Artillery and fortifications are manufactured structures, yet they evolve just like the weapons of animals do. Advances in the effectiveness of artillery select for new and better designs of fortresses, and vice versa, in a back-and-forth cycle that can spiral into an arms race. Because fortresses are fixed in place, rivals in these races have defined roles as attackers and defenders, much like animal predators and prey. The final examples pit attacker against attacker, in matched contests more reminiscent of battles between beetles or elk.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 525