Polyandry in animals

Polyandry in animals is when one female mates with several males in a breeding season. These males raise their own progeny without any help from females.
It is much less common than polygyny in mammals. It does occur in some primates, such as marmosets, and other members of the New World monkey family Callitrichidae. 
Another example is in those fish which store sperm for months. This tends to set up a kind of fitness competition between sperm. The system is called "cryptic sperm competition". 
Male "pregnancy" is found in seahorse, pipefish and sea dragons.