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Scientists say rhino horns have gradually shrunk over the past century, and hunting is the probable cause.聽The findings are based on an analysis of photos over the past 140 years.聽

Researchers say poachers target rhinos with long horns, which means that only smaller-horned animals survive, and then pass that particular trait to their offspring.

There is a fear that as individual rhinos continue to have smaller horns, more rhinos will be killed to get the same amount of horn.

Oscar Wilson is a doctoral student and first author of the research at the University of Helsinki:

"This is selection which has been introduced as a result of human hunting... we hope this is bringing attention to the plight of these endangered rhino species."

(Photo: Baby White Rhinoceros and mother (Ceratotherium simum). South Africa. Credit: Getty Images)