The Scientist: Evidence for empathy in mice
Adults become more sensitive to pain after
watching other mice in pain, the first sign of
empathy in non-primate mammals
[Published 30th June 2006 05:01 PM GMT]
Mice who watch their peers in pain are more
sensitive to it themselves, Canadian researchers
report this week in Science -- the first evidence
of empathy between adult, non-primate mammals.
There is an "increasingly popular" view that this
kind of basic, pre-cognitive response to social
cues may be present in all mammals, said Frans de
Waal at Emory University and the Yerkes National
Primate Research Center, who did not participate
in the study. "This "highly significant
[paper]confirms that empathy is an ancient
capacity," he told The Scientist in an Email.
The response mice showed to their peers in pain
is an example of emotional contagion, according
to senior author Jeffrey Mogil. (The best known
example found in both humans and chimpanzees is
the contagious yawn.) In higher primates,
emotional contagion can progress to the more
complex behaviors commonly associated with the
term empathy, such as when a human identifies
with a friend's pain and is driven to help.
Mogil and his team at McGill University became
interested in looking at empathy in mice after
they stumbled onto an interesting pattern in a
large data set suggesting that a mouse's
sensitivity to a pain test depends on its
exposure to others that have been through the
test. The pattern suggested that mice "might be
talking to each other" about their pain in ways
that changed their response to it, he said.
In this study, the scientists injected acetic
acid into one or both of each pair of same-sex
adult mice they were studying, causing them to
writhe in pain, and allowed them to observe each
other. An injected mouse writhed more if its
partner was also writhing, but only if the mouse
had previously shared a cage with its partner for
more than 14 days.
Observing another mouse also reduced a mouse's
response to pain. When the researchers injected
the paws of familiar mice pairs with varying
doses of inflammation-causing formalin, mice
whose partners experienced less pain tended to
show less pain sensitivity (indicated by how long
a mouse licked its paw).
The researchers also found that a writhing mouse
became more sensitive to the acetic acid while
watching its cagemate deal with a different
painful stimulus -- heat. These findings suggest
that mice experience a general increased
sensitivity to pain, and don't simply imitate
what they see.
To figure out what the mice were using to
communicate pain to their similarly distressed
peers, the researchers systematically blocked
each of their senses, using physical barriers or
rendering the animals deaf or unable to smell.
They found that mice appeared to depend primarily
on visual cues to generate an empathic response
-- a surprise, since mice are known to be reliant
on smell, along with ultrasonic vocalizations to
care for their offspring. "Given that rodents
don't use visual senses muchthat was our last
guess," said Mogil. However, it's "almost
impossible" to knock out pheromone circuits,
which mice use to identify their acquaintances in
the first place, so pheromones may also be a
significant mediator of empathy, he said.
There's a practical lesson here for mouse
researchers, according to Mogil -- mice who
observe each other during experiments may be
"contaminating" the data. He added that he and
his colleagues now routinely put up an opaque
barrier between mice being tested simultaneously.
Empathy is "an evolutionary mechanism to maintain
social cohesion. If you're evolving and you're in
a group, you're more sensitive to the pain of
other members in a group," explained James Harris
at Johns Hopkins University, who did not
participate in this study.
Greater empathy between individuals who are
familiar goes back to the early evolution of
maternal care in mammals, according to de Waal.
"This may have driven initial evolution of being
in tune with the emotions of others, after which
all the fancy stuff that we associate with
empathy came into play."
However, these findings in mice hinge on how one
defines empathy, which is still under debate, de
Waal noted. "Lots of psychologists think
top-down, hence equate empathy with complex
cognition which requires introspection," he
said. "In this view, mice shouldn't have empathy."
In 2004, British researchers used brain imaging
to pinpoint the empathy center in humans. The
next step here will be to find the mechanisms
behind the phenomenon in mice, according to Mogil
and Harris. Researchers "may have avoided looking
at altruism [in rodents] because it seemed too
ridiculous," Mogil said, but these findings have
"opened our eyes [about the] abilities of rodents
in terms of social interactions."
"If it turns out that the 'empathetic' effect in
mice is mediated by the same brain mechanisms as
human empathy," Jaak Panksepp at Washington State
University, not a co-author, told The Scientist
in an Email, "then the evidence would be truly
compelling that their model actually reflects
evolutionary continuity in a pro-social mechanism
among many different mammalian species."
Links within this article
D.L. Langford, et al, "Social modulation of pain
as evidence for empathy in mice," Science, June
E. Russo, "New views on mind-body connection:
Studies into placebo effect and empathy suggest
how the brain encodes subjective experience," The
Scientist, August 2, 2004.
S.D. Preston and F.B. de Waal, "Empathy: Its
ultimate and proximate bases," Behav Brain Sci,
Frans de Waal
T. Singer et al, "Empathy for pain involves the
affective but not sensory components of pain,"
Science, February 20, 2004
Panksepp, J. "Brief social isolation, pain
responsivity, and morphine analgesia in young
rats," Psychopharmacology, 1980.
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