Rhino horns have steadily shrunk across all species over the past century, which scientists believe may be due to hunting.
The study, published in the journal People and Nature, is based on an analysis of a large collection of animal photos collected over the past 140 years.
Rhino horn is often hunted or used in medicine in China and Vietnam, according to experts at the University of Cambridge.
After years of hunting rhinos with the longest horns, researchers believe those with smaller horns are more likely to survive and pass their genes on to their offspring.
The report’s lead author, Oscar Wilson, a former researcher at the University of Cambridge’s Department of Zoology, said: “We’re really pleased to find evidence in the photographs that rhino horns have shortened over time.
“They can be one of the most difficult things to deal with in natural history because of safety concerns.”
Mr Wilson, who is now at the University of Helsinki in Finland, added: “Rhinos evolved their horns for a reason – different species use them in different ways, for example to aid in grabbing or fend off predators – so we think we have more Small horns would be detrimental to their survival.”
The researchers measured the horns of 80 rhinos photographed between 1886 and 2018, including all five species (white, black, Indian, Javanese and Sumatran) found in the Rhino Resource Center’s online archive.
They scrutinized photos of rhinos shot by hunters, including one showing former U.S. President Theodore Roosevelt standing in front of a black rhino he had just killed in 1911.
The researchers also compared the length of the horn to the ratio of the rhino’s body.
Scientists even examined art and drawings accumulated over 500 years. They found that there was little conservation interest in the species until the 1950s.
“For decades at least, there has been an increased focus on the conservation of rhinos – and this is reflected in recent images related to their conservation in protected areas or their plight in the wild,” Mr Wilson said.
The rhino is an endangered species, with fewer than 30,000 living in the wild today—a sharp contrast to the more than 500,000 that survived in the early 20th century.
Black rhinos, Javan rhinos and Sumatran rhinos are listed as critically endangered.