Animal Weapons | Things to Know

Why do animals grow horns, talons, claws and other “weapons”? These impressive appendages are expensive to grow, and can slow creatures down. But these weapons can prove evolutionarily advantageous when it comes to one-on-one duels over mates. In this video, Douglas Emlen, an evolutionary biologist at the University of Montana and author of Beetle Battles: One Scientist’s Journey of Adventure and Discovery, explains the “arms race” that leads to these impressive appendages sported by elk, fiddler crabs, beetles and more.

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Video Transcript:

Knowable: “Across the animal kingdom, fierce creatures employ talons, fangs, horns and other weapons to hunt, defend themselves and battle for mates. But why do some animal weapons stay small, while others seem to grow wildly out of control? While useful, these weapons are expensive. Male elk, for example, leech calcium from their bones to grow their massive antlers, and the elaborate structures slow them down.”

Doug Emlen (biologist and author of “Beetle Battles: One Scientist’s Journey of Adventure and Discovery”): “It would be analogous to you or me having a coffee table fused to the top of our head and carrying that around with us with everything that we did.”

Knowable: “For this reason, massive weapons tend to be rare. So what drives their evolution? Often, it’s one-on-one duels over mates. Head-to-head battles can kick off an arms race.”

Doug Emlen: “In duels, the outcome is really predictable, and in that kind of a situation the better fighter usually wins, and that usually means the biggest males and the males with the most elaborate weapons.”

Knowable: “That’s especially true when the architecture of the habitat leads to frequent one-on-one standoffs. An amazing variety of beetle headgear, for example, has evolved for one-on-one combat on branches or in tunnels where females set up their nests. Duels also occur out in the open. In some species, males avoid fighting and instead wear their weapons as warnings to other males. Rather than lunging straight into battle, these males size each other up first, backing down more often than not.”

Doug Emlen: “These types of animal contests don’t get dangerous — really, really throw-down, knock-down, drag-out dangerous — unless the males are really evenly matched.”

Knowable: “The size of a weapon is usually an honest signal of the health and fighting ability of the male — good news down the line for the male’s potential mate. This signal is created by the very pathways that regulate the structure’s growth: the hormone insulin.

Doug Emlen: “We know if you’re stressed, your insulin levels go down. We know if you’re starved, so you’re running out of food, your insulin levels go down. And now we think the weapons are sort of hypersensitive to insulin.”

Knowable: “Perhaps displaying that honest signal of health to the competition helps explain why weapons show up in so many different animals and forms — and could play such a critical role in a male’s ability to reproduce. So what happens with the other less dominant males? Some species, they don’t enter the contest at all.”

Doug Emlen: “You reach the point where fewer and fewer males are dominating more and more of the reproductive success. They’re really driving the show — what does everybody else do? All the rest of the males in the population are effectively out of the game, because they don’t have the resources. They’re not big enough, they can’t produce the biggest weapons, they’re not going to win. And what we find in these populations is they cheat.”

Knowable:“While their macho brothers are in the throes of battle, smaller male bighorn sheep take that opportunity to steal a chance with one of the females. Cheating drops the value of bulky weapons and can collapse the arms race between males.”

Doug Emlen: “If you look at clades of things like beetles, you find if you look across species and you map the weapons, you find all these crazy examples of huge weapons, but you also find gain, loss, gain, loss — you find that there’s evidence that these things are disappearing just as often as they’re reappearing.”

Knowable: “Fellows of these species certainly invest a lot in their weapons, but it’s not always males stealing the show. Females sport massive appendages too, when weapons are used as tools or in ambush hunting, where the pros of size and power outweigh the need for speed. But whatever the use, the myriad contraptions are always pretty impressive.”

Doug Emlen: “They’re incredible from a biological standpoint because animals are investing stunning amounts of their resources into things that are so outlandishly big that they look insane. I mean, I think they’re amazing.”