This article in The Scientist describes a paper where the authors claim to have found empathy in mice. The problem is that what you define as empathy may be more a matter of semantics than of science:

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.

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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 much...that 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." (Emphasis.mine.)

The question for me about whether mice have empathy comes down to how the authors are choosing to define empathy.

If your defintion is that empathy is any commensurate change in behavior response to the percieved feelings of the other animal, then, yes, I would agree that the mice are feeling empathy. This model would hold empathy as sort of a sensory replay where you see someone else feeling something and you feel it yourself, as shown by the change in sensitivity to pain.

On the other hand, if you define empathy as the ability to abstract another individual's point of view from sensory data about them, then I don't think this proves mice have empathy. It is clear that mice do not perform this level of abstraction. Mice could not, for instance, feel empathetic about another mouse from a story they heard about it.

This is one of those arguments that you sometimes see in neuroscience that comes down to semantics. Do monkeys have technology? Well they can use sticks to get termites, does that count? Yes and no. It depends on how you define technology.

The point is that regardless of where you draw the line for technology beginning or ending, the level of complexity in humans is significantly greater than that of primates. While we can be comfortable in the evolutionary origins of the technological instinct, Homo sapiens still represent a clear advance.

Same deal with mice and empathy. I can totally understand how it would benefit a mouse to have a sensory replay for the feelings of other mice -- social cohesion and such. If you want to call this empathy in a rudimentary waym then great. But it doesn't change the fact that human empathy requires abstraction and complex cognition.