Sunday, September 07, 2008

Cattle shown to align north-south

A recent BBC article reported that a group of scientists had studied a large number of Google map images looking for cattle, and had found that most of them adopt a north-south orientation. The resolution of the images wasn't good enough for the researchers to distinguish which way the animals were pointing, north or south. They followed up with field studies of wild deer in the Czech Republic and determined that most of them faced north while about one third faced south. They checked, and the animals didn't track the sun as it moved across the sky, their orientation was consistently north-south regardless of the time of day. Their conclusion was that the animals possess a magnetic sense, and that they line up with the earth's magnetic field. We believe that migratory birds can sense the earth's magnetic field and use it for navigation but no organ has yet been found in large mammals that confers the ability to discern magnetic fields, but perhaps one will be found one day. The researchers do not offer any compelling explanation for why the animals would benefit from lining up with the earth's magnetic field.

It seems to me that there is another, simpler explanation for this behaviour. Each hemisphere of the earth undergoes seasonal variations - summer, autumn, winter, and spring. During winters sunlight is diminished and the growth of vegetation slows down. Animals deplete the available grazing, and in their natural state when unencumbered by fences they migrate in an equatorial direction to reach pastures where the daylight hours are longer and the vegetation more plentiful. During the winter they deplete much of the available vegetation, and when spring and summer returns, they reverse the process, migrating towards the poles to exploit the new vegetation that grows in response to lengthening days. So their orientation may simply reflect the general direction that they're migrating in when observed. If so, one would expect to see a strong correlation between the local seasons and their orientation - they should be pointing predominantly towards the poles in spring and summer, and predominantly towards the equator during autumn and winter.

It is not necessary to suppose that the animals have magnetic senses in order to point north or south. Bees navigate by the orientation of the sun in the sky, automatically making allowance for the apparent movement of the sun across the sky. If they can manage this calculation with their small brains then mammals should be able to do so as well.