the ability of south america's xenarthrans to compete effectively against the northerners represents a special case. the explanation for their success lies in part in their defence against predation. this was based on body armour and/or formidable claws. the xenarthrans did not need to be fleet-footed or quick-witted to survive. such a strategy may have been forced on them by their low metabolic rate (the lowest among the therians). their low metabolic rate in turn allowed them to subsist on less abundant and/or less nutritious food sources. unfortunately, the defensive adaptations of the large xenarthrans would have been useless against humans armed with spears and other projectiles.