Jacob Gyllenhaal is an American actor. The son of director Stephen Gyllenhaal and screenwriter Naomi Foner, Gyllenhaal began acting at age ten. He has appeared in diverse roles since his first lead role in 1999's October Sky, followed by the 2001 indie cult hit Donnie Darko, in which he played a psychologically troubled teen and onscreen brother to his real-life sister, actress Maggie Gyllenhaal. In the 2004 blockbuster The Day After Tomorrow he portrayed a student caught in a cataclysmic global cooling event, alongside Dennis Quaid as his father. He then played against type as a frustrated Marine in Jarhead (2005). The same year, he won critical acclaim as Jack Twist in the film Brokeback Mountain opposite Heath Ledger.

Gyllenhaal has taken an activist role in supporting various political and social causes. He appeared in Rock the Vote advertising, campaigned for the Democratic Party in the 2004 election, and promoted environmental causes and the American Civil Liberties Union.

Born December 19, 1980, his primary personality traits are represented by the Chinese Zodiac elemental animal sign, the Metal Monkey. Of all the signs the Monkey is most adept at bringing joy into their lives. He thrives in social settings and is not beyond playing little games with the rest of us. He also is often witty and a good conversationalist, the person we truly want to be around at any party or social function. He knows how to turn on the charm when he wants to, and that is most of the time. True, he often does it just to get his way, but when he adds in his sincerity and the affection he give us, we cannot resist him. The Metal Monkey is the strongest, most rigid of the bunch. He has a sharp edge compared to the other, more playful Monkeys. He prefers to work independently or be a small business owner rather than a cog in the corporate wheel if he can help it.