The adult males do have much longer pec fins than the females do, but your new pleco has several months of growing and maturing to do yet. Be great if the new one turns out to be a female. They are very pretty plecos! The one thing that still bothers me is that while you will find references on the internet that some females of some ancistrus species will show small facial bristles, it really is very rare and specific to the more uncommonly found plecos. Those lip nubs in your 3rd picture of your new one ..... I don't think you'd find nubs that size on a young female. 3" is around the size the male bristles begin to start showing and growing and the continue to grow very slowly for quite a long time. Even at a year of age you probably won't see full grown bristles yet on a male. And the red marking on the back of the head doesn't show up until the males are a bit more mature. It's a bit difficult to spot on a 3" light colored pleco so would be impossible on a 3" dark colored pleco. There is a body shape difference between the male and female bristlenoses that starts to become apparent around the 3" size. Looking at them from the top down, the males are very slender and tapered all the way to the tail. The females at 3" just begin to show a broader midsection.
