Sloth
The sloth demonstrates remarkable staying power. Individual specimens can live up to forty years in the wild, spending approximately 90% of that time in various states of motionlessness. Their metabolic rate is so slow that a sloth can take up to a month to digest a single leaf. In evolutionary terms, sloths have remained virtually unchanged for sixty million years, suggesting that doing almost nothing is, paradoxically, an excellent long-term strategy.
Love
Love has proven itself irritatingly persistent throughout human history. Archaeological evidence suggests romantic attachment predates written language, with cave paintings depicting what anthropologists diplomatically describe as 'intimate scenes.' Love has survived the Black Death, two World Wars, and the invention of dating apps. Individual instances of love can last anywhere from fourteen seconds (approximately the duration of eye contact with an attractive stranger on the Tube) to several decades of matrimonial endurance.
VERDICT
Whilst the sloth's forty-year lifespan is commendable, love has been causing human beings to behave irrationally since before the first sloth ever contemplated climbing a tree. Love takes this criterion by approximately sixty million years of documented emotional turmoil.