The hypothalamus is a way station where input becomes output. Specifically, it accepts smells, electrical signals, and blood-borne signals, and then based on these stimuli releases corresponding hormones (or prods the pituitary into releasing hormones).
A cool example of the hypothalamus linking input and output is seen in mice. After she mates with a male, a female mouse may run across a new male mouse in the course of her daily scufflings. Is he studlier than her previous mate? If so, she may choose to hang around him (or his droppings or urine), thereby inhaling his manly rodent odor. And this scent of a strange male, soon after mating, induces the mouse hypothalamus to release a hormone that aborts her developing pregnancy. Within one to four days, she’ll be in estrus and able to mate with the new George Clooney–mouse.
Interestingly, the hypothalamus also regulates food intake. Cranking up the hypothalamus’s activity creates overeating and obesity. And a mute hypothalamus can stop a creature’s food intake entirely.
Genius Tester #1: Well Hung
On Monday, a prisoner sits in a cell waiting to be hanged. He is told he will be executed before the weekend but that he won’t know the day of his execution beforehand. He reasons that if he were to be hung on Friday, he could deduce this from being alive on Thursday night. So Friday’s out. But if Friday’s out and he’s alive on Wednesday night, he can deduce that he must be hung on Thursday. And since he can’t know the day beforehand, Thursday’s out. He works back through the week and realizes he cannot be executed. And so he is unpleasantly surprised when he is hung on Tuesday.
What went wrong?