Hitting the treadmill voluntarily everyday is probably not everybody's idea of fun. However, for some house mice – and even humans – exercise seems to be an almost addictive activity. Clearly, these individuals are gifted with astonishing amounts of motivation and/or ability. These attributes, especially motivation, are controlled by the brain and nervous system, and increased brain size has been suggested to increase the ability to undertake and endure exercise. Bigger brains have also been linked with more complex and fascinating behaviour. This all begs the question – does selecting for mice with this intriguing ‘gym bunny’ behaviour also drive an increase in brain size?

Luckily, for nearly 20 years Theodore Garland and his colleagues at the University of California, Riverside, USA, have been selectively breeding exercise-loving mice and were able to start answering this question (p. 515). However, when the team analysed the total brain mass of these athletic mice they found no change in mass compared with their couch potato relations. Undeterred, the team decided to investigate further. They carefully dissected brains into two different regions, the cerebellum and non-cerebellar areas, and weighed these sections separately. This time, there was a difference. Surprisingly, however, the increase was not in the cerebellum – a region of brain crucial for controlling movement. To pinpoint the exact area in the non-cerebellar regions that was enlarged, the team turned to high resolution imaging to determine the volume of individual components. Interestingly, they found an increase in one specific area – the midbrain region – that occupied up to 13% more volume in the exercise-loving mice.

Whilst the cerebellum is important for co-ordination, the midbrain, as the investigators point out, is also important in controlling various other tasks. This region is essential for reward learning, motivation and reinforcing behaviour. Could it be that these active mice have a heightened reward system that motivates them to exercise? Alternatively, there is evidence that the midbrain is also involved in some aspects of controlling movement. Either way, it is clear that willingness to exercise can evolve from the enlargement of just one specific area of the brain rather than the whole structure.

E. M.
E. L.
S. K.
Mice selectively bred for voluntary wheel running have larger midbrains: support for the mosaic model of brain evolution
J. Exp. Biol.