Sexy Science: The Weekly Scientist on Sperm Wars

When it comes to sex, devious trickery is aplenty. Whether its killer semen, or barbed, multi-tasking penises; you name it, the randy male has it in his arsenal — truly, sadomasochism as only Mother Nature could have invented it. Take a look at the female fruit fly for example. It’s an established fact that Drosophila melanogaster females with higher mating rates die younger than their more celibate counterparts. For decades, scientists have been watching the promiscuous ones dropping, well, like flies. And you wouldn’t believe it, but apparently, it has all to do with the male’s sperm.

After investigating the fruit fly’s sexual troubles, biologists Linda Partridge and Kevin Fowler suggested that there was more to Drosophila’s seminal fluid than met the naked eye. They proposed that these so-called ‘accessory gland secretions”’were unintentionally laden with toxic chemicals. Unintentionally, because their original objective was to increase the female’s egg production and to reduce her receptivity to any potential love-rivals. The dying part just ended up coming in an unwitting package deal.

Now, this theory has gained worldwide acceptance. Molecular analyses have yielded a lot of supporting results. One chemical which has been discovered in the fruit fly’s ejaculate, ACP 26a, partly resembles the hormone which sea slugs produce to promote egg-laying. Then, there is ACP 62F, which looks like a toxin of the Brazilian armed spider. All in all, it sounds like Partridge’s and Fowler’s theory might not be that far off the mark.

Sperm are out to fertilise an egg no matter what, and that just means that there are no rules. It’s not so much a barbaric skirmish with everyone fighting everyone else. It’s a war, with bloodthirsty patriarchs, dispensable underlings and the occasional, undesirable armistice. Sperm from different males can resort to right-out bioterrorism if they find themselves bumping into each other; and they quite often do inside the female tract. To illustrate, we have the leafcutter ant queen, Atta colombica, who stores the ejaculate of each of her suitors in a special organ known as the spermatheca. Over the queen’s lifetime, these are used to give rise to offspring from multiple fathers. Yet, the sperm inside the spermatheca don’t just float around waiting for their turn to do the fertilising.

To ensure that his sperm get to fertilise the most eggs, the male leafcutter ant secretes seminal molecules that both nourish his own sperm and destroy anyone else’s. This assertion was made by scientists when they discovered that mixing together the accessory gland secretions from two different males lead to an overall lowered sperm count in the mixture. On an evolutionary level, this makes perfect sense. Selfish genes come into play here. Every male wants to pass on the most genes, and because insects have little means of keeping a libertine female in check, they’ll have to get good destroying the opposition.

The female isn’t just a bystander in this, of course. Because she has as much a vested interest in passing on her genes as the other guys, she wants to keep all of her precious little sperm intact. So, to avoid ending up with a bag full of dead spermatozoa, the ant queen produces chemicals that neutralise the negative effects of the different male fluids. Once again, it’s the women who do the peacekeeping while the men are going out and killing each other.

Still, if there is one thing this war doesn’t lack, it’s weapons. Male seed beetles, Callosabrochus maculatus, put a new spin to the sexual connotations of ‘hardcore’. These critters are the proud owners of a spiky, hard-shelled penis which leave the females seriously bruised after a round of violent copulating. Worryingly, it’s the beetles with the longest spikes on their penis that end up fathering the most young.

There are two main theories as to why the male seed beetle might have evolved such grotesque and unsightly genitalia. The first one needs a little bit of anthropomorphic stretching, so do keep an open mind: perhaps the purpose of this penis is to injure his partner to the extent of deterring her from mating with anyone else again. It sounds logical, in that twisted, sociopathic way that we scientists sometimes think, but we can’t forget that the female needs to stay alive to lay her eggs. There is no scope to how much damage these penises could inflict. Hence, the risks that come with this savage mate-guarding tactic are far too high for it to have come about on such terms.

Another more sensible theory goes back to sperm rivalry. Like most insects, female seed beetles are likely to have mated with different males within a short span of time. Maybe, the spikes are a feat of evolutionary craftsmanship; rather than hurting the female, these are supposed to ‘scoop out’ anyone else’s sperm from inside the female tract. By shovelling out the competition, the last male to mate with a female has the clear upper edge when it comes to fertilising her eggs. If this theory is correct, then the advantages of this clunky, sperm-displacing sex perfectly justify the casualties (injured female). It’s a no-holds-barred approach to procreation that leaves you feeling a little sympathetic for Mrs Seed Beetle.

The field of entomology is now prolific for its sexual tacticians, but it’s not just insects who show no scruples when it comes to prolonging their selfish genes. Even nature’s monogamist, the bird, is only out for him or herself on the evolutionary playing field. Approximately 10-20 % of a songbird’s brood are said to spawn from fathers outside a committed pair bond, which explains why, during a female’s fertile period, male songbirds become visibly more protective of their partners. They can’t afford to let them out of their sight, lest somebody else swoops along and fertilises what is rightfully theirs. Amongst the raptors, where hunting schedules make this twenty-four hour surveillance strategy unfeasible, males resort to a simpler option to avoid getting ousted by renegade sperm: they mate more frequently with their partners in a bid to ‘outdo’ any other sperm that might have been deposited inside their female.

