Wild Dogs's first ability will check life totals as your upkeep begins, before any player has the opportunity to take any actions. If no one player has more life than all other players at that time (i.e., if two or more players are tied for the most life), the ability won't trigger at all. If the ability does trigger, it will check again as it tries to resolve. If no player has more life than all others by that point, the ability won't have any effect. The player who had the most life as the ability triggered doesn't have to be the same player as the one who has the most life as the ability resolves.