A recent study of Google Earth images revealed a preferential spatial alignment for grazing and resting cattle. In a survey of over 8000 cows in about 300 pastures, the study found conclusive evidence of a significant deviation from random orientation with preference for a magnetic North-South direction. They were able to use global differences in positions to rule out the effects of wind and sun, and show a statistical correlation which favored magnetic north over geographic north as a predictor of cattle pointing direction. In other words, cows are dipoles. But they’re not the only ones. Deer show an even stronger alignment with the magnetic poles. Look at this diagram from the original paper:
A is cattle, B is roe deer, C is red deer.
That’s a great looking distribution in the middle. I wish they’d clean it up and fit to it. I expect to see a follow-up study calculating the effective magnetic dipole moment of the cow.
The theory. If you’ve ever played with two bar magnets you know there is an orientation to the magnetic attraction between them. Indeed, the magnetic field is a vector field and dipoles tend to line up along its field lines. The needle of a compass for example is a magnetic dipole. If allowed to spin freely, it will point toward magnetic north/south. A dipole in a magnetic field feels a torque which is proportional to the strength of the field and the magnetic moment of the dipole.
T is the torque, u is the magnetic moment, B is the external magnetic field (of the earth, in this case).
This torque will tend to spin it in the direction of the magnetic field line. Viewed another way, the energy of a dipole in an external magnetic field is:
The natural inclination of a system is to move from a higher energy state to a lower energy state. The lowest energy in this case is represented by the case where u and B are aligned (theta = 0), therefore their dot product is maximized, and the energy is most negative.
Now, cows are lazy, and they undergo some kind of random motion, so naturally there will be a width to the distribution of the probability that a cow will be pointing in a given direction. Now, if this random motion is associated purely with thermal energy, the pointing probability could be calculated exactly. My guess is that it’s more complicated than that. If I come up with a good way to model it, I’ll probably publish the follow-up paper.
An interesting side note is that many animals have already been well-documented to have internal magnetic sensors. Birds and salmon have been known to use magnetic field lines to navigate their migration. Apparently, even rodents possess some kind of internal magnetic compass. What they need it for is beyond me. Interestingly, some studies suggest humans who sleep oriented East-West experience shorter REM sleep periods than humans who sleep oriented North-South. REM sleep is apparently important for memory, alertness, creativity, and every other good thing our brain does. There’s something, after all, to be said for feng-shuing your room.
From randomness, order. (personal blog update)
My life has been evolving slowly to become more and more structured and less and less what I feel like I really ought to be doing. In addition I’ve experienced a noticeable decrease in REM sleep lately as my days and nights have really filled themselves up. This is only adding to my sense of confusion.
Mondays and Fridays I teach at the UC. Mondays are discussion sections and Fridays are labs. I’ve been doing this for 3 weeks now and it hasn’t gotten any easier carrying 50 lab notebooks across Science Hill.
Tuesdays and Thursdays I tutor middle and high school students in Aptos. This last Thursday one of my duties was to help a 7th grader finish his auto-biographical slideshow presentation. His accomplishments included being the last to be cut from the basketball team and almost making the honor roll in the 5th grade. His goals were one day be richer than Bill Gates.
Wednesdays I head up to Stanford to report on the meager progress I’ve made that week in my research. This wednesday I bring up the issue of funding for probably the last time.
Yesterday I helped bartend at a frat party. Someone stole a handle of vodka and a $1.50 jug of safeway brand orange juice. A vinyl record was pilfered from the DJs. Party was eventually broken up by the police around 11:30. Pretty classy stuff. I’ve been thinking about getting a job in sustainable energy and bartending on the side.
In other news I bought a used 2000 Volkswaggen Passat with a V6 engine and a moon roof. I’m going to use it to learn how to drive stick. Also, I’ll probably use it to get around.
I don’t know where I’m going to move to come spring but I will move somewhere.