Sperm Warfare (Or: Why it Takes 1 Billion Sperm to Make One Zygote)

The average man produces roughly 525 billion sperm cells over his lifetime and releases, in one way or another, more than one billion of them per month and anywhere from 40 million to 1.2 billion in a single ejaculation. The males of other species boast some equally impressive numbers: 280 million, 1 billion and 3 billion per ejaculate for rabbits, sheep and bulls, respectively. If it only takes one sperm cell to fertilize an egg, though, why produce so many?

The Seminal Wars

The females of many species mate with and receive the sperm of multiple males, often in quick succession. Deep in the lady’s nether regions, those sperm compete to fertilize the egg. Now, if you’re serious about winning a lottery or a raffle, you don’t buy just one ticket do you? No, you buy several to increase your probability of winning. Sperm, in a way, are a lot like lottery tickets. If you’re serious about passing on your genes, then you want to get as many sperm as possible near a fertile egg cell. (In other ways, they’re not like lottery tickets at all, and I would discourage you from trying to buy them in gas stations or convenience stores.) For a male, the more of his sperm going up against his rivals’ seed, the merrier.

Sperm competition is such a powerful selective pressure, in fact, that it influences the size of the testes and the volume of ejaculate of some animals and causes others to modulate the amount of sperm they produce based on the presence of a rival male. Male chimpanzees, who face high levels of sperm competition, possess the largest testes among the great apes. Gorillas, who face almost no sperm competition thanks to a rigid social structure where the dominant male alone gets to mate with all the females, don’t need to waste precious energy and resources on sperm production and hence have some downright dinky testes—almost 15 times smaller than chimps’ (relative to their body weight).

Male humans would feel somewhat embarrassed if they were naked in a locker room full of chimps, but still pretty good about themselves if they were naked and surrounded by silverbacks (nervous, too, perhaps). Evolutionary biologists are still trying to work out whether our relatively large testes are leftovers from some point in our evolutionary past, or if sperm competition was at one point an important factor in human reproduction.

It’s not a sprint. It’s a marathon

Sperm competition isn’t a prevalent problem among modern Homo sapiens and guys don’t really need a veritable army of sperm to race someone else’s genes to an egg. We still need an awful lot of those squiggly little cells, though, because even if there’s no other sperm to compete against, every man’s little swimmers still have to fight in a battle of the sexes. Females demand only the finest sperm for their eggs, and the war their bodies wage on sperm is one of attrition.

After insemination, the sperm cells of humans, and many other species, have a long trip ahead of them, relative to their tiny size. At every step of the way, many sperm cells run out of energy or die and their surviving brothers are forced to leave them behind: only a portion of the sperm that are deposited into the vagina make it to the uterus, an even smaller group get to the oviducts and a fraction of those make their way to the upper oviduct where the egg is actually located. Once the sperm reach the egg, things don’t get any easier. One does not simply walk into Mordor. The egg is covered by a thick layer of gelatinous, follicular cells called the cumulus oophorus, which acts as a barrier, and it often takes the assault of several sperm cells to break it down enough for one lucky one to get through and fertilize the egg. Charles Lindemann, who researches the mechanisms of sperm motility at Oakland University in Rochester, Michigan, likens the whole ordeal to a “marathon run in a maze filled with mucus followed by an obstacle course.”

The odds stacked against any single sperm cell making the grueling journey to the egg can be offset by producing a large number of sperm. While just a small fraction of the sperm will reach their destination and do the job they were made to do, having a few million more cells backing them up makes for a pretty good reproductive insurance policy.

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Medicine
Charles Dickens Museum Highlights the Author's Contributions to Science and Medicine

Charles Dickens is celebrated for his verbose prose and memorable opening lines, but lesser known are his contributions to science—particularly the field of medicine.

A new exhibition at London’s Charles Dickens Museum—titled "Charles Dickens: Man of Science"—is showcasing the English author’s scientific side. In several instances, the writer's detailed descriptions of medical conditions predated and sometimes even inspired the discovery of several diseases, The Guardian reports.

In his novel Dombey and Son, the character of Mrs. Skewton was paralyzed on her right side and unable to speak. Dickens was the first person to document this inexplicable condition, and a scientist later discovered that one side of the brain was largely responsible for speech production. "Fat boy" Joe, a character in The Pickwick Papers who snored loudly while sleeping, later lent his namesake to Pickwickian Syndrome, otherwise known as obesity hypoventilation syndrome.

A figurine of Fat Boy Joe
Courtesy of the Charles Dickens Museum

Dickens also wrote eloquently about the symptoms of tuberculosis and dyslexia, and some of his passages were used to teach diagnosis to students of medicine.

“Dickens is an unbelievably acute observer of human behaviors,” museum curator Frankie Kubicki told The Guardian. “He captures these behaviors so perfectly that his descriptions can be used to build relationships between symptoms and disease.”

Dickens was also chummy with some of the leading scientists of his day, including Michael Faraday, Charles Darwin, and chemist Jane Marcet, and the exhibition showcases some of the writer's correspondence with these notable figures. Beyond medicine, Dickens also contributed to the fields of chemistry, geology, and environmental science.

Less scientifically sound was the author’s affinity for mesmerism, a form of hypnotism introduced in the 1770s as a method of controlling “animal magnetism,” a magnetic fluid which proponents of the practice believed flowed through all people. Dickens studied the methods of mesmerism and was so convinced by his powers that he later wrote, “I have the perfect conviction that I could magnetize a frying-pan.” A playbill of Animal Magnetism, an 1857 production that Dickens starred in, is also part of the exhibit.

A play script from Animal Magnetism
Courtesy of the Charles Dickens Museum

Located at 48-49 Doughty Street in London, the exhibition will be on display until November 11, 2018.

[h/t The Guardian]

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Health
Feeling Down? Lifting Weights Can Lift Your Mood, Too
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There’s plenty of research that suggests that exercise can be an effective treatment for depression. In some cases of depression, in fact—particularly less-severe ones—scientists have found that exercise can be as effective as antidepressants, which don’t work for everyone and can come with some annoying side effects. Previous studies have largely concentrated on aerobic exercise, like running, but new research shows that weight lifting can be a useful depression treatment, too.

The study in JAMA Psychiatry, led by sports scientists at the University of Limerick in Ireland, examined the results of 33 previous clinical trials that analyzed a total of 1877 participants. It found that resistance training—lifting weights, using resistance bands, doing push ups, and any other exercises targeted at strengthening muscles rather than increasing heart rate—significantly reduced symptoms of depression.

This held true regardless of how healthy people were overall, how much of the exercises they were assigned to do, or how much stronger they got as a result. While the effect wasn’t as strong in blinded trials—where the assessors don’t know who is in the control group and who isn’t, as is the case in higher-quality studies—it was still notable. According to first author Brett Gordon, these trials showed a medium effect, while others showed a large effect, but both were statistically significant.

The studies in the paper all looked at the effects of these training regimes on people with mild to moderate depression, and the results might not translate to people with severe depression. Unfortunately, many of the studies analyzed didn’t include information on whether or not the patients were taking antidepressants, so the researchers weren’t able to determine what role medications might play in this. However, Gordon tells Mental Floss in an email that “the available evidence supports that [resistance training] may be an effective alternative and/or adjuvant therapy for depressive symptoms that could be prescribed on its own and/or in conjunction with other depression treatments,” like therapy or medication.

There haven’t been a lot of studies yet comparing whether aerobic exercise or resistance training might be better at alleviating depressive symptoms, and future research might tackle that question. Even if one does turn out to be better than the other, though, it seems that just getting to the gym can make a big difference.

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