Success in nature is measured in procreation, to sustain the species' population(s), hence parenthood is the key goal in life. Often social life, like a herd, helps protecting the vulnerable young, yet its own parents (mainly mothers) go the extra mile, like shielding a buffalo calf from a pack of wolves. Seniors help is often vital to survive on migration, especially the first year, as zebra foals needing to cross crocodile-infested rivers on the African savannah. Learning social skills can also be vital, as with bonobo apes. Fathers may fight off rivals for mating rites and to prevent a triumphant rival killing off their offspring.