One fish, two fish, dumb fish, dead fish

In the war for survival, it pays for males to choose their battles wisely, especially when food and females are involved. The poor guy who makes the mistake of picking a fight and losing is unlikely to attract many mates. For animals, the ability to determine one’s rank among competitors without direct contact reduces needless fighting and wasted energy. Now, Stanford scientists say fish are capable of deducing how they stack up against the competition by simply watching fellow tank-mates duke it out.