Now, we as humans tend to find other reasons for living. But on the basic biological level, once the next generation is off and running, there's no more need to hang about. Intelligence has nothing to do with this, for I believe documented cases appear at all levels. Mr. Silberman brought up a point about education and discrimination. Education has nothing to do with nature.
Discrimination is a natural reaction.
Only through education is modern discrimination taught. There is nothing natural about that type of discrimination, and when referring to the way of nature's discrimination versus human discrimination, one must realize the two are in no way comparable. There are many disabilities that leave the affected person with no outward signs. My father has Multiple Sclerosis, a severe neurological disease. Thankfully, the disease has not yet progressed enough to require him to use a wheelchair, and often not even a cane, but he still has trouble walking.