Those Selfish Armadillos

14 Aug

When I lived in South Texas I often saw armadillos, although very few were alive. Armadillos are one of the most common forms of Texan roadkill. They have an unfortunate tendency to jump at their predators; good for scaring off dogs, not so good against big rig trucks. The sad thing is, if they just stayed still the truck would probably just breeze over them. Most of my fellow neighbors probably weren’t that sad about it though, armadillos can be notorious agricultural pests. They love to dig and they really love to dig in gardens and fields.

But scientists are using them to test out an interesting idea. I’ve touched on kin selection in this blog before, the idea that evolution may favor strategies that wherein a single individual sacrifices their own fitness to benefit their family members. You can even try to predict how much someone might sacrifice. A famous equation is Hamilton’s rule, which goes that you would see altruistic behavior evolve where relatedness (r) times benefit (b) outweighs the cost (c): rB>C.

Thus, I’d sacrifice more for a brother than a cousin or for my child than my wife. An identical twin, who shares virtually 100% of my DNA, why for them I’d do nearly anything. At least, that’s the idea.

And guess which animal always gives birth to identical twins? The nine-banded armadillo. Every successful pregnancy results in four identical little pink quadruplets. At first they seem like they should be a great example of kin-selection in action.

At first.

The problem? How exactly does an armadillo act altruistically in the first place? None of the studies seems to have found any evidence of altruism. The observed armadillos tend to just go around doing armadillo-y things: eating, digging, scratching, moving, no matter who is around them. Turns out you need to use social animals before you can see social interactions. This isn’t saying the studies were a waste of time. They’re interesting, seem well-constructed, and help illuminate some of the challenges of studying animal behavior. It just seems that armadillos just never evolved much in the way of charisma.

Pity. Maybe it could have helped their public relations.

Sources:, Prodohl and Loughry et al (1996), Loughry and McDonough (2001), wikipedia

1 Comment

Posted by on August 14, 2012 in Natural History


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One response to “Those Selfish Armadillos

  1. Matthew Gray

    August 14, 2012 at 11:32 pm

    On my family ranch, I’d say 1 in 5 times I see an armadillo, there is another armadillo real close. That could mean sometimes it is coincidence or sometimes they are twins or sometimes it is distracting and I didn’t see the other armadillo. Can’t say much, but we have lots of armadillos. Generally when my dog approaches a dillo it jumps in fear, like, randomly up into the air, doing nothing to scare the dog. My dog always approaches the dillo from behind. It likes to chase dillos, but cannot really hurt them.


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