One of them must give way. In their earlier encounter, the first mouse exerted its dominance by forcing its rival to reverse down the tube. This time, things are different; the second mouse pulls rank and the first one backs down.
Mouse hierarchies don’t change this readily, but the second mouse has been given a boon by Fei Wang at the Chinese Academy of Science. By injecting a single gene into one part of its brain, Wang turned the subordinate animal into a dominant one.
The gene that gave the mouse a burst of social mobility is GluR4. It creates part of a protein called the AMPA receptor, which allows signals to flow quickly between two neurons. By injecting extra GluR4 into a mouse’s brain, and producing more AMPA receptors, Wang strengthened the connections between its neurons. The effect is like building expressways between two cities overnight – you can have a much larger and faster flow of traffic between them. [...]
By manipulating this signalling, he could push mice up or down the social ladder. With an extra dose of GluR4, the mice gained social standing. When they confronted other mice in a cramped plastic tube, they were more likely to force their rivals to retreat, even if they had previously given way. With their new rank, they were also more likely to court female mice with high-pitched ultrasonic songs.
Two mice meet in a narrow plastic tube that's not wide enough for both of them. That sounds like the opening of a really bad science joke, but stick with me. The punchline is downright amazing: