Eli5: why do chilli plants turn red to attract animals to disperse seeds but also produce the spicy chemical to put them off?
In: 6541
##Short answer:
Because birds have eyes too, and they see red better than most mammals do.
##Long answer:
First Factor: The spice is only effective on mammals.
Birds can’t feel it. It’s bad for the plant to have its seeds eaten by mammals (who destroy the seeds as they digest them), but good for the plant to have its seeds eaten by birds (who will poop them out whole and still in working order, and spread them farther away, as they fly.) Birds lack the reaction to the capsaicin (the chemical that makes chili peppers spicy) that mammals have. It’s not a *real* danger to eat capsaicin, but it tricks the nervous system of mammals to send the same pain sensation that actual dangerous things would send.
Second Factor: Birds can see reds better than most mammals can.
i.e. your dog is red/green color blind:
Most mammals only have 2 kinds of cones: a sort of green/blue vision. It’s not that they can’t see red, just that red and green are pretty similar as red things only register as being “very very green, as opposed to a green with some blue in it.” (i.e. red is far enough away from blue that it doesn’t give a residual tickle on the blue receptor like actual green colors would. That’s the only real distinction so it isn’t as stunningly obvious as it is to us.)
Some of the bigger primates, like us humans, have 3-color vision where we see red/green/blue. But that’s an exception among mammals.
And most birds are even better at it than us. They tend to have 4-color vision, a sort of red/green/blue/ultraviolet vision that gives them even more distinction.
So, basically, using a red color to stand out means you’ll stand out to most birds more so than you will to most mammals.
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