Cats once served as vital members of the US Navy. Old photos show these forgotten service felines.

Furry felines have been passengers aboard ships for thousands of years, traversing the world and spreading around the globe, according to Scot Christenson, director of communications for the US Naval Institute and author of "Cats in the Navy." 

Ancient Egyptian tomb paintings dating back 5,000 years show cats hunting birds from a boat, Christenson told Insider.

The Phoenicians and the Vikings, both master seafaring communities, later helped spread cats throughout Europe.

British, French, and Spanish explorers in the late fifteenth century carried cats to the Americas upon the discovery of the "new world" during the age of exploration.

The animals were so universally revered that local islanders visited by British trading ships would often sneak onboard the ships to try and steal a cat for themselves, Christenson said. 

Cats served a vital role: catching and killing rodents on ships.

During the Age of Sail, from the mid-16th to the mid-19th century, rats were known to leave an easily-ignitable trail of gunpowder aboard wooden ships as they scurried across the deck, posing a risk to the sailors on board, Christenson said, and cats could help stop the rodents in their tracks.

Even in the modern era, rats and mice remained an inherent danger to many ships, spreading disease, chewing through sails, and eating food supplies, according to Christenson

"But cats are effective predators," he told Insider. 

Cats were considered akin to crew members aboard the British Royal Navy, according to Christenson.

Some sailors would bond so closely with a cat that they would bring the animal home with them at the end of a voyage.

When the US Navy was founded in the 18th century, the military branch borrowed certain customs from its British predecessor, including a penchant for seafaring cats. 

"Cats have been on almost all ships," Christenson said of the world's navies. 

Long believed to be powerful and spiritual animals, cats served as omens and portents among early sailors, according to Christenson.

The Japanese believed cats could protect their ships from evil spirits, Christenson told Insider.

Sailors around the globe also believed that a cat's behavior could predict the outcome of a voyage.

If a cat jumped on board a ship prior to setting sail, seaman believed their vessel would be protected on its journey. But if a cat deserted a boat ahead of its departure, sailors thought themselves doomed, according to Christenson. 

The worst sign of all was the sight of two cats fighting on the pier ahead of a sailing, which some sailors interpreted as the devil and angel fighting for their souls, Christenson said. 

Sailors initially believed cats were in control of their fate, Christenson said. The animals were thought to have a gale inside their tail because they would begin shaking during storms.

Sailors interpreted this behavior as angry cats calling down foul weather.

Seamen later discovered that moody cats weren't in fact conjuring storms, instead, they were responding to the physical agitation they felt when the air pressure around them would drop.

Sailors started to watch cats' mannerisms to detect coming storms.

The animals are also sensitive to high-pitched whines, so cats helped Navy sailors detect coming air crafts during the World Wars, Christenson said. 

Feline members of the Royal Navy received a weekly allowance, which the sailors often paid themselves, contributing one shilling and sixpence to buy treats and milk for their cat friends, Christenson said.

The extra snacks helped make sure the cats were sustained on board even after they had caught all the rodents.

Most cats quickly learned to love their seafaring lives, Christenson said. Some learned to swim and dive for fish themselves, while others who never fully embraced the ocean, taught themselves to catch fish that jumped over the ship's bow.

The animals, which originally come from desert climates, can survive on very little water, obtaining most of their necessary moisture from their prey, according to Christenson. Cats have such adept internal filtration systems, that the animals could even drink a bit of salt water and be fine. 

During World War I, the US Navy scooped up hundreds of thousands of stray cats and assigned them to ships, Christenson said. 

The animals would typically stay on the same boat for long periods at a time, becoming territorial over their space. But every once in a while, a cat would jump ship if they determined they could get better food options on another boat, even if it was with another country's navy, according to Christenson.

The smartest cats claimed control of the ship's galley where they received extra treats and grew extra fat. Other felines opted to spend time in a boat's laundry room where there were plentiful soft and warm items on which to sleep.

The animals are known for having minds of their own, opting to remain loyal to territory, and often not to people.

"Cats are going to cat," Christenson said with a laugh.

A cat that chose to ditch its crew right before departure could be recaptured by a superstitious sailor, but would often run off again as soon as an opportunity arose. 

During prohibition, federal officials learned to look for cats scurrying away to hiding-holes on a ship, often inadvertently leading investigators to the ship's store of illegal booze, according to Christenson.

"Cats were a unifying friend to everybody on the ship," Christenson said.

Officers and enlisted men alike formed strong bonds with the animals, who offered them companionship during difficult journeys. 

Cats provided an emotional outlet for military men who could derive stress relief from a quick cuddle. 

Budget cuts after World War II dealt a death knell to Navy cats, Christenson said.

Advocates for the financial cuts ridiculed the Navy, accusing the military branch of complaining about a lack of funds while planning birthday parties for their cats.

The public relations aspect of the campaign embarrassed the Navy, even though more often than not, it was the sailors themselves paying for the upkeep of their feline friends.

But it was updated quarantine laws that ultimately led to the end of cats' seafaring days, according to Christenson.

For years, Navy cats were granted special permission to forgo most country's standard laws that required incoming private citizens to quarantine their accompanying cats and other pets for several months.

But as nations began cracking down on animal quarantine laws in the aftermath of World War II, ship captains faced serious repercussions if one of their boat's felines escaped and went exploring in a port city, Christenson said, leading to the demise of the practice altogether. 

Ships face hefty penalties for breaking the rules, according to Christenson.

But while most ships no longer carry furry seamen, anecdotal evidence suggests that the Russian navy may still employ cats on their boats, Christenson told Insider. 

Christenson spent years collecting stories of the cats who served in the Navy.

Of all the cases he found, his favorite anecdote is the tale of "Mis Hap."

Mis Hap was a tiny kitten found by a Marine during the Korea War, Christenson told Insider. The animal had just been orphaned, and her human rescuer named her Mis Hap because she had "been born in the wrong place at the wrong time."

The Marine was photographed feeding the cat with a medicine dropper in a heartrending photo that was picked up by dozens of newspapers around the globe, according to Christenson.

Prior to Mis Hap's discovery, the Marine in question was at risk of being court-martialed after submitting one of his own photographs of wounded Marines to a photo contest, flouting military censors that had banned the publication of such content at the time.

But the marine's newfound newspaper fame ultimately spared him from the charges and yielded hundreds of marriage proposals from women across the country who were moved by his tender care toward Mis Hap, according to Christenson.

Mis Map went on to become the mascot of headquarters in Korea.

Her Marine eventually brought her back to Chicago where she lived a long, happy life.