Size matters - a method of estimating the age of male mink In 1951, the Journal of Mammalogy published a paper: The Baculum as an Age Criterion in Mink by William H. Elder. Seventy years after Elder wrote that paper, we are finding a use for his detailed research.For those not familiar with the baculum, it is a bone found in many mammal species. Humans do not have one, although many other apes do. In those species where it does occur, it is only found in the males. The alternative name may give a clue if you haven’t guessed what it is yet – os penis.Elder dissected the bacula from 234 (dead!) farmed mink and measured them. He knew the age of each mink, so it was possible to see if the bone changed with age. He found that the length increased slightly with age, while the weight increased more noticeably. There was a lot of variation in each age class, so if you get a male mink and you want to use this feature to find out how old it is you need to accept that there is some uncertainty. How does knowing the age of a mink help with an eradication programme? Well, if we catch one juvenile mink it means there was a breeding female somewhere within a mink’s journey distance of that trap location, which may have had other offspring. Or if we have traps running in a place for three years and then catch a mink that is five years old, it means that mink have resisted our traps until that time, an indication of a potential flaw in the trapping strategy.I am dissecting the baculum of each male mink we catch. We will also have a second independent assessment of age based on a tooth x-ray for each one. These two sets of data will then be used to see if the age of wild mink can be assessed from the baculum. If it proves to be the case, we will publish this paper and scientists in another 70 years’ time might well find it useful! Bacula from six mink caught in Cambridgeshire in autumn 2021. The one on the right matches the maximum length (50mm) found in farmed mink so was likely to be in its third year or older. VL72 had the appearance of a juvenile, born in 2021. Photo by Vince Lea.