Britney Spears shaved her head because she was convinced her hair would be tested for drugs, according to a friend. The singer stunned fans in January (07) when she cut off her own hair in a Los Angeles salon in front of paparazzi photographers. She has never explained the gesture - but a friend claims she was worried she would be drug tested as part of her custody battle with ex-husband Kevin Federline over their two children. The friend tells British newspaper The Sun, "Kevin taunted Britney that even his smoking marijuana near her would leave traces that could be picked up by drug testing. "He threatened to use that against her in a custody battle." The pair are still battling over custody of Sean Preston, 23 months and 11-month-old Jayden James.