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Mice show pain in faces, study finds

May 9, 2010
Courtesy Nature Publishing Group
and World Science staff

Al­though it’s of­ten as­sumed that only hu­mans show their feel­ings through fa­cial ex­pres­sions, re­search­ers have found in a new study that mice make fa­cial ex­pres­sions that re­flect pain.

As the sci­en­tif­ic com­mun­ity in­creas­ingly re­jects en­trenched old­er views of an­i­mals as un­feel­ing, robot-like or­gan­isms, the re­search­ers are hop­ing the new find­ings will help elim­i­nate un­nec­es­sary pain in lab­o­r­a­to­ry ro­dents.

A mouse in a lab­or­a­tory maze. (Cour­tesy NIH)
The new study pro­poses a mouse “gri­mace scale” as a stand­ard­ized way to meas­ure pain based on fa­cial ex­pres­sions in mice. The find­ings are de­scribed in the May 9 ad­vance on­line is­sue of the re­search jour­nal Na­ture Meth­ods.

Squeezed eyes, bulging noses and cheeks, and changed ear and whisk­er car­riage are among the key signs of pain in mice, and the ex­tent of these changes ac­cords with the sev­er­ity of the stim­u­lus, said the re­search­ers. 

The in­ves­ti­ga­tors, Jef­frey Mogil of McGill Uni­vers­ity in Can­a­da and col­leagues, an­a­lyzed hun­dreds of im­ages of mice be­fore and dur­ing a mod­er­ate pain stim­u­lus.

“Dar­win fa­mously as­sert­ed that non­hu­man an­i­mals are ca­pa­ble of ex­press­ing emo­tion (in­clud­ing pain) through fa­cial ex­pres­sion,” they wrote. Dar­win, they not­ed, also pre­dicted that many of these ex­press­ions would be the same across spe­cies—as in­deed some of the mouse pain ex­pres­sions are si­m­i­lar to those in hu­mans.

But “de­spite ev­i­dence that non­hu­man mam­mals in­clud­ing rats ex­hib­it fa­cial ex­pres­sions of oth­er emo­tional states, there has been no study of fa­cial ex­pres­sions of pain in any non­hu­man species,” they added. “The abil­ity to re­liably and ac­cu­rately de­tect pain, in real time, us­ing fa­cial ex­pres­sion might of­fer a un­ique and pow­er­ful sci­en­tif­ic tool in ad­di­tion to hav­ing ob­vi­ous ben­e­fits for vet­er­i­nary medicine.”

Although the pub­lished study shows images of mice in pain, Mogil de­clined a re­quest from World Science for per­mis­sion to re­pub­lish those im­ages. He al­luded to the inf­lam­matory effect such pic­tures might have on ani­mal rights acti­vists, some of whom have phys­i­cally attacked re­search  build­ings in prot­est of ani­mal re­search. Sci­ent­ists ar­gue that ani­mal re­search leads to some­times life-sav­ing cures for di­sease.

Hu­man fa­cial ex­pres­sions re­flect­ing pain have been cod­ed and used for pain as­sess­ment in peo­ple who can’t com­mu­ni­cate in any oth­er way, such as in­fants, said Mogil and col­leagues. Wheth­er the same could be done for mice—which of­ten are im­por­tant mod­els for re­search in­to pain it­self—had been an open ques­tion.

The new sys­tem will al­low for ac­cu­rate pain as­sess­ment and ad­vances in pain re­search, said Mogil’s group. Pre­vi­ously, they added, sci­en­tists were largely lim­it­ed to meas­ur­ing with­draw­al re­sponses to pres­sure and heat, which mod­el only mi­nor as­pects of the chron­ic pain ex­pe­ri­ence.

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Although it’s often assumed that only humans show their feelings through facial expressions, researchers have found in a new study that mice make facial expressions to reflect pain. As the scientific community increasingly rejects entrenched older views of animals as unfeeling, robot-like organisms, the researchers are hoping the new findings will help eliminate unnecessary pain in laboratory rodents. The new study proposes a mouse “grimace scale” as a standardized way to measure pain based on facial expressions in mice. The findings are described in the May 9 advance online issue of the research journal Nature Methods. Squeezed eyes, bulging noses and cheeks, and changed ear and whisker carriage are among the key signs of pain in mice, and the extent of these changes accords with the severity of the stimulus, said the researchers. The investigators, Jeffrey Mogil of McGill University in Canada and colleagues, analyzed hundreds of images of mice before and during a moderate pain stimulus. “Darwin famously asserted that nonhuman animals are capable of expressing emotion (including pain) through facial expression,” the scientists wrote. But “despite evidence that nonhuman mammals including rats exhibit facial expressions of other emotional states, there has been no study of facial expressions of pain in any nonhuman species,” they added. “The ability to reliably and accurately detect pain, in real time, using facial expression might offer a unique and powerful scientific tool in addition to having obvious benefits for veterinary medicine.” Human facial expressions reflecting pain have been coded and used for pain assessment in people who can’t communicate in any other way, such as infants, said Mogil and colleagues. Whether the same could be done for mice—which often are important models for research into pain itself—had been an open question. The new system will allow for accurate pain assessment and advances in pain research, said Mogil’s group. Previously, they added, scientists were largely limited to measuring withdrawal responses to pressure and heat, which model only minor aspects of the chronic pain experience.