Indeed, no animal goes untarnished in this war trial; even in human relationships, there is an element of sperm competition. It’s nothing as drastic as chemical weaponry, but our sexual behaviours are often the result of our reproductive ambitions, such as the amount of sperm a man produces or how often he copulates with his significant other. Some women are even said to ‘shop around’ for better sperm if their current partner isn’t up to the job. How else do you explain rates of extra-pair copulation (or in layman’s terms, cheating) ranging from as far as 5% to 27%, and just for the under thirties? Or the fact that in the USA, ca. 1 in 400 twin births are fathered by two different men? Divorce lawyers are hitting it big in this day and age, but take solace from the fact that, adaptation-wise, we aren’t as equipped for a busy sex life as some of our primate friends.

On a scale of primate testes size, humans sit about midway between the lascivious chimpanzee and the monogamous gorilla. They might not look like it, but male chimps can ejaculate every one of five hours, which only slashes their sperm count in half. Compared to that, if a human man were to ejaculate an average of 2.4 times per day for ten days, his sperm output could stay lower than before the ten-day experiment for up to five months! Feel free to hang your head in shame.

Nonetheless, according to a book by more-science-fiction-author-than-scientist Robin Baker, human sperm are not as mediocre as we think. It’s called Sperm Wars, and if you’ve taken a peek in it before, (and you don’t just believe everything you read) you might understand why other scientists have been getting their knickers in a twist about it. Apart from raising pressing questions such as “Where on earth did he find women willing to do that?”, Baker’s book tries to make out of theory and scientific fact what butchers make out of dead cattle. Namely, minced meat.

Sounding more George Lucas than Charles Darwin, Baker purports that the sperm of a human male are divided into three separate infantries: the egg-getters, the blockers and the hunters. The egg-getters have the biggest and roundest heads, and are for this very reason assigned to fertilising duty. The blockers, or Kamikaze sperm, are marked by their more oval shaped heads and their coiled, sometimes malformed tails; as the name implies, their job is to sacrifice themselves for the sake of blocking any other guy’s sperm. Then, there are the hunters, who target any non-sibling sperm with deadly enzymes secreted from the tips of their heads (the acrosome). Image a scene not so unlike a galactic war; only instead of space, it’s happening inside of a woman’s fallopian tubes, and instead of a death star, there is the ovum.

Sperm Wars could have revolutionised the concept of human sperm competition, if only it were based on actual scientific evidence. Scientists have yet to make any observations of Chuck-Norris-style sperm-hijacking — and evidently, anything that Baker claims to have seen is taken with enough salt to induce a coma. Then, there is the fact that human sperm production is only about 70% efficient, and that any odd-looking sperm you’d see swimming about under the microscope are most likely no good for anything. In fact, almost twenty years before the publication of Sperm Wars, Roger Short discovered that the spermatozoa with the largest heads (the egg-getters, allegedly) carried twice the number of chromosomes as normal ones — rendering them incapable of fertilising anything, let alone an egg. Needless to say, for holes in scientific theory, this one is blatantly crater-sized. Baker clearly hadn’t been doing his homework before deciding to go ahead with publishing his book.

Certainly, a lot of guys would like to think of their sperm as ‘out-of-this-world’, but there are plenty of other, more remarkable sperm to find within the animal kingdom. Rat sperm, for example, can form a so-called ‘copulatory plug’, a glue that holds the female tract together and prevents her from mating with anyone else. Furthermore, a recent study by Heidi Fisher and Hopi Hoekstra discovered that sibling sperm of the deer mice can cooperate to out-swim any opponents. In a kind of two-legged race scenario, these related sperm join together and appear to drive each forward, such that they can swim faster toward the egg than any sperm on its own.

Though it sounds like something out of Channel Five’s late night viewing schedule, sperm competition is actually a heavily researched field of evolutionary biology. Over the years, scientists have come upon all kinds of surreptitious ploys the male animal resorts to in his campaign to father the most offspring. And with most of these running the wide gamut from treacherous to right-out spermicidal, there appears to be no limit to the measures these organisms resort to for the sake of procreation. This really is no laughing matter; these sperm mean serious business.

Eric John

(Image courtesy of the lovely Michelle Okane)

Editor’s Note: And thus begins the month of luuuurve. If you don’t know already, the Weekly Scientist shall be dedicating all four articles of this month to Valentine’s day. This means for the next three weeks you can look forward to another three articles about wonderful topics such as sex, love, dating, relationships, STIs, et cetera.  Thank you for reading this article if you have and please keep doing so. We always love to hear from you.


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