A new study involving injured squid and hungry sea bass may help explain why we are so grumpy and irritable when we are in pain.
For many of us, the unpleasantness of being in pain often goes beyond the agony of the injury. If we are in excruciating discomfort, suddenly it seems everything bothers us -- sounds are too loud, lights are too bright, and even a gentle touch can be uncomfortable.
"One of the effects of pain is the peripheral sensory system becomes hyperactive," said Edgar T. Walters, who studies pain and neural plasticity at the University of Texas Medical School at Houston. "People in pain are very easily irritated and we found that this fits in with a primitive pattern designed for an animal to be extra-vigilant."
Walters is the senior author of a new study in the journal Current Biology that looks at how squid react after having the end of one of their arms snipped off -- a relatively minor injury by squid standards.
The researchers put four injured squid in a tank with four squid predators -- in this case, sea bass. Then they compared the injured squids' behavior to four non-injured squid who were put in the same predator-heavy conditions.
The injury was subtle enough that it didn't effect the squid's ability to swim or maneuver in the water. However, the researchers found it did affect the behavior of the squid.
"The injured squid were really touchy," said Robyn Crook, an evolutionary neurobiologist at the UT Medical School at Houston who led the study. "They responded more strongly to visual stimulus than normal squid. So an encounter a normal squid might just want to keep an eye on caused the injured squid to start up their defense mechanisms."
Squid are the perfect animals to do this type of study on because their defense mechanisms are very specific. Their primary defense is to change color, either to a tan or to make light and dark stripes appear on its body. If they continue to feel threatened they stop swimming, then they swim quickly, and then they release ink.
The researchers also noticed that the sea bass were much more likely to hunt the injured squid. Even though the researchers could not discern any difference in the swimming ability of the injured and non-injured squid, the sea bass clearly could.
"Obviously the fish are evolved to detect the most vulnerable prey, which goes to show just how costly a really minor injury can be to an animal," Crook said.
In another experiment, the researchers anesthetized the squid before snipping its arm. These squid were not hypervigilant, but they were still more attractive to the sea bass, and therefore, they were more likely to be eaten than the injured squid who had not been anesthetized and were super sensitive to any perceived threat.
It seemed the hypervigilance of the injured squid served a useful purpose, leading the researchers to conclude that there is an evolutionary benefit to that hypersensitive sensory state we often find ourselves in after an injury.
"It is not an accident we feel really awful after we are injured," said Walters. "Those sensory changes are really important for increasing survival under conditions where an animal is extremely vulnerable."
So the next time you are around a friend or family member in pain who is complaining about the slightest disturbance in their environment, remember this: If they were in the wild, their hypersensitivity to light, sound and touch might keep them from getting eaten.