The Pak is a fictional race from the Known Space series by Larry Niven.
The Pak lived near the galactic core, where they evolved around 50 million years ago. The first Pak spaceflight is recorded as being 3 million years ago, and at 2.5 million years ago, a group of pak went out along the galactic arms, in an attempt to find another suitable planet.
During his life, a Pak goes through three stages, the child, breeder and protector stages. During the child and breeder stages, he has little to no intelligence. His only concern is growing up, and later multiplying. As he reaches the age of roughly 40 earth years, he yearns to eat a specific plant, paraphrased by the belter-cum-protector Jack Brennan as "Tree-of-Life", which grows on the Pak Homeworld. It contains a bacteria, which transforms the breeder into a protector (there is a very interesting relation between Earthlings and Pak, but I won't spoil that for those of you who have yet to read the books). During this process, he loses his reproductive organs, the skin grows thick and leathery, and his brain expands.
The protector has very strong instincts to protect his offspring and will kill anyone who poses (or might at a later date) a threat, small as it may be. Thus, the Pak are responsible for destroying several other races around their homeworld. A protector is very intelligent, and has an enormous attention span, so enormous, that he can fly a craft for several hundred thousand earth years manually (they wouldn't trust an auto-pilot anyway). Obviously, it is also virtually unaffected by what we humans call old age.
Audited January 27, 2003, sources: Protector, Ringworld