By scrutinising over a century's worth of photos, University of Cambridge researchers have made the first ever measurements that show rhinoceros horns have gradually decreased in size over time.

 

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.

 

Real rhino horns are so valuable that strict security protocols typically prevent researchers accessing them for study, so this is the first time that horn length has been measured over a long timeframe.

 

The 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.