Big scissors, nothing behind? Sexual selection among crustaceans gives males with large, dangerous-looking claws an advantage: With the apparently dangerous weapon, they can often impress a competitor enough to avoid a fight. But large weapons cost energy – especially if they contain a lot of muscles. A study now shows that the larger the scissors of the crab, the lower the proportion of energy-intensive material such as muscles. Compared to conspecifics with smaller claws, individuals with oversized claws construct them to a greater extent from energetically “cheaper” materials such as chitin.
Whether it’s the antlers of a deer or the claws of a lobster: many animals have weapons that are used to engage in battles with fellow animals in the fight for reproductive partners. In some cases, the opponent can be intimidated without a fight if the opponent’s weapons are particularly impressive – and in many cases females also prefer males with large weapons. Therefore, this trait has been sexually selected in many animal species. But the bulky, heavy appendages are a disadvantage in other situations. They can make the animal less agile, increase the likelihood that it will fall victim to predators, and also cost energy.
Muscles or exoskeleton?
“Most energy cost studies focus on weapon growth,” explains Jason Dinh of Duke University in Durham. “However, they overlook an energetic sink that is ubiquitous and unceasing: the basal metabolic rate.” These are the costs that are incurred even when the animal is sitting perfectly still. Certain tissues, such as muscles, require a particularly large amount of energy, while materials such as chitin, which makes up the shells of insects, spiders and crabs, hardly contribute to the basal metabolic rate.
Crab claws typically contain both muscle, which allows the animal to snap powerfully, and chitin, which forms an exoskeleton and gives the claws mechanical stability. To understand how crabs regulate the cost of maintaining their weapons, Dinh studied the size of the claws and the ratio of energy-consuming soft tissue to energy-conserving exoskeleton in two species of pistol crab and one species of crab.
Oversized weapons made from cheap materials
“Some individuals have oversized weapons that are larger than one would expect in relation to body size,” reports Dinh. If these oversized claws had the same muscle-to-exoskeleton ratio as smaller specimens, this would require a disproportionately large expenditure of energy for the individual. Dinh therefore hypothesized that larger claws have a lower proportion of “expensive” soft tissue than smaller claws in individuals of the same species.
And indeed: “The proportion of soft tissue decreased with increasing weapon size,” says Dinh. The difference was greatest in males of the crab species Uca pugilator studied. Found along the US East Coast, this shore crab has one claw that grows much larger than the other. At a good four centimeters, it is often longer than the entire body of the animal – but the crab apparently saves on its contents: “The largest individuals invest up to 62 percent less soft tissue than the smallest individuals,” reports Dinh. The two pistol shrimp species studied showed the same phenomenon, with a 33 to 58 percent reduction in soft tissue in larger scissors. This applied to both males and females.
deter opponents
“The individuals with over-the-top claws are pretty good at fooling their opponents,” says Dinh. “Their opponents have trouble judging whether they’re bigger or stronger, or just have over-the-top claws.” And while deterring the opponent doesn’t work, the big claw can still be an advantage in combat—even without a lot of muscle. Among other things, the chitin ensures increased mechanical stability of the scissors. “In this way, the animals can not only deceive, but also improve their performance during these fights, and apparently in a very cheap way,” explains Dinh.
Source: Jason Dinh (Duke University, Durham, North Carolina) et al., Biology Letters, doi: 10.1098/rsbl.2021.0550