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Rhinoceros horns appear to have become significantly smaller over the past 130 years, probably as a result of poaching.

Hunters kill rhinos to remove their horns as trophies or as high-value commodities for traditional Chinese and Vietnamese medicines and materials. The culling of rhinos with the biggest horns, which have the greatest value, may have encouraged the survival and reproduction of smaller-horned rhinos – an idea supported by analyses of photographs spanning more than a century, says at the University of Helsinki in Finland.

“This is bad news for hunters obviously, but unfortunately it’s bad news for rhinos as well, because if the hunters want the same amount of horn, they’re going to have to shoot more rhinos,” he says.

Because rhino horns are so valuable, even museum specimens are kept tightly locked away and are difficult to access for research purposes. So Wilson and his colleagues analysed 80 profile-view photographs of living rhinos, dating from 1886 to 2019, which rhino experts worldwide had uploaded to the database of the , based near Cambridge, UK.

The team used image software to calculate various anatomical measurements for each animal and then estimated its horn size relative to its body size. As horn size varies considerably among rhino species, the team created separate size charts for each of the five kinds of rhinoceros represented in the photos: the white rhino (Ceratotherium simum), the Indian rhino (Rhinoceros unicornis), the Javan rhino (Rhinoceros sondaicus), the Sumatran rhino (Dicerorhinus sumatrensis) and the black rhino (Diceros bicornis), the last three of which are critically endangered.

Most of the photographed rhinos had been born wild but were living in zoos, wildlife parks or sanctuaries at the time of the photos, while 12 were still living in the wild.

Mapping out the horn sizes along a time chart, the researchers found they had gradually decreased within each species. While the information isn’t sufficient to provide precise percentages of horn size changes, the downward trend is clear overall and is most pronounced in Sumatran rhinos, says Wilson.

Poaching pressure to reduce horn size could also have negative effects on rhinos’ behaviour and welfare, he says. “Rhinos do actually use their horns for a lot of different things, like defending their territory or finding a mate,” says Wilson. “We think [these reductions in horn sizes] must have some effect on the way rhinos will live their lives.”

Even so, the researchers’ findings “are not all doom and gloom”, he says. The team also analysed thousands of other images in the database, including artistic depictions, which suggest a growing improvement in people’s attitudes towards rhinos over the past few centuries. “We’re viewing rhinos way more positively than we ever have,” says Wilson. “We think this is real cause for optimism [concerning] rhino conservation.”