No, you don't necessarily need to use weights to improve the shape of your butt. Bodyweight exercises can also be very effective in building muscle and improving the appearance of your buttocks. Exercises like squats, lunges, glute bridges, and step-ups can be done without any equipment. To increase the challenge, you can use your body weight, focus on higher repetitions, and perform the exercises slowly to increase tension on the muscles. Incorporating variations and progressively challenging your muscles by changing the routine can also help. However, adding weights over time can further enhance muscle growth and definition.