Bradley Cooper's girlfriend Zoe Saldana "keeps him on his toes".

The pair split in March after three months of dating, but recently decided to give things another go.

Bradley appreciates the way Zoe challenges him.

"Zoe always keeps him on his toes, and he loves that," a source told Star magazine.

Zoe's intelligence is incredibly attractive to Bradley.

Apparently the astute pair are very much in tune with each other mentally.

"He tried dating other girls but was bored to tears halfway through dinner," the source explained.

"Zoe's the only girl in Hollywood who shares his interests in politics and the economy. They're each other's intellectual equal."

Friends of the couple can't believe that they called their relationships quit earlier this year.

Many were confident about Bradley and Zoe's compatibility from the outset.

"It was crazy that they split up in the first place," the insider said.

"They made each other very happy."

Bradley and Zoe met while working on romantic drama The Words, which was released in September.