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In human society, an “arms race” is an escalating competition between two or more rival nation-states regarding the destructive capacity of their military forces. This kind of one-upmanship can be quite costly, with little competitive advantage to show for it. Each side must invest more and more resources just to maintain their relative footing, and effectively, the status quo (Albeit a status quo with bigger and bigger weapons.) While this situation might seem uniquely human, it is actually widely documented in the rest of the animal kingdom as well.

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Douglas J. Emlen is an evolutionary biologist at the University of Montana who studies how these competitive dynamics play out in other species—his new book “Animal Weapons: The Evolution of Battle,” was published earlier this month. In this 2005 article, he reviews in depth the elaborate morphological weaponry visible across a wide variety of species, both modern and prehistoric. Emlen explains how this kind of exaggerated armor—everything from antlers (in both flies and deer) to disproportionately large jaws in fish—is more likely to evolve in certain contexts.  This armor has generally evolved to be most extreme in cases where a male’s reproductive opportunities increase. For example,  this evolution could occur when a male defends resources that are highly valuable and localized, but irregularly distributed—oftentimes egg-laying sites.

Bigger morphological weapons aren’t necessarily better in the battle for reproduction. In this 1997 article, Emlen reports on a counter-strategy, successfully employed by some males in a particular dung beetle species. In this species, only some of the males possess horns, while others are hornless. The mating strategy of the horned beetles fits exactly the pattern described in the previous article—they stand guard outside tunnel entrances, dug by females for mating and egg-laying purposes, and are hostile towards approaching males. The hornless beetles have a different strategy, however. Rather than defending tunnels, or confronting the visibly impressive guards, they gain access to females by digging side-tunnels into the prepared burrows. The hornless beetles are also at an advantage in that they don’t require the energy necessary for developing and maintaining more elaborate features. Consider, for example, how much energy it must require for a male fiddler crab to lug around claws that constitute half his body weight.

“Survival of the fittest” is a misunderstood term when it comes to evolution. The catch is that there is no way to be “fittest” in all contexts, so preparing for battle is only one strategy. It’s clear in the case of the dung beetle that bigger weapons are not always better, and that an arms race is not necessarily an endgame—it may represent an opportunity for alternative strategies to succeed.


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Annual Review of Ecology, Evolution, and Systematics, Vol. 39, (2008), pp. 387-413
Annual Reviews
Behavioral Ecology and Sociobiology, Vol. 41, No. 5 (1997), pp. 335-341