Men trying to look good for the ladies might pump iron, but when male mice are regularly exposed to male rivals, it results in thicker penis bones, potentially because wider girth might prove more desirable to females, a new study finds.

The intense rivalry between males over females has helped drive the evolution of many extraordinary male traits, from peacock plumes to the gigantic antlers of the extinct Irish elk. Evolutionary biologist Gonçalo Igreja André at the University of Western