The shape and size of genitals vary dramatically across the animal kingdom. From cats with a barbed penis to the spoonlike appendage of the damselfly, there seems to be few limits to their form. But why is there such variety, especially when considering some aspects of biology (such as body shape) remain fairly constrained? The answer might lie in the evolutionary tug of war that occurs between males and females, known as sexual conflict.
In an experiment involving the carnivorous burying beetles, researchers found that the frequency at which the insects have sex can physically change the shape of their genitals over a period of just 10 generations. For beetles that were selected to have more sex, the males evolved a longer “penis,” while females evolved larger “claws” on their genitalia. The researchers suggest that since these physical changes in genitals were seen in both sexes, they are likely the result of sexual conflict.
“It takes two to tango, so when changes in shape in one sex leads to corresponding changes in the other sex this is known as co-evolution,” explains Dr. Megan Head, co-author of the paper published in the journal Evolution, in a statement. There is often a conflict between male and female animals when it comes to how often to mate. For males, who generally put little energy into creating sperm, it is beneficial for them to mate with as many partners as possible, whereas for females it is often better for them to only mate a few times.
They found that the most dramatic changes in the shape of the genitals for both sexes were in those that were selected to have the highest mating rate. “Although we don't know the ins and outs of how these genital structures relate to the reproductive success of each sex, our results show that sexual conflict over mating can lead to co-evolutionary changes in the shape of the genitals of burying beetles,” says Dr. Paul Hopwood, another of the co-authors of the study.
Burying beetles (Nicrophorus) are unusual among insects as they actually exhibit parental care. They are named after their habit of fighting over and then burying small dead animals, in which they then raise their brood. Before burying the dead animal, however, they will first remove fur or feathers and then coat it in antibacterial and antifungal substances secreted from oral and anal glands, in order to mask the decaying smell and keep the meat fresher for longer. When the larvae hatch in the carcass, the adults then feed the larvae regurgitated liquid flesh.
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