Many animals carry weapons that are overly large and impressive, seeming to represent more a case of overkill than a genuine need for protection. Indeed, these weapons would be heavy and metabolically expensive for the animal to carry around and maintain. Whether they are giant horns, spreading antlers, claws, pincers or tusks, there is a disadvantage to the individual that has to look after these large weapons.
For example, in clawed crustaceans, such as shrimps, lobsters, and crabs, their weapons can weigh more than a third of the animal’s body mass. That tissue has to be fed with nutrients and oxygen, and maintained in fighting condition, which uses up some of the animal’s energy budget, even when it is sitting absolutely still.
“Some animals can spend 40 percent of their energy budget for the day just maintaining themselves sitting there doing nothing,” said Jason Dinh, Ph.D. candidate in Biology at Duke University, and author of a new study that shows animal weapons can be a lot like plastic guns: impressive on the outside, but ultimately cheap. “It’s a very slow and steady cost that’s happening throughout the animal’s adult life.”
Bigger animals often have disproportionately large weapons that would be very costly to maintain. However, it seems that appearances may be deceptive in the case of these outsized weapons. Although soft tissues, such as muscles, require lots of energy to remain viable, the chitin that forms the main component of a crab’s pincer is inert and low maintenance. It is the same for structures made of keratin, including rhino horns, bird feathers, claws and fingernails – they look threatening but cost almost nothing to maintain.
In the recent study, Dinh used two species of snapping shrimps and one species of fiddler crab to test whether the animals were building their weapons out of metabolically cheap tissues, such as chitin. He compared the ratio of soft, expensive tissue to cheap exoskeleton in the weapons present in each species. He was particularly interested in whether this ratio was the same in small and large weapons.
His results, published today in the journal Biology Letters, showed that the larger the weapon, the higher the proportion of exoskeleton it contained. In other words, large weapons were not packed with metabolically costly tissues but were instead constructed rather on the cheap. Clearly, these animals were cheating.
Even when comparing similar-sized individuals within a species, some can have markedly exaggerated weapons. Dinh looked at the relationship between exaggeration and the ratio of soft tissue to exoskeleton and found that, regardless of body size, exaggerated weapons also had disproportionately more exoskeleton. Once again, they were mostly show and little substance.
“These individuals with exaggerated claws are pretty good at deceiving their opponents,” said Dinh. “Their opponents have trouble assessing whether they’re bigger, stronger, or simply have an exaggerated claw.”
This does not imply that an exaggerated weapon is useless. To the contrary, larger weapons still have advantages. Dinh explained that among fiddler crabs who pinch and push each other, a bigger claw may have advantages in combat. In snapping shrimps, which fight by throwing extremely high-pressure bubbles at each other, larger claws may also present an advantage.
“It’s a way animals can deceive, but they can also improve their performance during these fights, and apparently they can do it in a really cheap way,” said Dinh.
Animal battles are usually not fought to the death but consist of a series of ritualized displays that aim to intimidate an opponent into submission and retreat. In these circumstances, an animal with an outsized but cheaply built weapon may escape detection. Cheating clearly pays in these cases.
And if an opponent really did enter into violent combat and a crab or shrimp lost its outsized claw, it could simply grow a replacement.
“We think of these weapons and ornaments as honest indicators of how good of a fighter an individual is, but animals seem to be able to play these nice physiological tricks to really cheaply deceive or exaggerate how strong they are during these fights, and it’s primarily by using cheap tissues instead of muscles,” said Dinh.