A sperm's quest to fertilize an egg is not easy. Of the roughly 250 million sperm ejaculated into the human vagina during intercourse, fewer than one in a hundred will survive the perilous trek up the hostile, acidic chamber to the cervix. If the female is set to ovulate soon, the cervix will allow the frisky, foreign interlopers inside. If not, they'll "drown" in a thick flow of cervical mucus. Sperm that enter into the cervix must next decide to go left or right – for a human female has two fallopian tubes with an ovary at each end, and usually just one releases an egg. Perhaps a hundred sperm will enter a fallopian tube, and maybe one or two will actually reach the egg for a chance at fertilization.

In this arduous race, all of the sperm independently race towards the same goal: reproduction. But what if the competition was more cutthroat?

In the 1990s, biologist Robin Baker put forth the idea that a significant proportion of human sperm are not actually capable of fertilization and instead only serve to block or attack sperm from rival males potentially present inside the female. These "kamikaze" sperm probably also exist in other primates as well as many other promiscuous animal species, he hypothesized.

But an experiment published in 1999 countered Baker's claims. Scientists Harry Moore and Tim Birkhead mixed sperm samples in settings similar to the female reproductive tract and watched if the individual cells went to war. The sperm flit about furiously in search of an egg, but there were no signs of microscopic violence. The findings shut the door on the "kamikaze sperm" hypothesis in humans. It remains open in other species, however.

Honeybees, for example, provide a prime arena for sperm combat. Before producing more than a million offspring, queen bees mate with up to twenty males over a matter of hours.

"The queen's body is an arena where sperm are allowed to fight it out for a while," Jacobus Boomsma, a Professor of Biology at the University of Copenhagen, told Nature in 2010.

Here, however, the sperm aren't actually fighting, but the seminal fluid is. Seminal fluid, also known as semen, is filled with enzymes, proteins, and sugars, providing a nurturing medium in which sperm can swim. In honeybees, however, seminal fluid from rival males seems to be toxic to foreign sperm. Floating proteins mercilessly attack unknown cells.

There is one group of animals in which genuine sperm warfare might actually occur: Lepidoptera. Butterflies and moths make up this order of insects, and males of almost all known species contain two different types of sperm called eusperm and parasperm. Eusperm are at least 50 percent longer and contain a nucleus, while parasperm lack a nucleus, and thus contain no genetic information. The former is capable of fertilizing an egg, while the latter is not. So what is the function of parasperm?

Biologists John Swallow and Gerald Wilkinson tried to answer this question in 2001 with a review of the scientific literature. Previously, other researchers suggested that parasperm might serve as "cheap filler," lingering in female moths and butterflies to delay them from becoming receptive to other males. Parasperm might also function as "blockers," getting in the way of rival males' sperm to prevent them from reaching the prized eggs. Lastly, and least likely, parasperm might actually attack rival sperm outright.

Swallow and Wilkinson found no consistent evidence from the published literature to clearly support a single hypothesis for the function of parasperm, but the "blocking" mechanism seemed to be the most likely.

Unfortunately, research on the topic has been fairly scant for the past twenty years, so we haven't learned much more on the matter of sperm combat.