For millennia, dogs have been our constant guardians, warning us when danger is afoot. However, canines aren't the only species that can act as sentinels. Check out other animals who have been used to deter thieves and protect something valuable.

Donkeys

Donkeys have a natural hatred of canids, and help protect sheep from predators like coyotes. Donkeys have a natural hatred of canids, and help protect sheep from predators like coyotes. (Photo: Lee Kay/Shutterstock)

Guard dogs aren't the only option for farmers. Guard donkeys can also be stationed with sheep to protect their woolly companions from harm.

Donkeys are an appealing option for ranchers because they don't require special feeding or care. They can be turned out with the sheep to graze the same pastures. These territorial equids will stand up to a threat like a coyote that comes prowling around the herd, and is likely to fend off the threat.

According to Modern Farmer, donkeys "are capable of dishing out crushing blows with both their front and hind legs as well as using their large teeth to bite raiding intruders. However, they cannot handle multiple canine attackers or larger predators like mountain lions, wild hogs or bears, and will rarely notify the farmer of any problems in the pasture — although their loud braying may indicate potential invasions."

While donkeys might not be the perfect solution to any danger befalling the flock, they have certainly earned a reputation as a great option for ranchers.

Dolphins

Dolphins have been used by the US Navy as underwater guards. Dolphins have been used by the U.S. Navy as underwater guards. (Photo: Tory Kallman/Shutterstock)

It's not just land-bound animals that can help us out. Smart and powerful, dolphins have been employed as guard animals under the sea.

Though controversial, the U.S. Navy Marine Mammal Program has trained dolphins for such tasks as protecting ships and patrolling harbors. The program has existed since the 1960s, but was kept secret for 30 years.

CNN reported in 2011:

"Dolphins most recently were deployed in the Iraq war, performing mine detection and clearance operations in the Persian Gulf to ensure safe passage for humanitarian ships delivering aid. Some of these Iraq war "veterans" are now back home, tasked with a new mission: guarding nuclear submarines in their homeports of Bremerton, Washington, and Groton, Connecticut. A key part of the training program is teaching these mammals how to intercept potentially hostile swimmers."

It isn't just dolphins who are part of the program either. The Marine Mammal Program also employs sea lions for similar tasks.

Llamas and alpacas

An alpaca seems an unlikely guard, but they're effective! An alpaca seems an unlikely guard, but they're effective! (Photo: fotobook/Shutterstock)

They might not look like the toughest of guard animals, but llamas and alpacas are actually pretty tough, especially when compared with the sheep they are enlisted to protect. Their territorial nature and fighting instincts are great deterrents for predators. After all, if you've ever been face to face with a tall and toothy llama, you'll know they can be pretty intimidating.

Kicking, spitting and screaming, they can chase off smaller predators like foxes, coyotes and weasels with ease. National Geographic reports, "Llamas react to canids threatening herds in a variety of ways, starting with a posture to alert others in the herd, then sounding a special alarm cry, and often running towards the threat, kicking and placing themselves between it and the herd. Dogs and coyotes have been injured and even killed by llamas. Farmers who pastured llamas with sheep discovered that fewer sheep were lost to coyotes."

Thanks to this tough, stubborn and fearless nature, llamas and their smaller alpaca cousins have become an increasingly popular option for ranches in the western United States.

Geese

Really, who doesn't have at least a small twinge of fear about geese? Really, who doesn't have at least a small twinge of fear about geese? (Photo: Modfos/Shutterstock)

If you've ever been chased by a goose at the park, it won't surprise you to learn that they've been used as guard animals throughout history.

Geese are credited with warning the Romans of a sneak attack by the Gauls. And much more recently, geese have been employed to stand guard at police stations in rural China.

Dr. Jacquie Jacob of the University of Kentucky writes, "Geese are able to distinguish regular everyday noises from other noises. As such, they are good as watch animals... During the Vietnam War, U.S. soldiers used flocks of geese to warn of enemy infiltration, with pens of geese encircling entire camps."

While they won't be able to fight off attackers of larger size (or sharper teeth), they can certainly scare off the more meek of intruders and are a highly effective warning system with their loud vocalizations.

