First, there were dung beetles both with and without horns living together, but fighting for the same resources
With limited resources, the different types of dung beetles fought over mates and food.
The Dung Beetles with bigger horns were stronger, so they got more food and mates.
Becuase the horned beetles got more food, mates, and won more battles, they had more offspring. These offspring in turn kept growing their horns.
The horned Beetles put so much energy into growing their horns that the rest of their body is impacted. The horns were so big and unwieldy that it affected their ability to mate. The number of hornless beetles started to increase.
Instead of having big horns, the other beetles' testes started growing. As a result, the hornless beetles were producing more offspring. Now, the horned beetles are less dominant and have fewer numbers. The hornless Dung Beetles are the more powerful and numerous.