Why Do We Love Our Pets?

VIDEO: Two women go to extremes for their pet chickens and snakes.

There's a deep-rooted reason why we humans surround ourselves with dogs and cats and other animals. New research finds we are hardwired to respond to other critters, and the mechanism that makes us do that probably dates back hundreds of millions of years to the time when vertebrates were first evolving.

The secret is buried deep in a very old part of the brain, called the amygdala, long recognized as the seat of emotional reaction. Scientists at the California Institute of Technology and the University of California at Los Angeles were able to measure brain activity in 41 persons on a cell-by-cell level and found that neurons in the amygdala became extremely active when participants were shown pictures of animals.

"Our study shows that neurons in the human amygdala respond preferentially to pictures of animals, meaning that we saw the most amount of activity in cells when the patients looked at cats or snakes versus buildings or people," Florian Mormann, lead author of a paper and a former postdoctoral scholar at CalTech said in releasing the study. The paper was published online in the journal Nature Neuroscience.

Mormann, who is now at the University of Bonn in Germany, said in an email that he and his colleagues were a bit surprised to discover that the type of animal -- whether it be a deadly snake or a cuddly puppy -- made no difference. They found the same level of brain activity for all animals.

"Given the amygdala's prominent role in fear conditioning, we were indeed expecting stronger responses to hazardous animals such as spiders or snakes, but it turned out that all kinds of animals, dangerous and cute, elicit responses in the amygdala," he said.

"This preference extends to cute as well as ugly or dangerous animals, and appears to be independent of the emotional contents of the pictures. Remarkably, we find this response behavior only in the right and not in the left amygdala."

That's significant because it supports other research indicating that early in vertebrate evolution the right hemisphere of the brain took on the role of responding to environmental stimuli, including the presence of other animals, which could be either prey or predator.

"This is somewhat in line with recent findings that the amygdala is also involved in reward processing and generally mediates vigilance," Mormann said.

The intense brain activity triggered by photos of animals "may reflect the importance that animals held throughout our evolutionary past," the study notes. It also suggests that early vertebrates, and by extension humans, really needed to know if there were other animals around, some of which could be eaten, and some of which could do the eating.

The researchers were able to carry out their work because 41 epilepsy patients at the Ronald Reagan UCLA Medical Center were willing to participate. They were already being monitored for brain activity related to seizures. So the researchers could use electrodes that were already in place to record single-neuron responses in the amygdala as the participants viewed pictures of animals, people, landmarks and other objects.

They also monitored two other areas of the brain, the hippocampus and the entorhinal cortex, as control regions because they "weren't expecting any stimulus preference there," Mormann said via email. As expected, there was no significant response in the two control regions, but the response in the amygdala was almost off the charts.

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