Rengar's abilities don't have any cost related to them and their use is only limited by cooldowns. Instead he uses Ferocity as his secondary bar.
Rengar builds 1 point of ferocity with each ability he uses on enemies (including structures and neutral monsters). Upon reaching 5 ferocity Rengar gains the ability to cast an alternate set of abilities, which are empowered versions of his basic abilities. Using an empowered ability consumes all 5 ferocity. The two ability-sets do not share cooldowns, and his basic abilities will continue to count down while his empowered abilities are available. His empowered abilities have no cooldown and can be used any time 5 ferocity is available.
While in brush or stealthed, Rengar gains bonus attack range and his attacks cause him to leap at his target. This bonus lasts for ~0.5 second upon exiting brush or stealth. This will happen regardless of whether or not his target has sight of him, and has no internal cooldown.
Additionally, Rengar builds 1 point of Ferocity with each ability he uses on enemies. When reaching 5 points of Ferocity, Rengar's next ability becomes empowered, granting it a bonus effect.
Active: Rengar activates his predatory instincts, stealthing himself and revealing all enemy Champions in a large radius around him. He gains movement speed and generates 1 Ferocity per 0.75s while stealthed. Lasts 5 seconds or until he uses an ability.