Mon. Sep 26th, 2022

Scientists from the University of Arizona founded their own “Bug Fight Club” in the lab, where they host insect wrestling matches to learn more about defense structures and the evolution of weapons in the animal kingdom. They outlined their findings in two separate papers, one published last fall in the journal Functional Ecology and the other published last month in Proceedings of the Royal Society B.

Intraspecies battles are common in nature, usually over competition for limited resources (partners, food, or shelter). According to the authors, it is widely believed that the creature that does the most damage wins the fight. That’s one possible explanation for why so many species have evolved different defense structures to protect them from damage during combat. For example, goats have skin shields, crocodiles have dorsal osteoderms, and mantis shrimps have armored telsons. But do these structures actually influence the outcome of battles?

“Biologists generally assumed that the person who inflicts more damage on their opponent has a better chance of winning a given fight,” said study co-author John J. Wiens of the University of Arizona, as he wrote the report. reasoning behind Bug Fight Club. “Surprisingly, this fundamental assumption had yet to be tested in an experimental study.”

Male and female giant mesquite insects (<em>Thasus neocalifornicus</em>) on a branch in a southern Arizona mesquite tree, their natural habitat.” src=”×426.jpg” width= “640” height=”426″ srcset=” 2x”/><figcaption class=
enlarge Male and female giant mesquite bugs (Thasus neocalifornicus) on a branch in a southern Arizona mesquite tree, their natural habitat.

Zachary Emberts/University of Arizona

So Wiens decided to design such an experiment in his lab, using giant mesquite insects (Thasus neocalifornicus† This is a species of the so-called coreid family, also called leaf-foot bugs. They have thickened forewings as defensive structures. For offense, most coreid types will attack, kick, wrap/squeeze or wrestle opponents. Co-author Zachary Emberts, a postdoctoral fellow in Wiens’ lab, likened it to a college wrestling match, in which the two critters approach and cling to each other, trying to deal damage with the enlarged spikes on their hind legs. “Think of it like a wrestling match where the opponents sneak in knives,” Wiens added.

To assess the impact of defensive structures on battle success, the team bolstered a group of insects’ protective gear by gluing small pieces of brown leatherette onto the combatants’ forewings using non-toxic Elmer’s glue. They chose that material because it was light and could withstand the type of attack typical of the mesquite bug. It was attached to the part of the front wing most likely to puncture during combat. Other insects had the extra “armor” glued to their pronotum (a plate that covers the thorax), which is not normally damaged during combat. A third baseline of bugs got no extra armor at all.

While the armor added extra weight, the authors noted that it shouldn’t be relevant to the experiment, because male mesquite insects “almost always left the fights by walking or running (rather than flying away).” , they wrote. “In addition, our previous observations in other coreids suggest that walking or running away is the most common form of withdrawal.”

To study the damage inflicted during wrestling matches between male giant mesquite insects, the researchers outfitted some of them with body armor consisting of pieces of leatherette glued to their backs.
enlarge To study the damage inflicted during wrestling matches between male giant mesquite insects, the researchers outfitted some of them with body armor consisting of pieces of leatherette glued to their backs.

Zachary Emberts/University of Arizona

The combat “arena” was a clear plastic cup with the walls lined with petroleum jelly to keep the buggy fighters from climbing out—not exactly the UFC octagon, but adequate for mesquite bug contests. The researchers placed a wooden dowel in the center of the “ring,” with a diameter comparable to tree branches, to give the insects something to fight over. For the battles, one armored beetle would be placed in the arena alone for 30 minutes. Then an unarmored rival was added, heralding the start of the trial, which lasted for the next two hours. Fights often started when both insects reached the top of the plug and usually lasted several minutes (the longest lasted 10 minutes).

The experimental observers tracked the insects’ behavior, the duration of all battles, and any attacks that caused damage to the opponent’s wing. When a mesquite bug’s wing was damaged by an opponent’s leg spike, it left a distinct hole that wouldn’t close, making it easy for researchers to count and measure the holes to assess damage to the frozen former combatants. . The fights were all videotaped and for each pair, the bug that withdrew the most was considered the loser (or “subordinate”). All bugs only fought once before being frozen for further analysis.

The team found that adding armor didn’t affect the duration of battles, but armor did increase a bug’s chance of success in battle. Bugs with extra armor were 1.6 times more likely to win battles compared to the control bugs (a 60 percent increase, according to the authors). “This tells us that damage matters in who wins the fights,” Emberts said. “This was rather a hypothesis, and it makes intuitive sense, but it hadn’t been demonstrated experimentally before.” The researchers also showed that defense structures can be an important factor in reducing damage.

A sampling of the damage done to the mesquite's forewings by the opponent's leg spikes.
enlarge A sampling of the damage done to the mesquite’s forewings by the opponent’s leg spikes.

Z. Emberts et al., 2021

Wiens and Emberts then turned their attention from defensive structures to the built-in weapons wielded by so many species. In particular, they wanted to explore the question of why weapons differ so much between species, from the pointed legs of the knife beetles — different species of leaf-foot bugs have different arrangements of the leg tips — to the horns of beetles and the antlers of deer. “Evolution has spawned an incredible diversity of weaponry in animals, but we don’t quite understand why,” Emberts said. “And if selection prefers weapons that do the most damage, why don’t all weapons look the same?”

The researchers carefully measured the shape and size of the leg spikes in 17 species of leaf-foot bugs, and they assessed the average amount of damage (the size and number of punctures) the different spiny configurations could inflict on the forewings of each species. Unsurprisingly, some weapon configurations were more effective than others, especially certain blade designs. “However, very different looking weapons can also deal the same average amount of damage,” Emberts said. “This tells us that there could be multiple solutions to inflicting damage.”

In short, “Much of the weapon diversity seen in animals fighting for resources and mates can be explained by how well different weapons perform at inflicting damage,” Wiens said. “How well the weapons perform — how much damage they inflict in combat — diversifies. This finding helps answer the question of why not all weapons evolve to look the same. Rather than evolving into one optimal weapon shape, there are very different shapes that perform almost as well, solving the mystery of why weapons look so different between species.”

DOIs: Functional Ecology, 2020. 10.1111/1365-2435.13730
Proceedings of the Royal Society B, 2021. 10.1098/rspb.2020.2898 (About DOIs).

List image by Zachary Emberts/University of Arizona

By akfire1

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