Rodents have lived on the planet for at least 56 million years, so we thought we would take a look at humans and rats shared history, and the role rodents have played in shaping aspects of society.
In this article we’ll look at:
- Rodents vs “Rodentia”
- Our shared history: rodents become pests
- The other side: rodents as pets
- An aside about cats and rats ….
- Rodents and medical testing
- Rodents doing good
Rodents are ubiquitous. According to the Encyclopaedia Britannica, they are indigenous everywhere except Antarctica, New Zealand and a few Arctic islands, although some species have been introduced by humans to even those far-flung places.
And they’ve been around for a long time. Rodents have lived on the planet for at least 56 million years: the oldest rodent fossil that has been found by researchers (in the Ucayali River in Peru) was dated as at least 41 million years old. Modern humans, by contrast, have been around for less than one million years.
But before I go any further, I should probably clarify my first statement that rodents are found everywhere, because it refers to the entire order Rodentia and not just to rats and mice. Rodentia is the largest group of mammals on the planet (making up half of Mammalia’s 4660 species) and encompassing 27 separate families.
“True” rats and mice belong to the family Muridae, but the order Rodentia also includes diverse groups such as porcupines, beavers, squirrels, marmots, pocket gophers, and chinchillas (some of which are definitely not native to South Africa).
So for the purposes of this piece, when I refer to “rodents” I’m using the term loosely (lower case as opposed to the upper case for Rodentia) and really only referring to the family Muridae - the true rats and mice.
A long shared history:
Whilst rats and mice have been around for far longer than we have, the consequences of our interaction have been significant. As humans became less migratory and more sedentary, we needed to keep stores of grain in order to survive without having to move from place to place. Rodents were attracted to these stores, and humans became a reliable source of food, water and shelter (the three things that all pests need to flourish) for the rodent species - such as the house mouse - that could adapt to man-made habitats.
Archaeological research in the eastern Mediterranean has borne this out: as the hunter-gatherers of that region started to become more sedentary, likely as a result of favourable climate conditions, the team found a rise in the amount of house mouse molar fossils in and around their human settlements.
However, when the climate shifted again and the region became cold and dry, the hunter-gatherers reverted to their migratory way of life. During these spells, researchers found that the more independent, short-tailed mice (considered to be more wild and less tolerant of humans) become dominant once again.
The team found a similar pattern of mouse interaction in modern hunter gatherer settlements. Research done with the Masai of Kenya showed once again that “the house mouse out-competed another mouse species by developing a one-sided relationship with humans”.
The impact of this shared history:
The impact that adaptable rodents had on the human population ranges from inconvenient to deadly, and quickly had them classified as pests. Wikipedia defines pests as “any animal or plant detrimental to humans or human concerns. The term is particularly used for creatures that damage crops, livestock and forestry, or cause a nuisance to people, especially in their homes”.
The entire order Rodentia is characterised by upper and lower pairs of ever-growing rootless incisor teeth. And it’s these teeth that are one of the reasons (there are a few) why rats especially have become such a problem for humans. They gnaw constantly to wear their teeth down, and this gnawing can cause structural damage to buildings. They have been known to chew wiring which causes short circuits or even fires. They also chew to create shredded up nesting material, and as upholstery, insulation, and newspapers are all fair game, this too can cause significant damage.
House mice and black and brown rats are considered to be “commensal”. Commensal is defined as “sharing one's table” - or living off human food. Unfortunately, commensal rodents are hosts to a variety of pathogens that can infect humans, the most deadly of which is plague - caused by bites from the fleas that live on rats. You can read more about the history of the plague, and the role of pests in pandemics, in our previous post. There are a host of other diseases which rodents can also transmit to humans, including leptospirosis and salmonellosis. Read more about how these diseases are spread in this blog post.
