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Poaching continues to be a severe threat to all rhino species across the world.

Their horns are still used in traditional medicines in China, despite having no proven medical benefit.

Because it is often the largest rhinos that are hunted for their bigger horns, based on photographs taken over the last 140 years, scientists now believe that Rhinoceros horns across all species have gradually shrunk.

And poaching is the most likely cause.

Rhinos are now passing on their smaller traits to future generations

Researchers at the University of Cambridge believe that as more smaller-horned rhinos are surviving into adulthood than larger ones, their smaller-horned traits are being passed on to future generations of rhino.

Whilst this might sound like this could have benefits to rhinos in the future, who may be seen as less of a target for poachers, rhino horns are really important to their survival.

So only having smaller horned rhinos may have a significant impact into the future.

What did the researchers study?

Rhinos are "probably one of the hardest things to work on in natural history because of the security concerns," says lead author Oscar Wilson.

"So we were really excited that we could find evidence from photographs that rhino horns have become shorter over time."

They measured the horns of 80 rhinos, photographed between 1886 and 2018 with their horns in full view.

These photos included all five species of rhino - White, Black, Indian, Sumatran and Javan.

They also measured other parts of the rhinos body to work out the right proportions to the horn length, so ensure their analysis was accurate.

And while artwork is not precise evidence, the researchers also considered drawings of rhinos that have been dated back 500 years. Back then, this was the only way of keeping records of any animal species - photographs did not exist!

Sadly, the information and evidence they gathered shows that very little was done to protect or promote rhino conservation until the 1950s but is showing the direction that the species is heading in.

Mr Wilson said: "For at least a few decades now there's been much more of a focus on the conservation of rhinos - and this is reflected in the more recent images, which relate to their conservation in sanctuaries or their plight in the wild."

As rhino horn and elephant tusks are still worth money for poachers, these creatures continue to be targeted.

Although a lot is being done to help protect these animals, evidence suggests their populations are still in decline.

Conservationists continue to do what they can to protect those that remain in the wild and help educate wider society about the threat these animals face and what can be done to better protect them.