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.