Commensal rodents also consume and contaminate human food and animal feed. In India, it has been reported that rats, mice and gerbils steal almost a quarter of all stored and standing grain. Because rodents tend to nibble on many foods, they destroy considerably more food than they consume. In fields, rodents can dig up and consume newly planted seeds. They also damage crops before harvest; their droppings and urine can contaminate stored products, their burrowing causes leaks and gnawing - as mentioned above - caused all sorts of damage.
The other side: pets, research and heroes
But this shared history is not all bad. Rodents have also shown to make extremely good pets, and rats especially are extremely intelligent. This makes them easy to tame and train. The domestication of rodents, however, is a relatively recent development, only becoming popular after the industrial revolution. Called “fancy rats”, the domesticated brown rat was a popular pet in Victorian England during the 19th century, being selectively bred for unusual colouration or temperament. Fancy mice (a domesticated form of the house mouse) were popular in China and Japan as early as the 12th century.
Prior to that, with Western society tending to be more agrarian than today, rodents were seen as vermin for the reasons listed above: they were carriers for disease and a threat to crops. Animals that hunted such pests, such as terriers and cats, were therefore prized.
An aside about cats and rats ….
Cats are actually not as good at catching rats as history would have us believe. An experiment conducted by Fordham University ecologist Michael Parsons to establish whether feral cats would have an impact on the rodent population in public buildings showed that whilst there were a couple of encounters in which the cats actively chased the rats, these were rare. In the hundreds of videos the team recorded, there were only three kills and 20 stalking events. The cats had no real effect on the rat population.
This may seem surprising, considering the popular view of cats as the enemy of rodents, but not to Gregory Glass, who has studied feline and rodent interactions for decades. Professor Glass says “once that rat hits puberty, they are way too big and nasty for the cat to deal with. You can watch a lot of cats and rats accommodating one another, easing by one another, eating out of the same trash bag.”
Rodents in medical research:
A piece on rodents would not be complete without a mention of the use of rats and mice in medical research. Ethical considerations aside, you can’t deny the huge role that rodents have played in the development of medical science - from formulating new cancer drugs to testing dietary supplements. In fact, 95 percent of all lab animals are mice and rats, according to the Foundation for Biomedical Research (FBR).
There are a number of reasons that researchers use rodents for medical testing, including that they are docile, adaptable, and breed quickly, but the most interesting (for me, anyway) is that their genetic, biological and behavior characteristics closely resemble those of humans, and many symptoms of human conditions can be replicated in mice and rats.
"Rats and mice are mammals that share many processes with humans and are appropriate for use to answer many research questions," said Jenny Haliski, a representative for the National Institutes of Health (NIH) Office of Laboratory Animal Welfare.
Rats doing good:
I mentioned before that rats are intelligent and easy to train, which is one of the reasons they make good pets. But would you believe me if I told you that there’s an NPO that puts this intelligence, trainability and keen sense of smell to use in saving lives?
A Belgian NPO called APOPO has been doing just this for the last 20 years. Started by Bart Weetjens, a pet rat owner himself, the organisation trains African giant pouched rats to save lives by sniffing out landmines and TB. APOPO’s HeroRATs are too light to detonate landmines and are very quick at finding them by smell, making them a perfect “tool” for speeding up detection and clearance. They ignore scrap metal and only detect explosive scent, making them much faster at finding landmines than metal detectors.
They can also sniff out disease. Tuberculosis is the world’s deadliest infectious disease, with approximately 10 million new global cases per year and around 1.6 million deaths annually. In most sub-Saharan African countries only about half of the patients with active tuberculosis are diagnosed. APOPO's Detection Rats can quickly test human sputum samples for TB, doing in a couple of hours what would take 4 days in a lab. Any suspect samples are then re-checked using WHO endorsed confirmation tests.
So do rodents deserve their bad press? The answer would appear to be both yes and no. Regardless of how you feel about them, our relationship with rodents is as complex as it is long. If you liked this article, and are interested in learning more about how pest control influences our lives, subscribe to our blog.