Painful truth is mice twist their faces in agony, just like humans

Researchers drew up a "mouse grimace scale" (MGS) for measuring rodent pain based on five distinct "pain faces".

They hope the findings, the first scientific evidence of facial expression of pain in a non-human species, will assist research and help prevent unnecessary suffering in lab mice.

Humans show pain through facial expressions which have been coded and used to assess the pain of patients who cannot communicate their suffering in other ways, such as babies.

A team of Canadian and Dutch scientists set out to see if a similar approach could be applied to laboratory mice, millions of which are used in scientific experiments. However, to carry out the study, they had to inflict pain on the mice.

This was done by injecting stomachs and paws with pain-inducing substances, including acetic acid, mustard oil, and capsaicin, the "hot" ingredient in chillies. Animals were also injected with a chemical that triggers bladder cystitis, and made to suffer nerve damage.

The "pain faces" were identified by comparing video images of non-suffering mice with those in pain. They were listed as: orbital tightening (eye squeezing), nose bulge – a rounded extension of skin on the bridge of the nose – bulging cheeks, ears drawn apart and back, and whiskers held against the face or standing on end.

Three expressions – orbital tightening, nose bulge and cheek bulge – were said to be "identical to those observed in humans".

The scientists, led by Dr Jeffrey Mogil from McGill University in Montreal, outlined their findings in the journal Nature Methods. They wrote: "Despite evidence that non-human mammals including rats exhibit facial expressions of other emotional states, there has been no study of facial expressions of pain in any non-human species.

"Considering the pain field's heavy and continuing dependence on rodent models and the paucity of usable measures of spontaneous (as opposed to experimenter-evoked) pain in animals, 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."

However, the study outraged anti-vivisectionists. Dr Ned Buyukmihci, veterinary adviser at the British Union for the Abolition of Vivisection (BUAV), said: "The BUAV is shocked by the nature of these experiments. These mice were subjected to atrociously painful situations without the benefit of any pain relief.

"Some were mutilated surgically, although while under anaesthesia, and then were allowed to recover without any further pain relief."