Courtesy of BBC Life, I just learned about a wonderful organism called the Dawson's bee, which lives in the Australian desert.
If you stumbled on these ground-dwelling bees during most of their life-cycle, you would think they were peaceful and rather sweet. But that's because most of the time the only adults around are females. The females are happily laying eggs in their burrows and storing up food for their babies.
But if you chanced upon them at the right time, you would witness a massacre. The male bees hatch first, and just hang around waiting for the females to emerge from the ground. As soon as one does she is swarmed by males, who fight each other so viciously for the right to mate with her that they sometimes tear her to pieces along with each other. The colony becomes one huge brawl as more females emerge and dozens of males battle to the death.
Once the mating is over and the males are all dead, peaceful life resumes.