Animals compete in all sorts of ways, even right down to the cellular level. Sperm competition became a prominent idea in ethology in the early 1980s, driven by theoretical considerations coming from sociobiology and the technical innovations of early DNA fingerprinting. It’s been long enough that you’d expect some fairly clear generalizations about sperm and genetics in the context of sperm competition, but Mossman and colleagues indicate there’s still work to do.
Mossman and colleagues argue that when you look across species and compare them, long sperm are faster – and you’d predict sperm competition would favour fast sperm. When you look within a single species, they claim, the pattern is not so obvious. They decide to look at this in zebra finches (Taeniopygia guttata), a well studied bird.
They had a colony of birds in which they knew all the relationships (for the genetics), they got sperm samples to look at the morphology of known individuals, and to measure the speed of the sperm.
The measurements of sperm are actually fairly complex. For instance, which sperm do you measure? They measured the speed of all sperm, the top 20%, and the fastest single one they measured. They took all those measures and ran them through stats to get a single value that incorporates information from all three measurements. Similarly, the authors didn’t just measure sperm length, but several features.
When you correlate all those features, the longest sperm turn out to be the fastest ones.
But in order for either sperm length or speed to be important evolutionarily, it has to be heritable. This involved looking at the relationships of hundreds of birds and linking those with the sperm characteristics they’d examined, and sperm length and speed are very heritable. Of course, since each is correlated with the other, heritability would be a package deal.
Nevertheless, as every researcher knows, correlation does not imply causation. Follow-up research might focus on an experimental test of the relationship between sperm length and speed. One way might be to see if breeding experiments, perhaps using an animal that’s a little faster breeding than zebra finches, could generate fast-and slow-swimming strains.
There’s also a prediction that males with large, fast-swimming sperm should have a selective advantage over other males if females are able to mate with multiple males. This seems to be true in “domestic fowl,” as the authors put it (I think that means “chickens”), but testing in zebra finches would be a logical thing to do.
Mossman, J., Slate, J., Humphries, S., & Birkhead, T. (2009). SPERM MORPHOLOGY AND VELOCITY ARE GENETICALLY CODETERMINED IN THE ZEBRA FINCH Evolution DOI: 10.1111/j.1558-5646.2009.00753.x
Picture by user marj k on Flickr, used under a Creative Commons license.