My opinion and observation is that it's only a guess. Can they really? Show us the math. It's as absurd as a guy I once watched in a BF video interview who claimed he could tell the weight and gender of a sasquatch based on their prints, step length, and appearance of the foot print. The person interviewing this guy just ate it all up hook, line and sinker. The bravado of some in the research community is astounding and laughable.