Rats! A new appreciation.

It is well known that humans often project some of our qualities – the best and the worst – onto other creatures. (Of course, we often do that to groups of people as well, people who differ in skin color, religious beliefs, national origins, or other properties they are born with or into, but let us not get into that but stick with animals here.) This has happened in many different cultures over many thousands of years. For instance, lions and eagles are noble creatures, e.g. lions as “the king of the beasts”, while dogs personify loyalty and friendship. Such assignment of certain qualities to particular animals seems a universal human trait even if different cultures focus on different animals and not everyone agrees completely with the characterization, e.g. dogs are not universally beloved and are denigrated by some people with some choice expressions.

Despite such differences, there is one mammal that has been nearly universally vilified. This is the rat, specifically the brown rat, also known as the Norwegian rat (Rattus norvegicus). This animal has made itself at home in all major cities of the world, indeed in most sites where garbage accumulates. Such places are ready sources of food for rats though for us they are associated with dirtiness. Indeed, there is a kind of disgust felt and often expressed when people talk about the rats in their city or building. I grew up in New York City and remember the hatred and disgust that rats inspired. No one, as far as I knew, ever applied the word “cute” to them. Rats were the classic case of “vermin”, namely undesirable animals deserving only extermination.¹

Beyond the association with filth, there was a mental connection between rats and disease, with the rats regarded as frequent carriers of disease. In particular, there was the Black Death (1347-1351), caused by the bacterium Yersinius pestus. This was estimated to have killed 25 million people, about 1/3 the population of Europe. For centuries, the Black Death had been attributed to being spread by rats, that is by their fleas, long before any understanding of the mechanisms of disease existed. Yet the evidence was weak. In 2018, a mathematical/theoretical study was published that concluded that the actual biological vectors of the Black Death were human-carried lice and fleas. In short, rats had probably taken a bum rap for the Black Death (as well as later outbreaks of the disease in Europe.)²

Distaste for rats even shows up in certain classic expressions, as when gangsters in old Hollywood movies curse out colleagues as “dirty rats” or even more pointedly, terminate them with “Take that, you dirty rat!” Less violently, rats were enlisted in a catch-all expression of dissatisfaction. “Rats!”, to indicate that something had gone badly wrong, though actual rats had nothing to do with the situation.

Yet, as is often the case with opinions that seem firmly set, new perspectives about rats have emerged. Indeed, in science and medicine, a more positive attitude toward rats began long ago. Rats have been highly useful experimental animals in medical research especially for more than a century. People who work with them often like them. Now, new studies of rat psychology are showing how psychologically sophisticated rats can be, amplifying that appreciation. In this article, I will first review the history of rats and then briefly discuss the new findings.

Today, rats are found around the world, in most of the big cities; it would be understandable to think that rats have always been around people. In fact, this spread and globalization of the rat has been largely a phenomenon of the last five centuries or so, as they spread around the world on ships first from Europe, then probably from North America and Asia.

Rattus norvegicus is a member of the genus Rattus, a genus of the rodent family Muridae, to which belongs that other key species of biomedical research, the mouse Mus musculus. From dated fossils, it seems that the Muridae originated about 10-11 million years ago in southeast Asia. From the kinds of analysis that can be done on DNA sequences and fossil evidence of this family (which is less reliable), it seems that this genus spread throughout Asia and then southwards into Australia and Melanesia (presumably on rafts of vegetation).

The species R. norvegicus probably arose in what is today southern China, about two to three million years ago and then slowly spread throughout Asia and eventually into what is today Europe. This expansion of range was almost certainly influenced and facilitated by the cycles of glaciation and retreat of the ice sheets during the Pleistocene. The brown rat is more cold-resistant than other rat species and would have better coped with the ice ages of the Pleistocene than other rat species. It is omnivorous and long before there were human cities and plentiful supplies of food refuse generated by humans, the brown rat presumably lived on wild plants, insects, and carrion of other animals.

Though of course it is impossible to reconstruct the detailed history of the population expansion of the brown rat, presumably a major phase occurred during the last two thousand years of settlement in Europe, with the growing number of villages, towns, and cities; this was almost certainly a golden age for this species. Needless to say, it is impossible to do a census of rats in cities today though of course it would be interesting to know the rat:human ratio in cities like New York and London.

The name “Norwegian rat”, incidentally, was based on the mistaken idea that some had arrived in the British Isles on ships from Norway early in the 18^th century. In fact, historical records indicate that the rat had not arrived in Norway until later in that century. (This mislabeling recalls the naming of the influenza epidemic of 1918 as the “Spanish ‘flu” because it was mistakenly believed that this virus had originated in Spain.)

