A University of Missouri study provides evidence that a man's body recognizes and tags defective sperm cells while they undergo maturation in the epididymis, a sperm storage gland attached to the testis.

The protein ubiquitin marks abnormal sperm cells, including cells that have two heads, two tails or are otherwise misshaped, which tells the body which cells need to be broken back down into amino acids.

The study, published in the Journal of Cellular Physiology, provides evidence that there is an active removal process or marking of defective sperm.

"In many cases, the cells that are tagged with ubiquitin are obviously abnormal with two tails or two heads, but many of them look like they don't have defects," researcher Peter Sutovsky said in a statement. "Oftentimes, these cells may look normal but lack proteins that are important to fertility."