Ostriches and emus

Wouldn't you run if you saw this coming at you? Wouldn't you run if you saw this coming at you? (Photo: Opalev Vyacheslav/Shutterstock)

Geese might not be able to fend off attackers, but an ostrich sure can! Ostriches can range from 6 to 9 feet in height, weigh between 140 to 320 pounds, and can run more than 40 miles per hour. They can kick like no one's business, too, and are willing to fight to defend themselves or their chicks. National Geographic notes, "An ostrich's powerful, long legs can cover 10 to 16 feet (3 to 5 meters) in a single stride. These legs can also be formidable weapons. Ostrich kicks can kill a human or a potential predator like a lion. Each two-toed foot has a long, sharp claw."

So if you make an ostrich mad, you better be prepared for the consequences. Their intimidation factor alone makes them helpful guard animals whether watching over smaller livestock or patrolling one's property.

From Softpedia: "In South Africa, ostriches are used for defending sheep and cattle herds... The same role is being held by the Australian ostrich, emu, in Australia. Their impressive bodies and attitude even makes thieves fear them. As an example of that, the owner of a deposit of old cars in Lambert, Mississippi, used an ostrich as guard, and the bird proved more effective than two Doberman dogs!"

Cobras

Snakes, especially cobras, are helpful theft deterrents. Snakes, especially cobras, are helpful theft deterrents. (Photo: Tlapy007/Shutterstock)

Snakes are scary to most people, especially the infamously deadly cobra. So it is no wonder that releasing a cobra to guard something valuable has been a strategy on more than one occasion.

In 1978, the Skansen Zoo in Stockholm decided to release a cobra to protect the zoo from an epidemic of animal thefts. Or at least, that's what the zoo told the press.

"We let the cobra loose among the cages, glass cases and fish tanks when we close at night. First thing in the morning, I put it back in its case... The king cobra measures more than 14 feet. If you get a bite in a sensitive place, like in a vein, you're dead within 15 minutes."

Needless to say, after this press release the thefts stopped.

Meanwhile, a cobra was also used to protect a 393 carat sapphire. In 1991, the New York Times reported:

"A poisonous cobra is guarding the world's largest sapphire at an international gem exhibition in the Sri Lankan capital, Colombo. "We thought this would be a better security system than anything else," a spokesman for the Sri Lanka Gem Corp. said. The snake, in a glass cage, is protecting two gems worth 45 million rupees ($1.1 million). The stones, which belong to the corporation and are not for sale, are a 393-carat star sapphire - the world's largest - and a 103-carat star sapphire, the spokesman said."

Alligators

Whether it's one alligator or a moat full, they're good at keeping people out. Whether it's one alligator or a moat full, they're good at keeping people out. (Photo: Heiko Kiera/Shutterstock)

Alligators are also effective guards if you want to intimidate potential intruders. After all, there's a reason why cartoons and campy shows feature a moat full of 'gators to keep people out of a stronghold. Interestingly, alligators seem to be a go-to guardian for drug dealers.

In 2011, a 4-foot long alligator named Wally was found guarding 2,200 marijuana plants worth $1.5 million. In 2013, police found 34 pounds of marijuana being guarded by a 5-foot-long (and very ill) dwarf caiman named Mr. Teeth in California. In 2016, two alligators were found guarding 500,000 euros worth of crystal meth, synthetic drugs, firearms and 300,000 euros in cash in Amsterdam. These are just a few of many instances in which the frightening reptiles have been used as sentries.

"Alligators have been found at drug busts in California, Ilinois, New York and Maryland," noted Huffington Post.

Screamers

The name says it all. Screamers make great alarm systems. The name says it all. Screamers make great alarm systems. (Photo: nwdph/Shutterstock)

If you need an alarm system, employing screamers seems like a good option.

Screamers are South American birds and they are made to be guardians. American Bird Conservancy notes, "Screamers are the 'guard birds' of their habitats; their trumpet-like calls can carry for several miles, warning other birds, such as Blue-throated Macaw, Orinoco Goose, and Streamer-tailed Tyrant, of approaching danger."

If they're this good at warning other birds of danger, then it's no surprise humans have used their watchful ways for our own purposes. The birds can be tamed easily enough to be used to warn farmers of predators such as raptors approaching their flocks of poultry.

They can also be quite aggressive and they're armed. The San Diego Zoo points out, "A sharp, keratinous spur on each wing helps screamers protect themselves by wing slapping and striking with these spurs."

Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.