Sunday 21 April 2019

Small British Mammals: Rats!

Brown rat
The term "rat", as commonly used, has no defined biological meaning. A "rat" is simply any rodent that looks at least somewhat like a mouse, but is larger. Animals fitting this description have evolved multiple times, so that rats do not form a single evolutionary group, and not all of them are even members of the mouse family.

Nonetheless, there is a biological group that we can call, for lack of a better term, the "true rats" (technically the "Rattini") which includes the animals that most people likely think of when the term "rat" is used without further qualification. There are two species of this kind of animal in Britain, not least because both of them are found just about everywhere else as well.

If you've ever seen a rat in Britain, it was probably a brown rat (Rattus norvegicus), also known as a common rat or sewer rat, among many other names. For much of human history, these seem to have lived only in northern China, Mongolia, and southern Siberia, where their native habitat was along muddy river banks. However, being naturally nocturnal and willing to eat just about anything, any subterranean water channel that also happened to be full of refuse was going to be, if anything, an even better place to live than a river bank. And so they followed human civilisation across the world.

The earliest such expansions were likely south towards South East Asia and Indonesia, and the second westward, possibly along the Silk Road. Perhaps surprisingly, sewer rats didn't reach Europe until some time around the early sixteenth century, and they may not have been particularly common until the Industrial Revolution really spurred the expansion of enclosed sewer systems. Today, the distribution of "native" wild brown rats is considered to cover essentially the whole of Europe, parts of the Middle East, Indonesia and the Philippines and a swathe of territory across central Asia, heading across to their original homeland, and Japan beyond it.

But that, of course, doesn't really mean much. In the 18th century, sewer rats followed Europeans into Africa and the Americas, and they are now found in urban areas worldwide. In North America, the oldest known remains of brown rats were uncovered at an archaeological site in Nova Scotia, and were dated to around 1745, suggesting that they arrived with the French. However, rats must have entered the continent multiple times, and it's likely that the English were primarily responsible for bringing them in the north, and the Spanish and Portuguese in the south. (The brown rats of San Francisco, however, have strong genetic links with eastern Asia as well as Europe, suggesting an effect from more modern trans-Pacific trade).

This relatively rapid dispersal has considerably blurred the genetic makeup of brown rats, and, if they ever had identifiable subspecies, there are none with strong enough evidence to be widely recognised now. Having said which, urban sewer rats don't tend to travel far from their homes if they don't have to so that, in places where rat populations are relatively high and food abundant, it's possible to quite precisely map their individual family histories across a city. This is less true in farmland, where food sources may be more distant from their burrows.

Brown rats are highly omnivorous, eating carrion, insects, and ground-nesting birds in addition to grains, other vegetation, and, of course, human left-overs. In fact, it seems that they'll pretty much try anything, although they are wary of food containers they haven't seen before, and prefer to eat things they've seen another rat eating before. And, apparently, their least favourite food is celery...

In fact, rats are often cooperative, sharing food among themselves rather than fighting over it, and helping to rear one another's pups... although males can certainly become aggressive towards intruders, especially when they've recently had sex. Indeed, their social behaviour is advanced enough to remember other rats which have helped them before, and to try to return the favour.

Like mice, rats communicate with ultrasonic squeaks beyond the range of human hearing, although they do make some audible sounds as well. These are used by pups to attract the attention of their mother, as alarm calls, when particularly happy, or, in the case of females, while mating.

While brown rats don't much like hot weather, and so are uncommon outside of urban areas in places such as Africa and southern Asia, they are capable of breeding year-round if the weather isn't too cold. They only live for about a year in the wild, but a female can produce up to five litters of typically around six or seven pups during that time, allowing the population to expand rapidly if there are sufficient resources to sustain it. This, of course, leads to brown rats being an agricultural as well as an urban pest, and is concerning given their ability to spread disease. On the other hand, they have been widely used in medical and other research ever since the standard "lab rat" was first bred in 1906 (mice were typically used before that) and, in the form of the "fancy rat" have even been bred as pets.

Black rat
The other worldwide species of rat is the black rat (Rattus rattus), also known as the ship rat, house rat, or roof rat. Colour isn't a very good way of distinguishing the two, since, while the black rat is, on average, darker than the brown rat, brown black rats and black brown rats do both exist. Black rats are, however, slightly smaller and more slender than brown rats, with a proportionately longer tail, as well as larger eyes and ears. They also have a rather different history.

