When the male penguin of a mated pair returned to their nest to find another male, he went on the offensive—with disastrous consequences for both male birds.

A Nat Geo Wild video showing the intense battle went viral Friday, resonating with hundreds of thousands of viewers.

Penguins are known for fidelity to their mates. An average of 72 percent of penguins return to mate with the same bird as the previous year.

This usually stems from “nest-site fidelity,” in which males return to the same nest site, and hope females will return as well.

For some species, a short three-week mating period starts in late October. At this time, males and females hope to return to their nests together.

Sometimes this plan goes awry.

“There are penguin breakups, but the reasons are often strictly logistical,” aviculturist Laura Dray explained in the video below.

The male of a mated pair will sometimes return after the female to find his mate has moved on.

Penguin “divorce,” when a bird chooses another mate, was only a secondary factor in mate change (death was the primary factor), at just 26 percent of separations.

The predictability of penguin mating has been challenged in the past. Scientists have observed female penguins turning to "prostitution" to help build their nests, offering themselves to male penguins in exchange for rocks.

When engaging in this behavior, female penguins usually target single males, so as not to be attacked by other females—unlike the male penguin in the video.

The females will occasionally take their payment and run, failing to complete the mating ritual with the new male and running back to their nests with their rocky prize once they have made their offer.

The world of penguin mating involves complex social structures, and when it goes wrong, can get bloody.