Rhino horns have been in high demand among hunters for centuries, while modern poachers sell them for use in traditional medicines in China and Vietnam. “In the case of rhinos, people have basically always wanted the bigger horn,” said Oscar Wilson, a doctoral student and first author of the study at the University of Helsinki, adding that bigger horns would command a higher price to modern poachers. Wilson and his colleagues say a study of archival images of the animals suggests that the size of a rhino’s horn, relative to its body length, has decreased over time. A similar trend has been recorded for creatures such as elephants and wild sheep. “Preferred hunting selection for individuals with larger horns or tusks resulted in individuals with smaller traits surviving and reproducing more, passing those traits on to future generations and driving an evolutionary shift,” the researchers write in the journal People and Nature. They made their discovery by examining artwork and photographs of rhinos held by the Rhino Resource Center (RRC) in Utrecht. Some of the art dates back to the 15th century. The team focused on 80 photos of rhinos that show the creatures sideways. They then calculated the length of the horn relative to that of the animal’s body before taking an average of these values ​​and calculating the difference compared to the ratio shown for each specific image. As a result, the team was able to investigate whether the relative size of rhino horns became larger or smaller over time relative to the average ratio. The results reveal that, relative to their body length, black and white rhinos had the longest horns, while Sumatran rhinos had the shortest. However, for all five rhinoceros species, relative horn length has decreased over time. While 65 of the rhinos photographed were in captivity, Wilson noted that the majority either came directly from the wild, or were born from rhinos that had previously lived in the wild. This, he said, suggests that the decline in horn length likely reflects the selection pressures the animals face in their natural environment, although the researchers are now conducting further work with wild populations to investigate whether the same trend is observed. While the impact of the shrinkage is not known, Wilson said it could be detrimental. “Rhinoceros use their horns, so they lose them [is] it probably won’t be good for them,” he said.