If you had to list the world's most vilified animals, rats would definitely make the cut – especially giant ones.

They are most commonly described as harbingers of disease that scuttle in swarms around our cities under cover of darkness.

The best known species are the brown rat (Rattus norvegicus) and the slightly smaller black rat (Rattus rattus), both of which live on every continent bar Antarctica.

Our worst nightmares seem to be realised every few months when frightening photos of supersized rats are splashed across the media. But in truth, cities are not the best places to look for the world's biggest rats.

"There is absolutely no evidence that brown rats found in the UK are growing bigger," explains Dougie Clarke from the University of Huddersfield, UK.

The camera trickery was undone by sceptics who measured the distances involved

Clarke is a leading expert on super rats. That does not mean giant ones big enough to train teenage mutant turtles to become ninjas, but rats that have become resistant to the poisons used by pest controllers. 

"In our study that surveyed over a hundred brown rats from all over the UK, the maximum body length of fully-grown rodenticide 'super rats' was 26 cm (10.3 inches) with a tail length of 25 cm (9.8 inches)," says Clarke. "So they are no different than what is expected for brown rats."

"Any reports in the media that they are growing bigger are either photo trickery by holding the animal at arm's length, or another species of escaped pet rat," he adds.

For example, the behemoth discovered dead next to a playground in Hackney, London in March 2016 was said to be as big as the children that played there. Thanks to forced perspective, where subjects closer to the camera appear larger than those further away, the rat did look enormous.

This bulkier rat can weigh as much as 8.8 lb (4kg), as much as a domestic cat

But the camera trickery was undone by sceptics who measured the distances involved and found the beast was only the standard length.

Still, if works of fiction are anything to go by, our fascination with giant rats runs deep. Even though it was only mentioned in passing in a single Sherlock Holmes story, the "giant rat of Sumatra" has become legendary.

In reality, there are two species of rat that could have inspired the reference.

The large Sumatran bamboo rat (Rhizomys sumatrensis) measures up to 19.7 inches (50cm) from nose to tail tip.

While this is similar to the brown rat, the large Sumatran bamboo rat's tail only accounts for around 4.7 inches (12cm) of its total length. So this bulkier rat can weigh as much as 8.8 lb (4kg), as much as a domestic cat, according to a review of the species published in 1936.

Gambian pouched rats that escape from private collections have caused concern in the Florida Keys

The other candidate mooted by some is the mountain giant Sunda rat (Sundamys infraluteus), described as a large, omnivorous species that lives in mountain forests.

"Sundamys infraluteus reaches 60 cm (23.6 inches), but is barely heavier than half a kilogramme (1.1 lb) because it has a different body morphology," explains Raquel López Antoñanzas of the University of Bristol, UK, who studies rodent evolution.

Any discussion of rodents of unusual size will include South America's capybaras, but they are more closely related to guinea pigs than rats. To avoid such red herrings, the family to focus on is the Muridae, also known as the Old World mice and rats.

One of the longest of these is the Gambian pouched rat, which can measure nearly 3 feet (90cm) from nose to tail end and weigh 3 lb (1.4 kg). Its size is one of the characteristics that has made it a popular pet, three times heavier than the standard fancy rats – which are actually domesticated brown rats.

They aren't heavy enough to trigger the mines but they are quite sizeable and easy to handle

As well as fuelling myths about giant sewer rats, Gambian pouched rats that escape from private collections have caused concern in the Florida Keys, where they have been declared an invasive species. They were also linked to an outbreak of monkeypox in the US in 2003.

However, the pouched rats are winning admirers back home in Africa. The non-governmental organisation Apopo pioneers work that makes the most of their intelligence and advanced sense of smell. Known as hero rats, the specially-trained rodents can detect landmines and even tuberculosis.

"Although most rats could qualify in terms of sensitivity and intelligence, we selected the African giant pouched rat because of its long lifespan and adaptation to the conditions in Africa," says Abdulllah Mchomvu, training manager for the mine-detecting rats in Tanzania.

"They have a very sensitive sense of smell and can be trained to detect specific target scents. As for the landmine detection rats I work with, they aren't heavy enough to trigger the mines but they are quite sizeable and easy to handle."

To find any species that might exceed the Gambian pouched rat's measurements, we need to look in Asia: specifically, on islands where unique ecological balances allow for extraordinary size variations.

The giants found on the island of East Timor, Indonesia were said to be the size of dogs

In the Philippines, there lives a group of murids known as giant cloud rats, thanks to their tree-dwelling lifestyle. Of these, the Northern Luzon giant cloud rat (Phloeomys pallidus)is the biggest at up to 2.5 feet (75cm) long, weighing as much as 5.7 lb (2.6kg).

Similarly large are the giant woolly rats of New Guinea, Mallomys. One species, discovered in an extinct volcano in 2009 and informally known as the Bosavi woolly rat, is 2.7 feet (82cm) long and weighs 3.3 lb (1.5kg).

Since discovering this giant, Kristofer M. Helgen of the Smithsonian Institution has been busy reviewing the giant woolly rats. "The biggest species is probably Mallomys gunung, which lives at very high elevations in the mountains of western New Guinea, and weighs 2 kilos (4.4 lbs) or more," he says.

The most recent discovery of giant island rats is the most impressive yet for size.

In 2015, Julien Louys and colleagues at the Australian National University made the biggest-ever find of the biggest-ever rats. The giants found on the island of East Timor, Indonesia were said to be the size of dogs. But these were not scurrying live rodents: they were fossilised remains.

Many of our remaining giant rats are vulnerable to the threat of habitat degradation

Researchers identified seven species of extinct giant rat, the smallest weighing an estimated 3.3 lb (1.5kg) and the largest 11 lbs (5kg); as much as a miniature dachshund.

The bones were discovered by archaeologists researching human activity on the islands. The early inhabitants obviously had a taste for rat, as the bones are charred and chewed. But according to Louys, even with hunting the rats lived alongside humans for 40,000 years.

He links the rats' eventual extinction with the arrival of metal tools, suggesting their forest homes were felled by human inhabitants exploiting the region's famous sandalwood timbers.

If that is true, it is a cautionary tale. Many of our remaining giant rats are vulnerable to the threat of habitat degradation.

Instead of our city dwellers being menaced by giant rats, it is human development that poses a risk to these living legends.