Tuesday, February 8, 2011

Why Animals Migrate

The humpback whale migrates up to 16,000 miles every year, from polar regions to tropical waters and back.

Humpback whale migration routes.

The arctic tern migrates over 44,000 miles every year, from the arctic to antarctic and back.

Bird migration routes. 1-Northern Wheatear, 2-Bluethroat, 3-Eastern Yellow Wagtail, 4-Dunlin, 5-Wandering Tattler, 6-Bartailed Godwit, 7-Arctic Tern, 8-Sandhill Crane, 9-Brant, 10-Smith's Longspur, 11-American Golden Plover, 12-Tundra Swan, 13-Semipalmated Sandpiper.

These are just two examples of the incredible migrations performed on earth every year.  What could possibly motivate these animals to do this?  Well, the exact reasons are slightly different for every migrating animal, but a unifying theme can be found in the video below.  This is a video of "Net Primary Production", which is essentially a measure of the growth of plants.

The animals are chasing the plants!  That's why they migrate*!  So simple!!!!!  And if they're not chasing the plants themselves, they're definitely chasing other plant-chasing animals.

Perhaps the most striking aspect of this video, which shows two years worth of data collected by NASA scientists, is the onset of spring/summer in the northern hemisphere where plant growth explodes in the boreal forest of northern Canada and Russia.  The same effect is visible in the southern hemisphere, though it is less dramatic due to the smaller percentage of land cover.  You will also notice similar effects in the oceans.

This whole pattern is of course due to the tilt of the earth's rotational axis relative to the plane in which it rotates about the sun, which causes the seasons.  This epic task undertaken by awesome numbers of animals is caused by the specifics of the way our planet moves.  Woah!  Earth = Life = Earth

As the search for extra-solar earth-like planets continues, this sort of makes me wonder how different life would be on another planet whose axial tilt were smaller or non-existent.  No migrations at all?  Smaller migrations?  No birds?  Unicorns??
*Nature is not quite as simple as I would like, as food is not the only answer.  Animals will often migrate away from food-rich areas in order to breed in places where their young will be safe from predators.  In addition, scientists still haven't completely figured out the rationale behind all migratory animals' journeys. However, food and sex are the common theme that motivates most animals' activities, including migrations.


  1. I sure hope there are unicorns. Speaking of greening/productivity maps, have you read this?
    I was talking to a backyard apiarist in the area last summer and he told me about this study he was participating in.

    Oh also, you should talk about how turtles and whales and birds migrate... I mean the magnetism part! The part where they have Magnetite in their HEADS!

  2. @Lora Amazing! Thanks for posting that. I hope the situation w.r.t. climate change is not as dire as it might be. However, the synchronous blooming and nectar-gathering is such an amazing example of the incredible coordination (and dare I say unity?!) of life on earth. The fact that so much data is being gathered by hobbyists also gets me really really psyched.

    Also you are right I should def do a post about that magnetic migration stuff! My friend Alex forwarded me this story a few weeks ago:

  3. That's pretty cool, I wonder if shepherds already knew this... I wonder if they used it to orient themselves on a cloudy night when they couldn't see the north star.

  4. Woah! Yeah, that would be the shit. Science finally catching up with and explaining folk knowledge. I need to make friends with some shepherds.