If you are out walking next to a river or lake at dusk or dawn and you spot a large, brown, fur-coated creature slipping into the water, or notice a whiskered muzzle bobbing along in the half light, you could be forgiven for asking the question – "Was that a beaver, or an otter?"
Both beavers and otters are moderately large, semi-aquatic, nocturnal mammals, native to the UK. Their shared water-based lifestyle has led to both species evolving similar adaptations – large lungs to allow them to hold their breath underwater for longer than other mammals their size (up to four minutes for otters and up to 15 minutes for beavers!); webbed feet for swimming; and dense, waterproof coats for warmth.
Despite the similarities between them, however, they are in fact very different animals.
Are the otter and the beaver related?
No the beaver and otter are not related. The Eurasian beaver (Castor fiber) is a rodent in the taxonomic family Castoridae. The only other living member of Castoridae, and the only other living species of beaver, is the North American beaver, Castor canadensis (Canada's national animal).
The Eurasian otter (Lutra lutra) is a carnivore in the family Mustilidae, a diverse group that includes weasels, badgers and 12 other species of otter.
How to tell the difference between an otter and a beaver
Like other rodents, the beaver is stocky with a rounded head and long incisor teeth. The beaver’s incisors are coated in a thick layer of orange, iron-rich enamel and are open-rooted, meaning they grow continuously. Beavers’ tails are long, flat and scaly, and are used much like a paddle, to help propel them through water.
Otters have long, sleek bodies and more pointed, dog-like heads. They have strong jaws, long, sharp canine teeth, and pointed molars, specially adapted for gripping fish. The otter’s tail also aids propulsion, but is long, thin, and pointed as opposed to paddle shaped.
Where do beavers and otters live?
Beavers are known as ‘ecosystem engineers’ due to their ability to modify a habitat to suit their needs. They use their extra-strong incisors to fell small to medium-sized trees to use as building materials.
For shelter, beavers either burrow into banks or, if the bank isn’t large enough, will build a lodge on the bank side. In both cases the entrance will be below the water’s surface, hiding it from view and making it inaccessible to predators.
This requires the water at the beaver’s chosen location to be suitably deep and slow-moving. Where this isn’t the case, rather than waste time searching for the perfect spot, the beaver will, very resourcefully, set about damming the watercourse to make it so.
Beaver dams and the ponds and wetlands that result from them can improve downstream water quality, lessen extremes of drought and flooding, and provide habitat for many other species, including otters!
Otters are more playful than industrious, preferring to spend their time juggling rocks and play fighting than building dams. They make their holts in existing crevices, under rocks, amongst tree roots, and in the abandoned burrows of other animals. They will also flatten down patches of vegetation to use as resting places called ‘couches’.
In both species territory size can vary greatly, with otters defending stretches of river up to 35km long, and beavers up to 20km. The size of a territory in both is dependent on food availability and the presence of other individuals of the same species.
Within a territory both beavers and otters will have several homes, abandoning each as food supply becomes depleted. Both use forms of scent marking to protect their resources from competitors.
Beavers create scent mounds – piles of mud mixed with a sweet smelling, oily substance called castoreum which is secreted from special glands. Male otters mark their territory with ‘spraints’ - faeces fragranced with scent gland secretions, deposited in prominent places to warn off other otters.
Do otters and beavers share the same diet?
Otters have a carnivorous diet of fish, crustaceans, amphibians and waterbirds. Beavers, on the other hand, are herbivores, eating aquatic plants and the twigs, bark and leaves of trees.
Beaver vs otter: reproduction
When it comes to their reproductive behaviour, beavers and otters are quite different. Beavers form monogamous breeding pairs and mate for life. Breeding pairs form a colony with their young of the year and the young of previous years that are yet to venture out to establish their own colonies. Beavers have strong family bonds and cooperate in building and maintaining dams, grooming, foraging and rearing young. Beavers breed once per year, between December and April, and litters of two to three kits are born in May or June.
Otters are largely solitary, apart from when they come together to breed, or when a female is rearing young. There may be several females living within a male’s territory, and they will use scent marking to signal to the male that they are ready to mate, which can happen at any time of year. The male and female will court by chasing and playing for about a week before copulating. Female otters rear litters of two to three cubs in the natal holt for two months without the male’s involvement. After venturing out, juvenile otters remain in a family group with their siblings for at least six months before dispersing to form their own territories.
Beaver vs otter: conservation status
Both beavers and otters have a chequered history in the UK. Otters were driven to the brink of extinction between the 1950s and 1970s due to water pollution from organochlorine pesticides, which had a detrimental effect on populations of fish that otters rely upon as prey. Now, thanks to improvements in water quality, and its protected species status, the otter is making a comeback across the UK.
Beavers were hunted to complete extinction here 400 years ago but have now been reintroduced to the wild in Scotland and trial reintroductions are happening in other parts of the UK.
If conservation efforts to restore these species continue to be successful, chances are it will become much more common to spot one of these elusive animals while out on an evening stroll.
Hazel Forrest is a species advocacy officer with the Scottish Wildlife Trust
Main image: Beaver swimming © Getty Images
