Option, both sexes will get horizontal lines when stressed or scaried. However its less likely to be seen in mature males or light colored bettas. Not sure why its seen less in mature males. Could either be that since they are "fighting fish" maybe nature makes it so they show no fear.? lol Or it could also just be that their coloring gets so intense that it doesn't show. So it's more common in females, but both have them especially when young. It is a great tell tell sign when spawning. If a female gets that line you know there is no question about pulling her.