In the past, there have been numerous instances of scientific discoveries being made independently by two research groups from different countries. For example, in the 18th century, two scientists—Joseph Priestley from England and Carl Wilhelm Scheele from Sweden—simultaneously discovered oxygen.
Now, in a relatively rare academic event, a new piece of knowledge has been uncovered almost simultaneously by two independent research groups. Interestingly, these groups were working on opposite sides of the Earth and were unaware of each other’s progress.
The mystery in question is about the orange fur of cats: Why are nearly all orange cats male, while calico cats are almost exclusively female? The relationship between cat fur color and sex has been an enigma for 10,000 years since humans first domesticated these animals. Sixty years ago, scientists believed they had an answer.
However, it turned out to be only half of the story. Now, two research teams in the United States and Japan have independently worked to solve this mystery completely.
The teams have jointly published their findings on bioRxiv, a preprint platform for scientific research. Interestingly, the Japanese team uploaded their work a day earlier, perhaps simply because the sun rises earlier in Japan than in the United States.
This event underscores the fierce competition among research groups worldwide.
From the Dawn of Agriculture
Around 10,000 years ago, during the dawn of agriculture, our Homo sapiens ancestors began cultivating crops in the region of Mesopotamia, situated at the intersection of Asia and Europe. It was there that a scenario reminiscent of Tom and Jerry unfolded.
A mouse fleeing from a wild cat accidentally entered one of humanity’s first grain storage facilities, which had been constructed after humans transitioned from hunting and gathering to farming and storing food.
The mouse, having evaded the cat, returned to its nest and invited its fellow mice to attack the grain store—a warm place where they could feast without fear of hunger or cold.
The invasion of the mice frustrated humans, who found the small, nimble creatures impossible to catch. Despite their efforts to secure the granaries, the mice always found a way in. Reluctantly, humans had to share part of their hard-earned harvest with these unwelcome guests.
That changed when wild cats began roaming human settlements.
Curious about the furry creatures meowing around their homes, humans realized the cats were there to hunt mice. Thrilled, they welcomed the cats as protectors of their granaries. From then on, humans revered cats, even worshiping them in temples.
During migrations, intercontinental moves, or seafaring expeditions to conquer distant lands, humans always brought cats along to safeguard their food stores.
Over millennia, cats transformed from tools into companions, symbols of good fortune, and, eventually, “masters” of their human companions.
Cat Breeding and the Calico Mystery
As cats traveled with humans across the globe, they didn’t just explore new lands and feast on birds and mice—they also encountered local cat populations. These meetings often resulted in fleeting romantic encounters.
For instance, an “playful” orange-coated Egyptian tomcat might travel on a trading vessel to Mediterranean port cities, where it would mate with local black-coated female cats.
A few years later, as the ship retraced its route, sailors noticed that the colors of the port city cats had changed. Rarely, a male calico cat would appear, or a female with entirely orange fur. The probability of such anomalies is about 1 in 3,000.
“It’s truly a genetic mystery, a real puzzle,” said Professor Greg Barsh, a geneticist from Stanford University. In the 20th century, part of the mystery was explained when scientists discovered that the X and Y chromosomes determine sex.
A gene linked to fur color is found on the X chromosome. Male cats, with only one X chromosome (XY), can have only a single fur color. Female cats, with two X chromosomes (XX), can have a mix of black and orange fur. When combined with the basic white fur found in both black and orange cats, calico females are born.
The Gene Behind the Mystery
But which specific gene on the X chromosome determines fur color?
This was the missing piece of the puzzle, and for 60 years, scientists couldn’t pinpoint the answer. The difficulty lay in the complexity of the X chromosome, which contains 155 million base pairs and over 1,000 genes.
In a recent study published on bioRxiv, Professor Barsh and his colleagues at Stanford announced they had identified the gene responsible for orange fur in male cats and calico fur in females.
Barsh collected eight feline embryos, including four with orange fur, from neutering clinics. By isolating RNA from their skin cells, he discovered that the gene Arhgap36 was highly expressed in orange-furred cats—13 times more than in other cats.
This gene, located on the X chromosome, appeared to be the key factor. To test their hypothesis, the team sequenced the genomes of 188 cats, including 145 orange cats, six calico/tortoiseshell cats, and 37 monochrome cats of non-orange colors.
The results confirmed that orange-furred and calico cats carried a mutated version of the Arhgap36 gene, involving a deletion of about 5,000 base pairs.
“Overall, these observations provide strong genetic and genomic evidence that the 5 kb deletion causes the sex-linked orange color in cats,” said Professor Barsh.
A Race to Publish
Interestingly, just one day earlier, an independent research team from Kyushu University in Japan had published a similar discovery on bioRxiv. They also identified the Arhgap36 gene as the determinant of orange fur.
Upon learning about each other’s work, the teams may have rushed to publish their findings to claim credit for solving the mystery of feline fur color.
This phenomenon isn’t new. In the past, discoveries like oxygen and Pluto were made independently by multiple researchers.
The advantage of such coincidences is that they eliminate the need for additional verification, one of the fundamental principles of science. Knowledge can thus be established immediately.
Just as the existence of oxygen and Pluto is now a certainty, we can confidently say that the orange fur of cats originates from the Arhgap36 gene on their X chromosome.
The mystery has been fully unraveled.
