How Cheating Helps Animals Win Battles

They create giant weapons without a lot of metabolic expense.

Fiddler crab with large claw
Elvira Draat / Getty Images

When it comes to fights in the animal kingdom, the size of a weapon really matters. Deer with massive antlers or lobsters with huge claws often get a less-endowed opponent to back down and scurry away.

But those large weapons are heavy and clunky and can take a toll on an animal’s metabolism as they maintain them. There’s extra tissue to feed and care for, even when the animal isn’t using them.

Some animals have come up with a crafty workaround. They build their scary artillery out of material that has less costly upkeep.

“Size is hugely important for many animal weapons, but why size matters varies depending on the species,” study author Jason Dinh, a Ph.D. candidate in biology at Duke University, tells Treehugger.

“In some cases, weapons are used as signals for fighting ability. If your opponent has a huge weapon, they are likely a formidable opponent. This kind of assessment can be useful in determining whether to persist in a fight or not.”

In other situations, weapon size is important because larger weapons are more effective.

“In fiddler crabs, for example, individuals with larger weapons can pinch harder and maintain high pinch forces for longer periods of time,” Dinh says. “By having a larger weapon, then, these animals can better engage in combat.”

Muscles need a lot of energy to maintain, but chitin—the main component of a crab’s shell—is mainly inert. That means it costs little to maintain. It’s the same situation for keratin, which makes up bird feathers and rhino horns.

Studying Exaggerated Weaponry

Dinh used two species of snapping shrimp and a species of fiddler crab to test how animals are creating exaggerated weapons out of tissues such as chitin.

He looked at the connection between the size of the weapon and the ratio of soft, expensive tissue to hard, cheap exoskeleton. Dinh found that the larger the weapon, the higher the proportion of exoskeleton. That means the muscle doesn’t grow in proportion. So the large weapons are made of less expensive muscle.

He found that some animals can create incredibly large weapons. Those exaggerated weapons also have more exoskeleton than muscle.

Those animals use those fake weapons to deceive their opponents. But researchers aren’t sure why every individual doesn’t build over-the-top weaponry.

“That’s the million dollar question! Animal weapons aren’t free. These exaggerated structures often have costs that can reduce an animal’s ability to survive and reproduce,” Dinh explains.

For example, when animals spend their limited resources on the growth of weapons, they could sacrifice the growth of other key parts such as eyes, muscles, or reproductive organs.

Large weapons can also be costly to use.

“It takes a lot of energy for a fiddler crab to pinch a claw that is close to half of its body weight,” Dinh says. “Finally, weapons could increase predation risk. They are super conspicuous, and might attract attention for eavesdropping predators.”

That’s why the benefits of having a weapon have to outweigh the risks. Typically, that means attracting a mate or the resources that attract mates.

“The cool part about this study is that they use physiological tricks to improve this cost-benefit ratio,” Dinh says. “Snapping shrimp and fiddler crabs reduce the costs of growing and exaggerating weapons by investing disproportionately into metabolically cheap tissues.”

The findings were published in the journal Biology Letters.

Backing Down or Fighting?

In most cases, the animal with the smaller weapon will just back down when faced with an opponent with an exaggerated one.

“Snapping shrimp, for example, use lots of visual displays and posturing before they engage in any injury-risking combat. In most cases when there is a large size difference, the smaller individual will retreat before engaging in physically demanding fights,” Dinh says.

If they do come to blows, the large weapon is capable of inflicting damage.

“When they do fight, the large weapon is certainly not useless. Larger weapons could hit harder than smaller weapons, meaning that they are more effective in physical combat,” Dinh says.

Understanding Deception

Dinh says he became fascinated with animal weapons because they are “charismatic, beautiful, and bizarre.” He wanted to investigate what contributed to their evolution.

The study findings are important for two reasons, he says. It helps researchers understand how these extreme weapon sizes are created and it helps understand how exaggeration and deception can occur with them.

“In many species, weapon size relative to body size is WAY larger in large individuals compared to small individuals. However, these weapons are costly to grow, and it’s not really clear how large individual can bear the costs of growing a disproportionately large weapon,” Dinh says. 

The study results found that one way to deal with those costs is to use low amounts of metabolically expensive tissue and higher amounts of metabolically cheap tissue.

“One of the best examples of deceit we have are in snapping shrimp. Male snapping shrimp with exaggerated claws use high frequencies of visual displays during contests, and this really confuses their rivals who are trying to assess them,” Dinh says.

“However, smaller individuals should be limited in their ability to grow deceptively large weapons because they are less able to deal with the costs. The finding of this study—that exaggerated claws comprise disproportionately high amounts of energetically cheap tissues—demonstrates that animals could play physiological tricks to get around the costs of exaggerating.”

View Article Sources
  1. Dinh, Jason P. "Large and exaggerated sexually selected weapons comprise high proportions of metabolically inexpensive exoskeleton." Biology Letters, vol. 18, no. 2, Feb. 2022. doi:10.1098/rsbl.2021.0550

  2. study author Jason Dinh, Ph.D. candidate in biology at Duke University

  3. Chelini, Marie Claire. "In Animal Battles, Cheaters Can Win." Duke Today. 10 Feb. 2022.