Mammals of the River

Emily Cooper


This blog is written by Jenny Pearson, one of our fantastic Ambassadors.

Britain has some fantastic wildlife. Most of the fauna biomass in the UK is made up of invertebrates. If you haven’t heard the term biomass before, it simply means the collective mass of biological matter. This is a good way to compare the prevalence to two species who differ in size, for example, comparing populations of mice and elephants!

Now, I don’t know the exact figure but I would bet my hat, that the Scottish midge population, in August, outweighs all the other animals that live in Britain. Invertebrates truly do make the world go round and are vital to the health of a river ecosystem. But the animals that most people are most familiar with are vertebrates, after all, they are usually massively charismatic and very recognisable.

After a quick research session, I discovered that in Britain, there are 622 bird species; 66 resident mammal species; 42 species of freshwater fish; 7 amphibian species; and a mere 6 reptile species. This blog series will focus on some of the greatest river vertebrate species we have here in the UK.

All mammals require a source of freshwater to stay hydrated and so most, if not all mammal species, could be found near rivers - but a few of these are river specialists. The following list celebrates 5 of these river swimmers that you are most likely to encounter around your local river.


Rats have earned themselves a bit of a poor reputation, associated with disease, I bet that if we took a survey, rats would come out on top as the least popular species. Rats have also, unfortunately, been the demise of many bird species, as one of their favourite meals includes bird eggs.

And while, yes, many of our disease related concerns involving rats are valid, rats are very intelligent and adaptable animals. They can be found in almost any habitat as they aren’t particularly picky - all the need is somewhere warm and dry, and some food. If these conditions are met, rats can unleash their superpower… breeding!

There are 2 species of rat in the UK: the black rat, or what is sometimes known as the ship rat; and the brown rat, or common rat. The brown rat is the most populous of the two species, and is not in fact a true British species. It came to the British Isles around 1700 from Asia! And its adaptability meant that it settled in very well, and to this day its populations are still going strong.

Skilled animals, the rats are strong swimmers and can often be found around water ways, particularly urban waterways (urban meaning there sure is a very easy food supply around… our waste!). This increases the likelihood of encountering one of these riverine mammals.

Water voles

Water voles are one of the UKs most endangered species and have disappeared from 90% of their original inhabited rivers; they are the fastest declining mammal species in the UK! The main threats to water voles are disruption to their habitats, pollution and predation (mainly by the invasive american mink). There are many conservation efforts underway to protect this fuzzy mammal which deserves a whole blog in itself.

Water voles, therefore, are far more difficult to come across. And if you do come across one of these cuties, you may confuse it with the common rat (as they are quite similar and inhabitat similar areas). And do keep in mind that water voles are protected species and therefore disrupting them in any way is against the law.

Key things to look out for sightings of water voles include:

  • Latrines i.e. piles of little droppings, that may resemble brown tic tacs.. (rat droppings are larger, about the size of a 1 pence piece)
  • Feeding piles - water voles carry vegetation to their preferred feeding spot and will often cut the grasses at a sharp 45 degree angle!
  • Burrows and nests - water voles spend a lot of time underground and they create burrows at the water's edge (they are often wider than they are tall). And they will build themselves woven nests of grass amongst the rushes (these can be as big as a football!).

If you are sat by the river's edge, take a moment to be still and listen. Water voles are often heard and not seen; they will make a characteristic ‘plop’ noise as they slip into the water.

Water voles may be confused with rats - a key difference is that water voles are easily frightened and will dive under out of sight, while rats will confidently swim along the surface.

Daubentons bat


There are 17 species of bats that live in the UK, but it is very difficult to tell these species apart without listening to their echolocation calls. Unfortunately, these calls are so quiet, we can’t hear them unless we have a specialised piece of kit called a bat detector... but knowing some clues about the various species’s behaviours and habitats might help the investigation.

The Daubentons bat is often known as the water bat. If you see a bat skimming over wetland in the twilight, there is a good chance it is a Daubentons! They are fast and agile and feed on the insects that hover above the surface of the water. Therefore, these bats may act as an indicator species for the health of the wetlands - healthier rivers will have more insects, and in turn, more bats! They will make the most of the surrounding habitats and will roost in bridges above waterways, or in old, hollow trees on the river banks!

These bats can be found all across the UK, usually between April and September when there are plenty of insects to eat!



Otters may be, in my opinion, the most beautiful British mammal. They are slender and sleek and there is nothing like an otter sighting to absolutely make your day. Otters are a conservation success story. After the industrial revolution, many of our rivers were completely uninhabitable and otters disappeared from the river banks. There has been a significant improvement in otter populations around the country as industrial pollution has reduced, and many are near towns and cities, making it easier than ever to spot them.

Otters are top predators, and are fantastic at fishing. Their bodies are well designed to make them efficient swimmers and fish hunters - their teeth, feet, slender bodies and tail. But unlike other fish eating mammals, otters are not great at holding their breath and can only dive for 30 seconds.

Otters have fantastic senses - strong eye sight, a great sense of smell and hearing - they are often on alert. It is said that there is a greater chance of an otter spotting you, than you spotting it.

Unfortunately, otter populations are still at risk. As top predators, they require a healthy and sufficient food supply, and so will be sensitive to pollution and prey interference. Otters are also very susceptible to road collisions and many are killed each year by cars.



Beavers are magnificent animals (and the largest rodent in the country). They were once hunted to extinction for many reasons, one of which being that they possess a gland in their anal region that produces a chemical that was very sought after for perfumes!

Slowly but surely, beavers have started reappearing in Britain. Trial re-introduction projects have shown that the beavers brilliant dams have extensive benefits including (but certainly not limited to) the creation of fantastic wetlands that support many species, filtering pollutants, and slowing heavy water flows to the extent that large flood events are significantly less damaging to businesses, homes and peoples lives. The beaver has become the poster animal for rewildling, a concept that involves humans taking a step back and letting nature do its thing.

The reintroduction of beavers to British waterways is a somewhat controversial topic. While many of us have come down with a serious case of Beaver Fever, not everyone is catching the bug. Beavers have a negative reputation amongst some communities and are associated with land damage and flooding. In order to go ahead with a wide scale beaver reintroduction, we need more people on board, including land owners. This means having conversations, supporting land owners and helping people understand the magnificent benefits of these large rodents.

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