One female will mate with a number of males. There can be even up to 8 males at a time trying to mate with one female. During mating, there is a lot of activity on the surface (splashing, pushing, shoving, large and frequent blows). The males producing the most sperm is probably the father of the calf. This mating strategy is known as sperm competition.
Surprisingly, the testicles of the right whale are likely to be the largest of any animal. Each weighs around 500 kg. At 1% of its total body weight, this size is very large even taking into account the size of the whale. This suggests that sperm competition is important in the mating process.
The same phenomenon comes into play with some species of bats too:
For some male bats, sexual prowess comes with a price ? smaller brains. A research team led by Syracuse University biologist Scott Pitnick found that in bat species where the females are promiscuous, the males boasting the largest testicles also had the smallest brains. Conversely, where the females were faithful, the males had smaller testes and larger brains.
More here: http://dml.cmnh.org/2006Jan/msg00359.html
Sperm competition is reasonably common in nature.