Pak Protector

Pak Breeders and Pak Protectors are two generic forms of life in Larry Niven's Known Space universe. They are a dichotomy brought about by the Tnuctipun's breeding policies and the plant known as Tree-of-Life.

Tree-of-Life is a bush that smells repulsive to breeders until they reach about 25 Pak years (or 42 human years.) After that, the smell suddenly becomes irresistible. The breeder eats the root, infecting himself or herself with the virus and triggering the transition.

After consuming Tree-of-Life, Pak breeder forms (such as humans) turn into the Pak Protector form, which involves reconfiguration of the anatomy, including the acquisition of a leathery armor or exoskeleton, strong enough to turn a knife. Joints swell until the creature becomes "a parody of the human form done in cantaloupes and coconuts." This expands the leverage available to muscles by increasing the moment arm: the protector can lift 10 times his or her own weight. Genitalia vanish, and a second heart appears in the groin. Teeth fall out, and lips fuse: the resulting structure is a sort of bony beak. All hair falls out and the head acquires a bony ridge to protect the cranium. The brain grows to an enormous size; the resulting mind, even starting from something as dumb as a chimp, becomes literally too brilliant for human comprehension.

Pak Protectors have an in-built need to look after (or protect, hence the name) their "parent" breeder species.