A short video of an unidentified animal swimming in the River Thames near Old Windsor Lock in Berkshire has prompted debate among naturalists and experts after a family posted the footage online.
Ryan Linehan, 21, filmed the animal while walking with his parents; his mother, Bernie, 63, said the family's attention was drawn by the lock keeper, who suggested they "keep an eye out for what he thought was a seal." In the clip the animal's dark head appears to bob in and out of the water; Linehan estimated its size as similar to a medium to large dog.
The family shared the footage with members of a naturalist group on Facebook; the post drew tens of thousands of views and a range of suggestions from commenters, including seal, beaver, otter, coypu and escaped capybara. Some users joked about "Nessie," while others argued strongly for particular identifications.
Local wildlife contacts and museum staff examined the clip but reached different conclusions. A representative from a local wildlife trust told the family she and colleagues believed the animal was an otter and said a seal or beaver was unlikely to travel that far upriver or to negotiate locks. Florin Feneru, identification and advisory officer at the Angela Marmont Centre at the Natural History Museum, said he could not tell "with any confidence" from the footage and that seeing the animal's tail would have helped determine the species. He added: "I can't see this as a seal at all. Between beaver and otter, I lean towards beaver. Their upper line of the cranium is flatter, a bit convex even, like a bull terrier's for example. Otters have a stop – the concave dip at the base of the muzzle, like a beagle for example."
Richard Bennett of the Canal and River Trust also said he could not be 100 percent sure but described the head shape and movements as "more beaver than otter," and dismissed the possibility of a seal.
In the footage the animal appears to have a dark, slightly furry head and a relatively flat nose; Linehan said it "looked slightly furry in its appearance, and it had quite a flat nose." He estimated the length at about that of a German Shepherd or Alsatian. Because the sighting was a long way from the sea, some observers questioned whether it could be a seal, while others pointed to non-native rodents such as coypu or escaped capybara as possibilities.
Beavers, which are native across parts of the northern hemisphere, are strong swimmers and are sometimes mistaken for other semi-aquatic mammals. They are known for felling trees and constructing dams and lodges, creating still-water habitats with underwater entrances. Scientists have speculated beavers build dams for shelter, warmth and predator protection. The largest beaver dam discovered was measured at 2,790 feet (850 metres) in northern Alberta, Canada.
Authorities and experts reviewing the footage stressed that short, low-resolution clips can make definitive identification difficult. The Natural History Museum and local wildlife organisations did not announce a final determination. The family posted the clip to social media and has not reported contact from wildlife services about capture or rescue.
The sighting adds to occasional reports of unexpected or non-native animals appearing in waterways near urban and suburban areas, which can draw public interest and raise questions about wildlife movement, escapees from captivity and the capacity of rivers to support different species. For this Thames sighting, the absence of clear views of the animal's tail or full body left the question open, with experts and the public offering differing interpretations based on the available footage.