As Europe began its exploration of and conquest of much of the world, rats began travelling as stowaways on ships that travelled from Europe to other parts of the world. The rat invasion of North America is believed to date only from about the time of the American Revolution though of course it is possible that it had begun earlier. Previous rat migrants might not have established colonies in their ports of entry. There is now much evidence that rats adapt in various specific ways to their new locales, hence it is likely that there had been earlier migrant rats who had failed initially to establish footholds in the New World. By the 18^th century, however, the brown rat had made itself at home in North America.³

But the history of the rat in the last few centuries has involved more than their spread around the world and adaptation to new environments. Rats had also begun to be adopted as pets in a few places. In particular, in 18^th century Japan, there was a very active “rat fancy”, in which color variants were deliberately bred and widely adopted as pets. And, as noted above, rats began to be recruited for biomedical research from relatively early in the 20^th century in North America and Europe. Today, they are a favored mammal for medical research, second only to their cousin the house mouse, Mus musculus domesticus. Furthermore, they are not rare as pets, often found to be not just cute (unlike their feral city relatives nosing around in garbage) but friendly, curious, and intelligent. Give a rat a proper home and he can be a good friend; context, as in so many things, is crucial.

A comparatively recent development is the expansion of research on the rat as a social animal, as noted earlier. Although inferring an animal’s mental attributes from its behavior will always involve some ambiguity, the findings indicate that the rat has unexpected emotional depth and cognitive capacity. Although there were some experiments exploring these qualities in rats in the 1950s and ‘60s, the bulk of the work is from the last two decades. It has involved lab rats, often the albino strain, which may have mild deficiencies or weaknesses that wild brown rats do not have. Hence, it is possible that the emotional and cognitive capacities that wild rats possess and use in a state of nature are even greater than what the recent lab experiments indicate. Nevertheless, the results reveal an animal that is highly sociable, capable of empathy, and often interested in helping its fellows, even when there is no gain for the helper.

A lot of the work involves tests on what is termed “targeted helping”, namely situations in which rats are given an opportunity to help fellow rats, either involving a task to be completed or to alleviate their distress. This used to be assumed to be a property specific to humans or, after a few tests on chimpanzees, shared with apes but not possessed by other mammals. That was, however, an assumption and work in mice showed that these small rodents spontaneously or, in an experimental situation, help their fellows. (For one kind of situation, where a mouse seems to be unconscious and needs help, see On altruism, a tale of mice and men).

Much evidence now suggests that rats too have a large capacity for such targeted helping, directed to either their fellow rats or the human experimenters who are working with them. The state of mind of the helping rat is judged in many of the experiments not just by its actions and expressions but by monitored endocrine and neural responses. Older behavioral studies on rats often focused on the fear response and its physiology while most of the more recent work is about prosocial interactions such as those involved in targeted helping. The tasks involved such things as getting access to food in otherwise impossible to reach places or pressing levers to do so.

Cumulatively, the work from many labs shows that not only can rats engage in trying to help their fellow rats, or their human scientist coworkers in experiments to test this but exhibit the emotions and personality traits that we associate with humans in many social situations. These include happiness, playfulness, boldness, and persistence, as judged from their facial expressions and body language. These visible markers of emotional state are seen to be accompanied by appropriate neuroendocrine responses, when the latter are measured. In humans, prosocial emotions are underlain by a complex neural network involving many parts of the brain. Much of this neural network has now been shown to be involved in rat sociality.

All this emotional, neural and behavioral complexity must be underlain by a sophisticated cognitive apparatus and this has been increasingly delineated. The findings indicate that rats can learn sequences of rules for doing things and, apparently, judge the intentions of other rats from various kinds of indications. The collective evidence that rats employ sophisticated emotional and cognitive capacities to deal with the world cannot be summarized here but has been covered in a very helpful fairly recent review article in Science.⁴

This new appreciation of the rat as a sentient and feeling animal is part of a larger movement that is taking place in the world, as humans more generally re-evaluate animals and our relationships with them. Although there is much in our world that is deeply disturbing and worrying, we are, to some degree, I think, becoming a more humane version of humanity than we used to be in our perceptions and attitudes and, even in many cases, our behavior. Moving on from seeing the rat not as “vermin” but as a fellow creature with its own capacities for feelings and thought is part of that process of becoming a more humane species ourselves.

1. The word “vermin” in connection with animals has, fortunately, fallen out of use. After all, a word that can encompass animals as diverse as cockroaches and rats or indeed any animal one does not like – raccoons exploring your garbage bin? – does not have a lot of descriptive power. Unfortunately, it also was used occasionally to describe people, as Hitler did with his least favorite ethnic group, and is being used by certain prominent right-wing politicians today in a similar way (though for different groups). One can hope that this usage will fall by the wayside too.

2. For this study, see Dean, K.R. et al. (2018). Human ectoparasites and the spread of plague in Europe during the Second Pandemic. Proc. Natl. Acad. Sci. USA 115: 1304-1309. Doi/10.1073/pnas.1715640115

3. The history of the rat is described in a very good review article, Munshi-South, J. et al. (2024). Science 385: 1292-1297.

4. A broad-ranging and excellent review of all this material in a companion article to that of Munshi-South et al. is Ben-Ami Bartel, I. et al. (2024). The complex affective and cognitive capacities of rats. Science 385: 1298-1305.