Study suggests hunters may have caused rhinos to evolve smaller horns

If certain animals posses a trait that decreases their chance of survival, then that the trait is less likely to be passed along to offspring. Such appears to be the case with rhinos hunted for their large horns, according to a recent analysis of photographs.

For the study, scientists from Britain’s University of Cambridge scrutinized 80 historical photos of rhinoceroses taken over a 132-year period, between 1886 and 2018. The images were part of the Rhino Resource Centre’s online repository, and depicted all five rhino species – white, black, Indian, Javan and Sumatran – photographed in profile view.

It was found that for all species, horn size (relative to body size) got progressively smaller as the years went by. The researchers believe this trend is due to the fact that rhinos with large horns have been targeted the most by hunters, leaving smaller-horned survivors to reproduce and pass along their traits to future generations. Similar trends have been observed before, in other threatened animal populations.

Rhino horns are utilized mainly in traditional Chinese medicine, although they are also becoming increasingly popular as display items meant to symbolize the owner’s wealth and success. Because they are so valuable, direct access to them – even by scientists – has long been very restricted. That is the main reason the Cambridge team was limited to examining photos, and is reportedly also why such a long-timespan study of rhino horn size hasn’t previously been conducted.

The scientists also analyzed 3,158 drawings and photos of rhinos in the repository – dating from 1481 to 2021 – to see how the portrayal of the animals has changed over time. Perhaps not surprisingly, earlier images tended to depict rhinos as scary, fearsome creatures. Starting around 1950, however, they were increasingly portrayed as threatened animals requiring protection.

And ironically, although large horns may have put the rhinos who had them at a disadvantage, today’s mostly smaller horns also have some major drawbacks.

“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,” said Oscar Wilson, first author of the study.

The team’s report was published this week in the journal People and Nature.

Source: University of Cambridge

If certain animals posses a trait that decreases their chance of survival, then that the trait is less likely to be passed along to offspring. Such appears to be the case with rhinos hunted for their large horns, according to a recent analysis of photographs.

For the study, scientists from Britain’s University of Cambridge scrutinized 80 historical photos of rhinoceroses taken over a 132-year period, between 1886 and 2018. The images were part of the Rhino Resource Centre’s online repository, and depicted all five rhino species – white, black, Indian, Javan and Sumatran – photographed in profile view.

It was found that for all species, horn size (relative to body size) got progressively smaller as the years went by. The researchers believe this trend is due to the fact that rhinos with large horns have been targeted the most by hunters, leaving smaller-horned survivors to reproduce and pass along their traits to future generations. Similar trends have been observed before, in other threatened animal populations.

Rhino horns are utilized mainly in traditional Chinese medicine, although they are also becoming increasingly popular as display items meant to symbolize the owner’s wealth and success. Because they are so valuable, direct access to them – even by scientists – has long been very restricted. That is the main reason the Cambridge team was limited to examining photos, and is reportedly also why such a long-timespan study of rhino horn size hasn’t previously been conducted.

The scientists also analyzed 3,158 drawings and photos of rhinos in the repository – dating from 1481 to 2021 – to see how the portrayal of the animals has changed over time. Perhaps not surprisingly, earlier images tended to depict rhinos as scary, fearsome creatures. Starting around 1950, however, they were increasingly portrayed as threatened animals requiring protection.

And ironically, although large horns may have put the rhinos who had them at a disadvantage, today’s mostly smaller horns also have some major drawbacks.

“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,” said Oscar Wilson, first author of the study.

The team’s report was published this week in the journal People and Nature.

Source: University of Cambridge