Strange: Find out the cause of the bird's loss ...

Researchers at the University of Florida, USA have found the reason why more than 10,000 birds lost their penis.

Researchers have examined the embryonic development of penile soil birds, such as ducks and birds without chickens like chickens.

Penis of chickens (red) before atrophy.

They found that these birds have a normal penile development process as a fetus. However, at the end of the process, the gene named Bmp4 develops, causing the species' penises to shrink.

For birds with a penis, this gene does not work, causing their penis to have a full development opportunity. It is thought that in waterbirds, the penis still exists to keep sperm from swept away by water.

'Genitalia is one of the fastest-growing organs in animals, from molluscs to mammals,' said Martin Cohn of the Hughes Institute of Medicine and the University of Florida, USA.

Although there is no penis, males can still mate with females through insemination. Many scientists believe that it is possible to lose penis to help them control their reproductive processes effectively.