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Hank explains the evolutionary basis for altruistic behavior in animals, including vampire bats!

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Ever wonder why we, like, do things for each other? Like, why we help our friends move, even though we hate doing it? And for that matter why a bee will sacrifice it's life by stinging an intruder to protect the hive? Why would a vampire bat regurgitate blood into the mouth of another bat that hadn't eaten that day? Why? It's a very nice vampire bat, I guess. Eugh. That's gross in, like, so many ways!

It turns out, that this is actually, a pretty big question. Things like this have been stumping scientists for years. Charles Darwin thought that altruistic behavior was a potentially deal breaking flaw for his theory of natural selection. If the game was survival of the fittest, natural selection couldn't possibly favor a behavior that made us less likely to survive. Or could it?

Darwin studied bee hives and realizes that since sterile worker bees were helping their blood relatives, especially the queen, natural selection might favor altruism within related groups.

100 years later in 1964 a British scientist named William Hamilton actually came up with an equation to explain this.  He figured altruism could evolve as a trait, if genetic relatedness times the benefit of the action was greater then the cost of the individual.  In other words, since some behavior is hereditary, the genes responsible for altruistic behavior could evolve if it's benefit exceeded whatever cost had for the individual, because it helped the individual's relatives enough to make it worth while.

Hamilton called this idea inclusive fitness, expanding Darwin's definition of fitness, basically how many babies somebody's making, to include offspring of other relatives. Hamilton's ideas were a huge hit among other scientists because, for starters, it explained stuff like ant colonies.  Ants have virtually no personal lives, everything they do, they do for the good of the colony.

And worker ants share three quarters of their genes.  They're actually more closely related to their sister ants than they would be to their offspring.  So, according to Hamilton's equation, each individual ant has an exceptionally large genetic stake in the survival of the colony. But Hamilton's ideas do not explain why some animals help others that they're not even related to.

Take humans for example.  We are deeply social animals, also exceptionally altruistic. In fact, a lot of scientists think that humans evolved our huge, super smart brains in response to the overwhelming benefits of engaging in selfless behavior. Because it turns out sharing and cooperating are very mentally taxing. Seriously, ask any three year old.

But if you think about it, you can see how we evolved to be altruistic for really self serving reasons.  Like, helping a friend move totally sucks, but you do it, because you can conceive of a time in the future when that friend will help you move, or when that friend will drive out into the country to help you change a flat tire, or whatever.

Same with the bats.  Barfing some blood into your neighbor ensures that someday if you don't get something to eat, somebody will come vomit blood into your mouth, and they'll expect the same from you in return, and so on and so on.  Hamilton's equation does not explain that behavior. Which complicates things because, equations are nice and comforting to have if you're a scientist.

But, if you can cooperate with others and resist the urge to hoard resources, knowing you'll be rewarded later, you'll be acting a lot like ants, bees, and vampire bats, which makes me just feel all warm and snuggly all over.

Thanks for watching this episode of SciShow, I hope you have friends good enough to vomit blood in your mouth when you need it.  If you have any questions, comments, or ideas, please connect with us on Facebook or Twitter or in the YouTube comments below. And if you wanna keep getting smart with us here at, head on over and subscribe and we'll see you next time, goodbye.