Competitive Synchronized Swimming in Mouse Sperm

 Competition in the natural world is not a novel concept.  It is well-studied in biology, and well-televised by Discovery Channel.  The first kind of competition that many people think of (or at least, the favorite of Animal Planet) is the flashy, mano a mano combat of wild animals fighting for a mate.  

[1]

This is clearly not the only kind of biological competition.  We know it shows up in less exciting contexts: like plants beating each other out for sunlight, or between two species trying to eat the same food.  The sexy fighting example is useful, though, because it reminds us that competition is about being the most fit.  And being the most fit is about having the most sex just as much as it is about surviving and eating.  Without kids, your impact on the next generation's gene pool is zero.  (Even if you lived to be 110 by eating nothing but kale.)

[2]

Just mating may not guarantee you offspring, though.  In some species, the competition doesn't end at ejaculation.  Research in the field of behavioral ecology is looking at competition among sperm.  Is there pressure for males to have the 'best' sperm?  A new study published in the Proceedings of the Royal Society B indicates there may be. 

Researchers looked at the sperm of two species of mice: one that is generally monogamous (the oldfield mouse), and one within which females mate with many males (the deer mouse).  Presumably, the sperm of males of the promiscuous species is subject to more sperm-on-sperm competition than those of mice that have a dedicated partner.   

Oldfield mouse [3]

Deer mouse [4]

Does this show up in their ejaculate?  Does the deer mouse have some sort of buff super-sperm as a result? 

[5]

Here's a side-by-side comparison;  oldfield mouse spermatozoon on the top, deer mouse speratozoon on the bottom.  Not seeing anything too impressive (or even anything different between the two). 

The hooks are pretty interesting, though.  The heads hook together, allowing the spermatozoa to travel in clumps.  Mice are not the only animal to employ this technology.  Sperm 'bundles' have been observed in many other species, including the fishfly, rats, and the short-beaked echidna.

[6][7][8]

The first order of business (after taking pictures) was to better understand this clumping.  Does it confer a competitive advantage?  To do this, the researchers formulated a mathematical model using physical motion laws to describe how the spermatozoa clumped and unclumped in the dynamic environment of the mouse vagina. 

The output from this model suggested that, up to about six, more spermatozoa in a bundle meant less time for the sperm to get from point A to point B.  When compared to observations of the actual mouse sperm under the microscope, the pattern held up.  Bundles of about seven got from A to B the fastest.  Video capture revealed that both the oldfield mouse and the deer mouse have sperm that employ the seven-gets-there-fastest strategy. 

So what about the competition?  Is there any evidence that deer mouse sperm have been pruned to swim-to-win once inside the vaginal canal?  This figure suggests so:

[9]

The deer mice sperm are a little bit faster than their less-stressed oldfield brothers.  But, once again, the best evidence is in the clumping: this time, of data points, not the sperm. 

[10]

The variance in deer mouse sperm bundle size is much lower than in oldfield mouse sperm bundle size.  This aspect of the data says more about competition to the researchers because it looks like stabilizing selection.  Although both species use, on average, the optimal bundle strategy (about 6-7 sperm per bundle), more of the deer mouse sperm 'get it right'. 

[11]

Oldfield mice males aren't under much pressure after ejaculation; in a monogamous society, their sperm aren't likely to be competing with the sperm of other males once inside their female partner.  Therefore, it's not critical that their sperm swim in the fastest possible arrangement.  Deer mouse males, on the other hand, can't be sure that one of their sperm will inseminate an egg even after it's been deposited in the female's vagina.  The female deer mouse may have just mated with another male, and/or be on her way to mate with more.  The presence of other males’ sperm makes it imperative that every spermatozoon uses the best strategy to avoid losing out on inseminating an egg. 

[12]

Over time, deer mouse sperm have honed the best practice for swimming in the competitive environment of the deer mouse vagina.  Promiscuity in deer mouse societies elicits real differences in sperm fitness, and effects of generations of sperm-on-sperm competition can be seen in microscope video of plated mouse ejaculate.

Photo Credits

[1] Photo: Mike Baird http://en.wikipedia.org/wiki/File:Elephant_seals_fighting.jpg

[2] Photo: Karl Bulla http://commons.wikimedia.org/wiki/File:Bulla_Hercules.jpg

[3] Photo: Jackie Isaacs http://commons.wikimedia.org/wiki/File:Peromyscus_polionotus_ammobates.jpg

[4] Photo: 6th Happiness (Wikimedia) http://commons.wikimedia.org/wiki/File%3ADiGangi-Deermouse.jpg

[5] Figure 1 from http://rspb.royalsocietypublishing.org/content/281/1790/20140296.full

[6] Photo: goldentakin (Flickr) http://commons.wikimedia.org/wiki/File%3AFishfly_-_Chauliodes_sp_Family_Corydalidae%2C_Subfamily_Chauliodinae.jpg

[7] Photo: AlexK100 (Wikimedia) http://upload.wikimedia.org/wikipedia/commons/f/fd/Fancy_rat_blaze.jpg

[8] Photo: Periptus (Wikimedia) http://commons.wikimedia.org/wiki/Tachyglossus_aculeatus#mediaviewer/File:Tachyglossus_aculeatus_-_Anstey_Hill_-_profile.JPG

[9] Figure 4 from http://rspb.royalsocietypublishing.org/content/281/1790/20140296.full

[10] ibid.
[11] Image: Azcolvin429 (Wikimedia) http://commons.wikimedia.org/wiki/File:Selectiontypes-n0_images.png#mediaviewer/File:Selectiontypes-n0_images.png

[12] Video: NPR Science https://www.youtube.com/watch?v=rk3S7zz2TiM

Journal Reference

Fisher, H.S., Giomi, L., Hoekstra, H.E., Mahadevan, L., 2014. The dynamics of sperm cooperation in a competitive environment. Proc. R. Soc. B 281, 20140296. doi:10.1098/rspb.2014.0296