Male or female is determined by the strings. Male tanpuras are set up with light strings, generally around 9 gauge to 15 gauge. Male tanpuras are set up with heavier strings. The suitablility of the tanpura is determined by the player to be accompanied. At one of the concerts I attended in Delhi last winter the female vocalist had two tanpura players that used enormous male-voiced tanpuras and just to be safe, she had a Raagini Pro too. The sarangi player Ram Narayan is generally accompanied by someone on a small female-voiced travel tanpura.