The basic decision-making circuitry underlying social behaviors such as fighting and mating is incredibly similar in all vertebrates, from fish to mammals, new research suggests. These networks may be 450 million years old, the researchers said.
This means that while the input (whether, for example, it is sight or smell that the animal uses to find its mate) and output (how it performs its courtship rituals) may be different, the process the brain goes through to decide to pursue a certain mate is the same in many different species of animals, the researchers said.
"How these animals make decisions about whether to fight and how much to escalate their aggression may be made at least in part on pretty similar mechanisms in different species," said study researcher Hans Hofmann, of the University of Texas at Austin.
"It does make sense when you think about it because if you think about the tasks that animals have to solve, whether it is dealing with the risk and challenges of reproductive or other kinds of opportunities, they are fairly similar across species," Hofmann told LiveScience.
The researchers examined decades of research on genes known to be involved in these social behaviors in 88 species of vertebrates — including birds, reptiles, fish and mammals — and used slices of their brains to look at the genes' expression in 12 different brain regions associated with the social decision-making network.
They analyzed this huge data set to see how similar genes expressed in this network look across species. While species within a group – say, reptiles – were expected to be similar, the researchers also found a large similarity between even far-ranging species, such as mammals and fish.
Because these networks are preserved so far back in the vertebrate lineage, they must have been there since the fish split from four-limbed animals 450 million years ago, Hofmann said.
While these processing networks seem to be very similar, the actions that come out are different. For example, some species may use their eyes to spot a mate, while others rely on pheromones, which send a signal through the nose. [Top 10 Swingers of the Animal Kingdom]
Whether it comes from the eyes or the nose, the signal that a mate is present is sent to the social decision-making network, the researchers found. This network processes the risks and rewards of mating at that time, and it signals other parts of the brain as to what to do.
If the animal courts its mate by flying, swimming or walking, different motor areas of the brain would be activated by the decision-making network. What remains the same, in all the different animals tested, was the network itself.
Humans weren't included in the analysis because not enough data on behavioral genes and samples of human brain were available to analyze. The researchers are hoping to eventually have that information and incorporate it.
"My prediction is it will be very similar to other mammals. But we don't know at this point," Hofmann said. "The human brain and human brain function didn't just start a couple hundred thousand years ago when modern humans appeared.We share a lot of our brain and brain structure with animals, and apparently this may be true at a fairly deep level."
One thing that does separate mammals from other vertebrates is the presence of the central cortex, which adds a layer of brain between the social decision-making network and behaviors. It's difficult to say how much input the cortex has into behavior, and more research is necessary to see how it impacts these behavioral decisions.
The study was published today (May 31) in the journal Science.
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