The D.C. Zoo Has Its  Own Police Force—So Does the Bureau of Engraving and Printing.

“Folks,” the officer warned, gesturing to a painted sidewalk, “you might want to stay clear of the gray concrete.”

The subject of interest—a sometimes moody resident known as Bonnie—squatted. A long, steady stream sprayed onto the sidewalk 50 feet below. The crowd squealed upon realizing that Bonnie, an orangutan at the National Zoo, was peeing.

“That looked intentional,” the patrolman said. Could she be arrested? “No statute for that one,” he deadpanned.

It was a typical summer day for Sgt. Gaskins of the National Zoological Park Police, a 123-year-old law-enforcement agency that is a rare breed: a fully armed police department run by a zoo, complete with a fleet of squad cars, an armory and a jail cell.

Wearing a SIG Sauer .357 caliber sidearm and a badge, Sgt. Gaskins is a sworn peace officer with the full arrest and use-of-force powers of most municipal police officers. He is one of thousands of police officers in Washington assigned to as many as three dozen police departments that patrol the nation’s capital, serving and protecting constituents of all stripes, from cabinet secretaries to lawmakers to justices.

This being Washington, law-enforcement agencies have blossomed alongside the growth of government, with many agencies, from the Bureau of Engraving and Printing to the National Zoo, embracing their police squads as sources of pride, prestige and protection.

“We’re knee-deep in police,” said Chuck Wexler, executive director of the Police Executive Research Forum, a think tank in Washington. “Only here in the capital of the free world do you find local police and federal police tripping over each other.”

The plethora of police in the capital is rooted in history, the U.S. Constitution’s separation of powers and the establishment of the District of Columbia as the federal city, which is neither a state nor a federal entity. Neither is the zoo, a part of the Smithsonian Institution, which is a federal trust instrumentality created by Congress but not a part of the legislative branch. In other words, the National Zoo is neither fish nor fowl.

The Federal Reserve Board is also a different animal, a creation of Congress that is independent. The 2001 Patriot Act gave the Fed power to create its own police, which also patrols the 12 regional banks of the Fed.

Some police even have their own police. The nation’s premier enforcement agency, the Federal Bureau of Investigation, operates the FBI Police as a separate department. The Secret Service runs a uniformed Secret Service Police.

In 1791, President George Washington created Park Watchmen, now called the U.S. Park Police. In 1861, President Abraham Lincoln formed the Metropolitan Police Department to keep order as the Civil War began.

MPD is the main force handling municipal policing, major crime and protests, and plays a key role in motorcades of presidents and heads of state.

“When I first came, it was confusing,” said Charles Ramsey, D.C. police chief from 1998 to 2006. “It seemed like each agency wanted to have their own separate department, which doesn’t really make sense to me.”

There was a running joke, Mr. Ramsey said, that if a man got shot in front of the White House “and staggered and fell in the street, MPD would investigate. If he fell on the sidewalk, U.S. Park Police would investigate, and on the grass, the Secret Service would.”

It’s not a joke at Union Station. Amtrak police guard the inside of the train depot, MPD watches the front drive, and the U.S. Park Police patrol the station’s grassy plaza.

D.C. police handle most major crimes. The MPD has agreements with the Zoo Police and many of the 30 or so other departments in D.C. The MPD will determine who will take the lead in the investigations, the agreements state.

In 1948, the District’s top attorney tried to rein in all the agencies, ruling that special police departments were unauthorized, according to a history by Smithsonian researchers. “This created quite a stir at federal facilities that had commissioned Special Officers,” the history says. The Supreme Court, the Smithsonian and the Library of Congress quickly got acts of Congress to authorize their police.

Once created, police departments rarely shut down, although that happened in 2003 when Congress directed the Library of Congress Police to rejoin the U.S. Capitol Police and it folded in 2009 after 59 years.

National Zoo Police Chief Tim Kildea was hired about eight months ago after military service and 17 years in the Department of Veterans Affairs Police, yet another force in D.C. and nationwide. His department has “a long, tortured history,” Chief Kildea said. “We’re very low on the federal totem pole, and that doesn’t help with recruiting.”

The Wall Street Journal requested all National Zoo Police incident reports for a week in May. Lost children, overheating, falls, seizures and calls for medical aid are among common incidents.

Near Lemur Island, a boy “face planted,” and was treated at the zoo’s medical station.

Outside the Reptile Discovery Center, a woman got her head wedged in a bench. “She was feeling dizzy and overheated,” the report said. “She laid down to rest on the bench. She woke up a few minutes later with her head stuck.” D.C. firefighters sawed off the armrest to free her.

One police report detailed the escape and capture of a white-naped crane at the National Zoo’s research campus in Front Royal, Va.

Officers are called for the rare jailbreaks, Sgt. Gaskins said, but zookeepers do the heavy lifting to corral fugitives. “Usually,” he says, “once we’re needed, it is not about recapture.”

Sgt. Gaskins has never fired his sidearm in the line of duty, he said, and he hopes he never has to.

Night patrol is different. “It feels like you’re the one who’s being watched,” he said. Cheetahs race alongside squad cars. One gorilla bangs a window twice when officers lock up.

On a recent day patrol in the Small Mammal House, there’s a loud screech. Sgt. Gaskins stops. A male red ruffed lemur, a critically endangered animal native to Madagascar, strides across a branch and embraces a female red ruffed lemur.

“Uh oh,” Sgt. Gaskins says. “They need some privacy.” He walks on. Zoo-goers gape.

Before joining the zoo police, Sgt. Gaskins worked for nearly 17 years for the Smithsonian Office of Protective Services, a separate police agency. One assignment was at the National Museum of Natural History, which features gems, dinosaur skeletons and stuffed animals.

“At Natural History, I was watching the animals that were dead, and here we watch the animals that are alive,” Sgt. Gaskins said. “Here, it is completely opposite. But I love it.”

James V. Grimaldi is a Pulitzer Prize-winning senior writer for The Wall Street Journal based in Washington, where he has written about money, politics, lobbying and campaigns.