I have heard the same...and I know the same for males. My neighbor got a male boxer puppy, Gino, a few years ago. Harley was 4yrs old at that time. He and Harley got along great until Gino turned 2yrs old. After that, Gino just didn't tolerate having Harley around. On the other hand, my sister's male boxer and Harley have been friends since they first met at the ages of 2 and 2 1/2yrs old, and remained friends until the end. I would have loved to have gotten an adult brother for Harley, but there was potential for disaster there and it just wasn't worth the risk.