Size matters to the small but powerful mantis shrimp, which shows a clear preference for burrows in coral rubble with volumes that closely match their own body size or are just slightly larger – in other words, large enough to accommodate their bodies, but small enough to defend the entrance. But according to a new paper published in the journal Animal Behavior, sometimes a mantis shrimp will compromise. If a den is already occupied and close to ideal size, or a little smallerthe mantis shrimp will fight longer and harder for that burrow – and have a better chance of winning the match.
As we reported earlier, mantis shrimp come in many different species: there are about 450 known species. But they can generally be grouped into two types: those that stab their prey with spear-like appendages (“spear pins”) and those that smash their prey (“smashers”) with large, round, and hammer-like claws (“raptorial appendages”) Those attacks are so fast — up to 23 meters per second or 51 mph — and powerful that they often produce cavitation bubbles in the water, creating a shock wave that can serve as a follow-up attack, stunning and sometimes killing prey. Sometimes a blow can even produce sonoluminescence, with the cavitation bubbles producing a brief flash of light as they collapse.
A 2018 study found that the secret to that powerful punch doesn’t seem to come from bulky muscles, but from the spring-loaded anatomical structure of the shrimp’s arms, similar to a bow and arrow. The shrimp’s muscles pull on a saddle-shaped structure in the arm, causing it to bend and store potential energy released by swinging the club-like claw. And earlier this year, scientists found that, counterintuitively, the mantis shrimp beats into the air at half the speed, suggesting that the animal can precisely control its striking behavior, depending on the surrounding medium.
“Resource Value Review”
Patrick Green of the University of Exeter and JS Harrison of Duke University – authors of the new paper in Animal Behavior – were interested in examining what is known as “resource value assessment” in mantis shrimp of the crushing variety (Neogonodactylus bredini† Both male and female mantis shrimp in this genus are known to compete over coral debris burrows, which provide protection from predators and a safe space to mate and hatch eggs. If a preferred den is already occupied, this can cause a fight over who gets the lair. Those matches usually involve a ritualized exchange of powerful attacks (praying mantis shrimp SMASH!), with the defending mantis shrimp also using its armored tail plate to block the entrance to the burrow from intruders.
These kinds of animal competitions are common in nature, and animals seem to be able to appreciate the value of such “contested resources” and modify their behavior accordingly. Such encounters are usually described in terms of a linear or categorical valuation, where, for example, men will fight more aggressively in the presence of women. Likewise, female parasitic wasps will compete for the most desirable hosts in which to lay their eggs. For example, the larger the host, the more food is available for the offspring when they hatch. Previous studies have suggested that a female’s egg load appears to be a contributing factor (or selective force) in how aggressively they fight for a potential host and how likely they are to win such a competition.
Previous studies have shown that mantis shrimp pick burrows whose dimensions (volume) are a good fit for their own body size (mass), much like hermit crabs. In the case of hermit crabs, there seems to be a trade-off when it comes to resource assessment: towing a larger shell takes more energy but offers more protection from predators, while the reverse is true for smaller shells. Green and Harrison suggest that when it comes to competing for a desired shell, hermit crabs prefer shells of the desired size or slightly larger, while placing less value on shells much larger or smaller.
This would be an example of a quadratic resource value assessment, where resources are valued highest at a particular peak level. That value drops in both directions from that peak. In other words, there is an optimal sweet spot, or “Goldilocks zone”, where an asset is considered “just right” and the animal will adjust its behavior accordingly, for example by fighting more aggressively when such a desired asset is challenged. Green and Harrison thought that a similar quadratic resource value assessment might also apply to mantis shrimp, namely that mantis shrimp would value burrows of ideal volume and be more aggressive, and more likely to win, when they fight for control over such dens.
To test their hypothesis, the researchers conducted two sets of experiments: “choice experiments,” in which mantis shrimp could freely choose unoccupied burrows of various sizes, and “staged competitions,” where “defending” and “invading” mantis shrimp were randomly matched in a competition over a preferred cave. Green and Harrison predicted that their experiments would show that competitors would fight longer, harder and be more likely to win when their body size closely matched the volume of the contested den — and that these factors would decrease as the match deviated further from the ideal, both ways.
“This study is an example of maximum effort being reserved for something that is ‘just right’.”
The researchers built fake burrows from clear plastic tubes with a single opening, wrapped in black vinyl, with a clear area at the top so they could observe what was happening inside. The mantis shrimp were collected from burrows in seagrass beds along the Caribbean coast of Panama. The researchers also filmed the staged matches (36 in all) and intervened when it appeared that one of the fighting shrimp was in danger of serious injury or death.
They found that, in general, the occupying mantis shrimp successfully defended their burrows against invaders in 69 percent of the fights. But those odds changed dramatically in cases where the invading mantis shrimp competed for burrows slightly smaller than their ideal size; invaders won 67 percent of battles in those conditions, usually by striking first and striking more often.
“We know that animals can judge several factors, including the size of the opponent and the value of the prize, when deciding whether to fight and how hard to fight,” Green said of the results. “In this case, since a smaller burrow is likely to be occupied by a smaller opponent, it seems that praying mantis shrimp will compromise on house size if it means an easier fight. It can be assumed that animals fight the hardest for the greatest assets, but this study is an example of maximum effort being set aside for something that is ‘just right’.”
There were some caveats, especially sample size limitations. Green and Harrison also recognized that the fake burrows were standardized, with fixed lengths and diameters, unlike naturally occurring burrows, which tend to have more variable dimensions. And the smooth tubes are distinctly different from the natural burrows formed in rock and rubble.
“Mantis shrimp are adept at modifying natural burrows, using appendage attacks to widen burrows that are too narrow, and using rock and sand to fill in burrows that are too large,” they wrote. “Although the individuals we tested could not widen the fake dens by hitting, perhaps with more time to establish their abode, individuals would have filled in larger fake dens.”
DOI: Animal Behavior, 2020. 10.1016/j.anbehav.200.09.014 (About DOIs).
List image by Roy Caldwell