Black rats originated further south than the brown sort, originally being native to India and Pakistan. Like the brown rat, they rapidly followed humanity across the world, but they started doing so much earlier. Quite how early is a matter of debate, but they had certainly started doing so by the last few centuries BC, probably after stowing away on ships sailing the trade routes between India and Egypt. From there they entered the Roman Empire, and we know of rat remains in Britain dating back almost to the Roman Conquest in the first century.

Oddly, they seem to have vanished from at least some parts of northern Europe after the fall of the Empire - although they may have remained in port cities, and were certainly common in the Mediterranean. They began to return to the north around the ninth century, and became common once cities rapidly expanded again from about the eleventh century onwards. It's likely the unsanitary conditions of medieval European cities helped significantly with this, and it is, of course, this rat, not the brown one, that helped bring the Black Death to Europe in the fourteenth century.

Just as the brown rat was entering Europe, however, the ship rat was already reaching the New World, with sixteenth-century remains being known from both the southern and northern continents. The earliest record of black rats in North America pre-dates the brown rats arrival by almost two centuries, with them being noted in Florida in 1565; within five years there were apparently enough of them that the Spanish military was complaining about their presence. (Black rat remains were also unearthed from a shipwreck off Labrador that sunk in the same year, but they presumably didn't yet live onshore in that area).

Rats repeatedly crossed the Atlantic over the next hundred years. Jamestown was said to be infested by the animals in 1609, just two years after it was established, and the oldest rat remains in New York date to around 1650, when it was still Dutch. Presumably, the Spanish first brought rats to California, as they did to South America, but it's not clear how long it took them to do so; we know they were there by the time of the Gold Rush in 1849, but if they were there any earlier, seemingly nobody bothered to write any letters complaining about the fact.

The subfamily Murinae
Black rats are now almost as widespread as the brown sort, but in many places where the two compete, it is the black rat that loses out. Black rats are relatively rare in modern Germany and Poland, for example, but they haven't always been so. In Britain, they are practically extinct, surviving only around London, Bristol, Dublin, and a few small islands off the west coast of Scotland; they likely visit other port cities, such as Liverpool, from time to time, but haven't kept a foothold there.

In warmer climes, black rats do slightly better, however and, in the absence of competition from other rats, can become significant pests in their own right; they have caused considerable damage on remote subantarctic islands that previously lacked any land-based mammalian predators. Their native habitat was likely subtropical forests, although they have repurposed their tree-climbing adaptations to buildings and other artificial structures since ancient times - it's unclear how ancient, but it's plausible that this may date back to the Bronze Age Harappan culture of northern India.

In biological terms, black rats are not that different from the larger brown rat. They eat marginally less meat, while still being omnivorous, and their habit of nesting in trees in the wild goes a long way to explaining why they are called "roof rats" and not "sewer rats". Their lifespans, reproductive behaviour, and communal living habits are all similar to those of brown rats, including the use of ultrasound to communicate.

As currently defined, there are no universally recognised subspecies of black rat, but that's partly because genetic analysis recently resulted in the elevation of the rats of eastern Asia to a new species, the oriental house rat (Rattus tanezumi), which doesn't seem to have spread as far as its cousin. It's likely not alone, with other species in the same general area having a virtually identical appearance that so far concealed their separate existence.

In fact, by our latest estimate, there are over 60 species in the genus Rattus, most of them native to southern Asia or parts of Indonesia. Most are quite localised, and live only in the wild, although the Polynesian rat (Rattus exulans) is widespread across Pacific islands that it can only have reached with human assistance. Now found in human settlements from Bangladesh to Easter Island, it represents a third widely invasive species - if not quite on the scale of its better-known kin.

Given its great number of species, the genus is surprisingly recent, in geological terms, being no more than 3 million years old, as indicated both by genetic studies and the fossil record. Perhaps being so successful in part due to its great adaptability, and in part to an unusually variable chromosomal structure that makes the formation of new species relatively easy, it's part of an even larger group of "true rats", with over 100 species in total.

And, as I noted at the beginning, not all animals we would describe as "rats" belong to this group, and some, such as the pack rats of North America, belong to a different family altogether. In the case of the pack rats, that family is the Cricetidae, the second-largest mammal family after the mice. While there are no mouse or rat-like cricetids in Europe, that continent is far from devoid of other members of the family... and I'll be looking at some of those next time.

[Photos by Reg McKenna and H. Zell, from Wikimedia Commons. Cladogram adapted from Steppan & Schenk 2017.]

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