The heavier the half of the body of the handcuff

For the handcuffs (Uca crabs), the more important the role is.

The bigger the size, the more advantage they have in attracting their partners . The two more of the male cuffs are very different in size.

The smaller one is used to search for food, the larger the body is about half the size of the body, used as a weapon to fight other males and as a tool to appeal to partners.

Scientists have experimented on a robot, the results show that females are attracted to males with "waving" or "tapping" fast and strong dance.

Shackles also prefer males with larger females because it shows that the males are more mature and have a larger nest where females can lay eggs.

So, in the battle for a partner, the male with the biggest and most moving will win.