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Sexing the red tail black shark is pretty tough, it is difficult to be 100% sure of a RTBS's gender. Males are rumored to have darker bellies than the females (I have not been able to verify this), and males do have more pointed dorsal fins than the ladies. The genders really don't have many external differences, the dorsal fin is probably the most telling of all the ones I've heard.

As per your other point, most of these sharks are actually pretty peaceful. They really tend to bother each other more than anything else, as they are hostile to other red tailed sharks (and similar-looking fish, such as rainbow sharks). They are usually good citizens in community tanks without other freshwater "shark" Cyprinids.
Still, you're correct in that some are more aggressive than others. I believe that it has more to do with the fish's personality than its gender, though. This holds true for many freshwater fish. Take German rams, for example. They tend to be very peaceful little fish, except when breeding. However, I have heard from several aquarists and seen for myself that there are "mean" rams, ones that do not fit their peaceful description. Sometimes males are the aggressive fish, but females can be nasty, as well. Bettas are another good example. Though male bettas tend to attack other male bettas and females as well, there have been many cases in which males have shared a tank with females, or other males. On the flip side of the coin, the usually-peaceful female bettas can turn on tankmates, and become little terrors. They are not common, but these ill-tempered girls do exist. It is just that they have different personalities than some of their peers.

And, in many species of fish, males are more aggressive/territorial than females, this is true. Male plecos often fight, and males are the only egg-guarders. Male Characins often display and spar with each other, and chase females around. Male cichlids often battle over females, etc. etc. You can see that much of this behavior is centered around reproduction. Males hold territories of their own, and keep other males out. Males fight for mates, males chase girls to persuade them to spawn, males guard spawning sites and eggs, males guard fry. In some fish, females participate in these activities as well, but it is mostly male behavior. Again, much of this is connected to propagation. If the males weren't so competitive, there wouldn't be as good a chance for them to pass on their genes. Another, more relentless male would win the girl, or successfully protect its spawn from predation, and so on. It is basically a survival mechanism, one that ensures the future of their offspring.

Is that what you were looking for, Keith?

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