University of Cambridge researchers scrutinised over a century's worth of photos of rhinos' horns and discovered that they have shortened over time - perhaps as a response to poaching.
The researchers measured the horns of 80 rhinos, photographed in profile view between 1886 and 2018.
The photographs, held by the Rhino Resource Centre - an online repository - included all five species of rhino: white, black, Indian, Javan and Sumatran. Horn length was found to have decreased significantly in all species over the last century.
Researchers think rhino horns have become smaller over time due to intensive hunting. Rhino horns command a high price and are in demand both as a financial investment, and for their use in traditional medicines in China and Vietnam.
Hunting has not only caused severe declines in rhino populations; the researchers suggest that shooting rhinos with the longest horns has increasingly left smaller-horned survivors - which have reproduced more and passed on their smaller traits to future generations. This has been shown for other animals before, but never rhinos.
"We were really excited that we could find evidence from photographs that rhino horns have become shorter over time. They're probably one of the hardest things to work on in natural history because of the security concerns," said Oscar Wilson, formerly a researcher in the University of Cambridge's Department of Zoology, first author of the report.
"Rhinos evolved their horns for a reason - different species use them in different ways such as helping to grasp food or to defend against predators - so we think that having smaller horns will be detrimental to their survival."