Burrows are probably the most widespread ecological situation leading to the evolution of extreme weapons. They are localized and readily defendable. Males can guard the entrances to tunnels where females reside, and in so doing, block rival males from approaching the females. But tunnels also physically restrict access in a way that aligns the interactions of opponents. A rival male dung beetle has to enter the tunnel before he can challenge the guarding male. Ten males couldn't attack at once even if they wanted to, because there isn't space for more than one rival to enter at a time. The restricted confines of tunnels align battles so that they necessarily occur as a series of successive duels. Ball-rolling species, on the other hand, face no such restricted access. Males can challenge from all angles at once, and very often battles among ball rollers entail chaotic scrambles between three or four males. In dung beetles, species that fight one-on-one often have elaborate horns; species that fight in chaotic scrambles do not.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 247