When females shop around, sperm evolve to be faster and fitter in just about every way.
It鈥檚 known, for example, that sperm from promiscuous chimps move faster than those from relatively monogamous gorillas, but no-one knew if this pattern was common among animals. Studies in other species have produced conflicting results.
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To settle the issue, of McMaster University in Hamilton, Ontario, Canada, and colleagues studied a variety of species of cichlid fish living in Lake Tanganyika in central Africa.
Cichlids form a major family of fish, with species employing the full range of mating behaviours 鈥� from monogamy to 鈥渟perm shopping鈥�, in which a female gets several males to ejaculate into her mouth, which is where she carries her eggs.
Watch a video of sperm shopping here
鈥楽uper sperm鈥�
The researchers ranked 29 species of fish from zero to four according to how promiscuous they were, and recorded sperm characteristics such as size and speed.
They corrected for the degree of relatedness between species, which might otherwise suggest a relationship between mating behaviour and sperm characteristics that did not exist, by constructing a 鈥渇amily tree鈥� from cichlid mitochondrial DNA sequences.
The team found that, the tougher the competition among sperm, the faster, bigger, more numerous, and longer-lived they become.
Sperm of strictly monogamous fish were small and slow, at around 50 microns per second. 鈥淏ut as you move up the scale, species have more competitive ejaculates, with the most promiscuous producing 鈥榮uperman鈥� sperm 鈥� they were almost twice as fast, larger, there was more of them, and they lived longer,鈥� says team member John Fitzpatrick of the University of Western Australia in Perth.
Surprise result
鈥淚t demonstrates for the first time that sperm traits such as speed and size can evolve together, and that those traits are related to the risk of sperm competition with rival males. Elegant,鈥� says of the University of Wollongong in New South Wales, Australia.
Sperm competition theory predicts a trade-off between sperm traits, so the finding that all traits improved was a surprise, says Fitzpatrick. Presumably, another trait unrelated to sperm is being adversely affected, he says. For example, a fish making more sperm might have less energy to spend on muscle formation.
The Balshine team also used the fish family tree to work out how the traits evolved over time. They found that the ancestral species was monogamous, with tiny gonads, and small sperm, and that more promiscuous behaviour preceded the evolution of more competitive sperm.
Journal reference: (DOI: 10.1073_pnas.0809990106)
