant a few metres off her track.The ant continues walking in the same direction and for the same distance she had calculated, missing the nest by exactly the same amount that she was displaced. Only when she reaches the end of her calculated journey home does she try to use visual landmarks to guide her back to the nest.

The ant and the bee both use a compass that depends on the pattern of polarisation in daylight. Light from each different direction has been bent a specific amount by the atmosphere polarising it in a particular way. Both the ant and the bee have a special set of eye facets that detect the pattern of polarisation in daylight. These facets give the biggest response when the insect is aligned with the solar meridian.

Characteristic errors in navigation that happen when the insect can only see part of the polarisation pattern allowed scientists to work out the details of the polarisation compass. The odometer has been more difficult to understand. For a long time scientists were confused by early results suggestiing that distance was calculated by the amount of energy used on the journey. However neither ants nor bees overestimate distances when they are loaded with weights to increase energy consumption.

Now it is clear that in both cases it is the optic flow – the movement of patterns across the eye as the insect walks or flies through the world – that tells the insect how far she has moved. When ants walked on a transparent floor overlying a patterned “carpet”, moving the carpet along with the ant to reduce the optic flow caused her to underestimate the length of her journey. Moving it in the opposite direction made her overestimate.

When bees return to the hive from a food source they do a dance to signal the direction and the distance to the food. Harald Esch and John Burns at the University of Notre Dame [the zip code is IN, is that Indiana?] put a beehive on top of a skyscraper and trained the bees to fly to a feeder on top of another tall building.

Because they flew high over the ground the optic flow was much less than when they flew the same distance at ground level. As predicted, when they danced on return to the hive on top of the skyscraper the bees signalled a much shorter distance than when they flew the same distance at ground level.

Esch and Burns also trained bees to fly to a feeder suspended from a balloon. When the balloon was raised to a higher altitude, reducing the optic flow but increasing the energy needed to fly to the feeder, the bees signalled a reduced distance.

Quite apart from their scientific importance these findings will be good news for beekepers planning looking for novel holidays. It seems that sightseeing in New York, or hot-air ballooning would both be possible.

 Posted by at 20:06