This may result in inability to get penile erections and erectile dysfunction. Prostate stimulation can produce more intense orgasms than penile stimulation. Nocturnal penile tumescence is believed to contribute to penile health. It has been theorized that pearly penile papules stimulate the female vagina during sexual intercourse. The cause of nocturnal penile tumescence is not known with certainty. The urogenital folds form the labia minora in females, or penile shaft in males. An increase in penile curvature can be caused by Peyronie's disease. Papillomaviruses have been associated with the development of cervical cancer, penile cancer and oral cancers. Retractable nipples, internal testicles, and an internal penile sheath provide further streamlining. Three cases of penile cancer have occurred on the scar. HPV is associated with approximately 50% of penile cancers. It is important to exclude other causes of similar symptoms such as penile cancer. Around 95% of penile cancers are squamous cell carcinomas. Aardwolves also have scent glands on the forefoot and penile pad. Variations in the positions may vary the angle and depth of penile penetration.