The term vestigial is used to describe behaviors or organs no longer useful due to evolution. The most famous vestigial organ is the appendix, which appears to have no function in humans. During early embryological development, all humans have a vestigial tail. The tail can be seen via ultrasound between 31 and 35 days gestation. In most cases, the tail disappears when the spinal column develops, but a residual portion of the vestigial tail may be present at birth, in some infants. Here is a case report of a vestigial tail or caudal appendix in a newborn, and here is another case report from 2011 of surgical treatment.
All humans are created with a small tail that is later absorbed by the body and developed into the tailbone. In some rare cases, small amounts of tissue are left hanging on the tailbone area of the body. Most often, the small piece of skin contains no bones, but does contain nerves and blood vessels. In some rarer cases, there are up to five vertebrae in the small tail – resulting in a true human tail or vestigial tail.
The true human tail is not really a tail at all, according to experts. It is thought to be linked to spina bifida or a hiccup in the natural human development process. Typically, white blood cells degrade vertebrae six to 12 in the 8th week of gestation. The 4th and 5th vertebrae are shrunk during the process and eventually skin covers the new spinal cord. If white blood cells do not absorb the latter vertebrae 100%, a tail could be left.
The longest known tail is 13 inches long and belongs to a man in India.
There are no known risk factors of the vestigial tail. All infants develop similarly, independent of genetic conditions or disease, and the vestigial tail present in early development eventually ends up as the coccyx bone – the lowest part of the spinal column. Male children are twice as likely to be born with a vestigial tail as female children, according to a 1984 study in Human Pathology.
Other than a protrusion at the top of the buttocks, there are no symptoms of vestigial tail. In most cases no vertebrae are present in the tail, though nerves and blood flow are present. If the tail is not removed, it could interfere with sitting. The tail can be upo to 13 cm (about 5 inches) long and contract and move.
There are no known complications of the vestigial tail. Fewer than 50 cases of the condition have been reported since 1884. Some cases are associated with neural tube defects, which would account for complications, but not associated with the vestigial tail. In an extremely small number of cases, vertebrae have been found in the vestigial tail.
Testing and Diagnosis
Diagnosis of a vestigial tail is made visually. The tail is clearly visible via ultrasound, in extreme cases, and during the initial physical examination after birth.
In most cases the vestigial tail is removed soon after birth. Some parents choose to leave the tail in place, especially in cases where the tail appears as a small nub near the lower spine. Surgical removal of the vestigial tail causes no long-term side effects in most patients.
The vestigial tail does not impact lifespan, in most cases. The number of cases with vertebrae present in the tail is too small to determine prognosis.