The animal kingdom is filled with examples of major physical differences between the sexes, known as sexual dimorphism. Among spiders, it is common for females to have 10 times the body mass of males, while elephant seals reverse this, with adult males three times the weight of females. It would be easy to think of this as a successful adaptation, yet the reverse is true, a new study finds. The greater the sexual dimorphism, the greater danger a species is in.
The conclusion is based on a specific group of crustaceans known as ostracods. Although these small creatures, also called seed shrimps, are low profile, they provide a good sample for survival studies since we have a fossil record for them stretching back 450 million years. Moreover, the males grow longer shells because their sex organs are so big they need extra space – sort of like inbuilt crustacean codpieces.
The Smithsonian's Dr Gene Hunt looked at 93 late-Cretaceous ostracod species and reported in Nature that the most sexually dimorphic species - which for these crustaceans mean the ones where the males have to expand their shells to fit their enormous genitalia - were 10 times as likely to go extinct within a period as their more egalitarian equivalents.
This raises the question of why, if extreme sexual dimorphism is so hazardous, it is also so common. Apart from size differences, nature is filled with examples where one sex, usually the males, puts great effort into plumage or inbuilt weaponry while the other sex prioritizes survival.
However, this confuses benefits to the individual and the species. An individual male is more likely to pass on his genes if he develops a huge tail to impress females or mighty antlers to fight off rivals, what Darwin called "sexual selection". It's sexual selection that drives sexual dimorphism, with one sex becoming ever more extreme in its features in order to either woo partners, or make more and better sperm. However, the arms race that develops as a result does no good to the group as a whole, wasting energy that could be in short supply when times are hard.
The male ostracods' expanded carapaces make room for what the paper coyly refers to as "large sperm pumping and copulatory apparatus". Co-author Professor John Swaddle of the College of William And Mary called them "the Don Juans of the crustacean world," in the college magazine. In fact, the oldest fossilized penis known belonged to an ostracod. Among some animals, bigger penises may impress the females and increase mating opportunities, but in ostracods the major benefit is probably that by releasing more sperm an ostracod boosts its chances of successful insemination when females mate with multiple males, as ostracod females often do.
However, while some ostracod species were investing all that energy in their nether regions, others were more focussed on other things, with natural selection taking precedence over sexual selection. The males in these cases had smaller genitalia, so their carapaces were shaped more like the females. From a species point of view, this was a much better strategy.
“It’s not as if they’re sexing themselves to death,” Swaddle said. “At issue is the intense investment in sexual selection and mating strategies. It’s making them vulnerable to other things going on in their environment. It puts them at risk, and then when the environment changes in different ways, the species that are investing more heavily in reproduction are the ones more likely to go extinct.”
Given the highly specific nature of Hunt and Swaddle's sample, we need to look more widely before accepting this conclusion as a general rule, particularly for species where sexual selection is about display or fighting ability, rather than sperm production. Studies in birds have tended to produce a similar result, while at least one in mammals found no increased risk of extinction from sexual dimorphism. There is even an argument that sexual dimorphism encourages faster removal of harmful mutations from the gene pool. However, these studies looked at contemporary animals and the authors acknowledge their results may be distorted by human-induced extinctions, hardly an issue during the Cretaceous.
The good news is that, while humans are clearly sexually dimorphic to a degree, relatives like gorillas or chimpanzees have us beat. Sadly, it's unlikely any of this will be acknowledged by the armchair evolutionary psychologists who love to proclaim male dominance is an adaptive trait.