Sperm Wars

Publisher: Globe and Mail

Byline: Jacob Berkowitz

Source: Globe and Mail

How tight is the link between the male mind and testes? Sperm competition research is revealing that how males, including humans, think and behave can have a profound and near instantaneous impact on the quality and quantity of their sperm — and reproductive success.

The term "sperm competition" was coined in 1970 by British zoologist Geoff Parker. He was the first to report the phenomenon, a result of his work on yellow dung fly reproductive behaviour. He found that among the polygamous flies, the male flies altered their ejaculate in relation to the perceived level of competition from other males. Biologists have since documented sperm competition in dozens of animals, from bats to beetles and mice to men. It's now clear that for these species, the Darwinian struggle to reproduce continues after copulation.

In the latest report from this hot field, a team of Norwegian researchers reports that dominant, aggressive behaviour in Arctic char reduces their sperms competitive edge.

As with salmon, Arctic char return annually to their natal rivers to spawn. Males arrive first, dominant ones guarding prize territories. When the female char arrive, a dominant male fends off other fish and spawns in synchrony with the female. However, subordinate males quickly squeeze in and spawn several seconds afterwards.

The new research, by graduate students at the University of Tromsø, shows that the subordinate male fish, rather than simply capitulating, are actually saving up their energy for the struggle that really counts — the cellular-level race for the eggs. Arctic char eggs are fertilized within the first ten seconds of spawning. Thus with multiple males, it's literally an Olympic-style sperm sprint to the eggs and the evolutionary goal line. So if there are several competing males, sperm speed is everything.

The Norwegian researchers discovered that when their sample of 48 male Arctic char were initially netted, the fishes' sperm were all out of the gate with about the same speed. Then pairs of equal-sized fish were put in cages for four days and observed to see which fish, and one always did, displayed dominant behaviour, such as chasing and biting.

After this forced socializing, the researchers again measured sperm speed for each fish. Now the fastest ten percent of the dominant males sperm — literally its star sprinters — had slowed significantly, while the subordinates remained the same. Thus, even with a delayed start, the subordinate fishes sperm were fast enough to in some cases beat the early spawners sperm to the eggs.

"I find this quite remarkable," says Gary Burness, an evolutionary biologist at Trent University in Peterborough. "(We don't know) how they do this mechanistically. But this kind of information does demonstrate that what males do is very context specific."

In 2004, Burness and a team including Queen's University biologist Bob Montgomerie were the first to demonstrate that so-called "sneaker" male fish were getting a leg up on the competition by producing faster sperm. Their study showed that in bluegills, members of the sunfish family, younger sneaker males waited in the weeds until older, "parental" males were spawning and then darted out to spawn at the same time. On further investigation, the Canadian researchers discovered that the sneaker males packed sperm cells with 50-per cent more energy than the older competition, ensuring a powerful kick out of the starting blocks.

"Subordinates in nature are not less adept at surviving and reproducing, otherwise this behaviour wouldn't occur so frequently in nature," says Jonathan Vaz, a graduate student at the University of Tromsø and the lead author of the char study, published this past summer in the Canadian Journal of Zoology. "The subordinate fishes win the sperm competition because they have evolved to the extreme that they allocate all their resources directly into the sperm production."

Research with Atlantic salmon has also shown that among competing males it's not sperm length, number or longevity that matters, but it's "the fastest sperm that have the highest probability of success."

Burness notes that sperm speed is the latest on the list of sperm adjustments that scientists have found that males make, a list that includes adjusting sperm density, the percent of sperm that are swimming (motility) and the ejaculate volume.

So what about us? There's initial evidence that men are also sperm war participants — the lasting effects of what Geoffrey Parker, the father of the sperm competition says via e-mail is probably the "ghost of sperm competition past", a hold-over from our polygamous primate ancestors.

Last year, an Australian team led by Dr. Leigh Simmons, author of Sperm competition and its evolutionary consequences in the insects, found that simply the perception of rivals causes men to subconsciously adjust their ejaculate. In the research, 52 male university students "provided a sample" while looking at pornography. Half of the participants viewed images of women alone; the other half viewed images of women with two male partners. The men who viewed images involving men produced significantly fewer sperm, but more active sperm, than their counterparts — presumably a strategic trade-off of quality over quantity.
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