Jake Young is a 
It would appear that today will be Sex Day at Pure Pedantry. So be it.
I didn't know this but mouse and rat sperm have funny shaped hooks at their tips. To the right is a picture of sperm from a variety of mouse and rat species (click to enlarge).
A) Variation in hook design across nine murine rodent species: (1) Bunomys fratrorum, (2) Mus musculus, (3) Rattus norvegicus, (4) Dasymys incomtus, (5) Pseudomys oralis (6) Maxomys surifer, (7) Melomys burtoni, (8) Apodemus sylvaticus, (9) Apodemus speciosus.
Interesting, but scientists didn't know what this significance of the hooks were. However, Immler et al. publishing in the journal PLOS One may have found the answer.
It turns out that the hooks cause the sperm to cluster in groups together while they swim. The researchers show that at least for the Norway rat, group swimming is faster than swimming alone.
Why would they want to swim together? Aren't the sperm competing? Well, in situations where females are mating with a single male, the sperm are competing and should not want to swim together. However, in situations where the females is mating with multiple males, the researchers speculate that selection will favor cooperation among the sperm. The sperm are related to each other with 50% of their genetic material. This means that they are essentially siblings. Normally, we would expect them to compete. However, when the selection pressure is enough it may force them to behave cooperatively.
In their words:
Sperm competition may be divided into (i) the competition between sperm of rival males (inter-male sperm competition due to diploid selection), and (ii) the competition among the sperm from a single male's ejaculate (intra-male sperm competition due to haploid selection)...If sperm cooperation is costly to some sperm and beneficial to others sperm within one ejaculate might compete for the benefiting position. Therefore, if sperm cooperation increases the fertilisation success of a male in sperm competition, diploid selection is expected to drive the evolution of sperm cooperation, whereas haploid selection opposes sperm cooperation if cooperation is costly. The genetic relationship between the sperm of one male is 0.5 which is the same relationship as between full siblings. Therefore, Hamilton's rule for the evolution of cooperation applies and sperm cooperation may still evolve despite haploid genetic influences if the selective pressure (e.g., due to high risk of sperm competition) is intense enough. (Emphasis mine. Citations removed.)
More information on Hamilton's rule here.
Here is an awesome video of rat sperm swimming in groups. If you look closely, the groups move faster than the individual sperm.
Hot.
Hat-tip: Eurekalert.
