at the fusion of the femoral veins. The arms lengthen. Fingernails turn into retractable claws. Teeth fall out and the lips and gums fuse, the mouth forming a horny beak (flat in protectors transformed from humans, non-flat in protectors transformed from Pak). All the breeder's hair falls out and the head acquires a bony ridge to protect the newly expanded cranium. The expanded skull allows the brain to grow to an enormous size; the resulting mind, even starting from something as "unintelligent" as a chimpanzee, becomes far more intelligent than a typical human. Pak Protectors also acquire an extended lifespan, and can live tens of thousands of Earth years (a common element in Niven's stories). Once the transformation is complete, a Pak Protector must periodically consume more Tree-of-Life root to maintain the virus in its body. Without the virus, a Protector will weaken and die as its DNA is degraded; the virus supplies replacement DNA. The Tree-of-Life crop on Earth failed due to there being insufficient thallium oxide (which particular oxide is never explained) in the Earth's soil; the plants grew but did not support the virus. As a result, the Protectors that led the colony to Earth died of starvation when their store of roots ran out. Protector behavior Pak Protectors have an innate need to protect close relatives. When a Pak Protector has no bloodline to protect, it usually stops eating and starves, though some childless Protectors can adopt the entire Pak race as their family. Niven states that Protectors on the crowded Pak homeworld would constantly war against each other to gain advantage for their family and that alliances would last only until one ally sees advantage in betrayal. Human Protectors, and those on the Ringworld, seem to be less warlike and better able to work for the betterment of the entire species (or all of the Ringworld hominids) rather than just their bloodline, though this may simply be a function of how few Protectors with families encounter each other. Protectors typically die from starvation (from lack of will to live, e.g. if the Protector's bloodline has died out) or violence. Although Protectors have a vastly extended lifetime, it is not infinite; during the original half-million year journey to Earth some Protector colonists did die from old age. Because of their enormous intelligence and instinctive need to protect their family (or their species, etc.), Protectors are efficient,