It makes sense that the male and female members of a species should look or smell or sound a little different — enough that they can tell each other apart during mating season. But extreme differences are hard to square with the principle of survival of the fittest. Surely if a species has an optimal size and shape for keeping fed and evading predators, then something has gone wonky when massive males loom over streamlined females, or when one sex gets protective camouflage while the other flounces around as bright as a new sports car. Scientists have been investigating this matter, and one group has concluded that indeed, big sex differences can be hazardous to the survival of a species.

What does this mean for us humans? Our sex differences are relatively minor, especially compared with our closest relatives’. Male but not female chimpanzees have prominent fangs, while male gorillas are more than twice the weight of the females.

The new research, published this month in the journal Nature, documented the fates of a class of shrimp-like animals called cytheroid ostracods, among which there are dozens of different species — some living, some extinct. These creatures make bivalve shells, and in living species, some of the males have larger or more elongated shells than those of the females. When the scientists look inside those shells, they find that the larger or longer male shells are housing larger sex organs. There are also plenty of extinct species whose sexual endowments can be inferred from their fossils — and that’s just what researchers did.

Roughly speaking, they found that the more different the shells, the more oversized the male genitalia — and the higher the risk of extinction. Those with the most pronounced sex differences faced 10 times the odds of going extinct as those with the least.

This is perfectly in keeping with today’s understanding of evolution. Charles Darwin recognized two drivers of evolution — the struggle for survival (natural selection) and the struggle to mate and reproduce (sexual selection.) Biologists today are debating not just the relative importance of sex and survival, but whether they work in constant harmony or occasional opposition.

Some animals show almost no sexual dimorphism — certain Amazonian monkeys, for example, and emperor penguins, whose males and females look so similar that naturalists aren’t sure how they tell the difference. Such animals tend to be devoted co-parents, with moms and dads putting equal energy into raising their progeny. But in nature, there’s no free lunch: As soon as one sex starts putting less energy into the offspring, a competitive situation ensues.

Peacock dads, for example, are the opposite of penguin dads. They don’t do anything at all for their eggs or chicks, and so they have to compete with their looks for the chance to have sex. Peahens are happy to line up for the prettiest in the flock while all the other males go home alone. Other animals compete by singing, dancing, or, like alley cats, trying to claw each other to pieces. (It’s usually the male who invests less in offspring, but not always. In a fish called the bristlenose pleco, the females give their fertilized eggs to the males and then swim off.)

The subject of the new study — the ostracods — are probably not engaging in any form of display, but manifesting what the biologists call sperm competition. Rather than co-parent, or fight or compete with their looks, male ostracods compete in “quantity, size or transfer efficiency of the sperm,” as the researchers put it in their Nature paper.

Longer shells, it turns out, function as a sort of ostracodpiece. They go with males who’ve focused on sperm transfer efficiency, developing a strong “pumping mechanism” in their proportionally oversized ostracod penises. Larger shells, meanwhile, belong to those who are either competing on the number of sperm they produce, or making giant sperm.

High sperm counts sometimes evolve in species where females mate with multiple males. In that case, passing on genes becomes a sort of lottery in which it helps for males to buy more tickets. Giant sperm represent another strategy. The fruit fly species drosophila produces sperm with a tail that’s 100 times the length of a human sperm. Somehow the tail balls up neatly enough to fit inside the fly. Smithsonian Institution biologist Gene Hunt, who was a co-author of the Nature paper, said giant sperm can help a male’s chances by blocking sperm from competitors.

The ostracods with larger shells may have been making either more or bigger sperm, he said. They looked at 93 species whose fossils, left in eastern Mississippi, dated from 84 to 66 million years ago. While having outsized genitalia was advantageous to individual males, something about the sexual asymmetry may have been detrimental to their species, as they tended to go extinct the fastest.

These new findings don’t mean that all species with big sex differences have in any way failed, or that sex differences don’t provide advantages under certain kinds of conditions. The researchers say their findings could point out which other species most need extra protection.

The most extreme known case of sex difference occurs in the deep sea, where it’s frigid, lonely and dark, and there’s hardly anything to eat. There, the female anglerfish outweighs her male counterpart by a factor of 500,000 to one. The gnat-sized male latches onto the tuna-sized female, where he stays on as a lifelong partner and parasite. He feeds off his mate’s blood, and, in return, sends her sperm. From a human perspective, it doesn’t sound like fun for either partner, but the system has a certain elegance about it.

This kind of weirdness continues to reveal the subtleties of evolution. People often try to understand our species by looking for what makes us different from other animals, but there’s a lot to be learned from understanding the evolutionary process that gave rise to us all.

Science writer Faye Flam is a Bloomberg View columnist.

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