When it comes to a dung beetle's junk, size doesn't matter. At least, not to the process of rapid evolution that creates new species. Researchers say that what matters there, as males and females evolve together and distinguish themselves from related species, is shape.
Intercourse between Onthophagus beetles--the genus used in the current study, which includes the dung beetle O. taurus as well as other scarab beetles--is far from simple. It begins with the male climbing onto the female's back and inserting his penis's grabbing appendages (called parameres) into special pits on her underside. "For simplicity," the authors refer to the female's collection of equipment here as the "pygidial flap." Once the male has gripped the female tightly, he inflates his endophallus and--well, you know the rest.
Beetles and other arthropods are known to have quickly evolving genitals; often, the genitals are the key to telling similar species apart. So researchers wanted to know how this genital arms race affects the development of new species. In some cases, it really is an arms race--males evolve mechanisms that are more efficient at, say, piercing the female's abdomen and delivering sperm directly to its target, while females evolve mechanisms to protect themselves or regain control of the situation. Even in less sexually aggressive species, male and female genitalia should evolve together to ensure a good fit.
The authors looked at several species of Onthophagus with different degrees of relatedness. A species, despite what you were told in ninth-grade biology, is a bit of a squirrelly thing to define. It's often said that animals belong to different species if they are unable to interbreed--but in the wild, this breaks down. Three of the beetle species studied here belong to a "species complex," a closely related group whose members haven't totally separated from each other. The other species were O. taurus, an Old-World dung beetle, and its sister populations that have evolved since the beetle was introduced to other regions in the 1960s.
Since the paramere and pygidial flap are tightly locked together during beetle sex, the researchers looked here for the signature of evolution--rather than at the beetles' other copulatory organs that don't need to interact as closely. Across the several Onthophagus species in question, they analyzed the shape and size of parameres and pygidial flaps from several hundred beetles. Analysis was done by mapping "landmarks," such as boundaries between segments or inflection points of curves, onto images of the organs.
For consistency, poor lead author Anna Macagno mapped the landmarks on every one of these male and female bits (top and bottom, respectively) herself.
The tiny variations between all these very tiny private parts added up to a clear big picture: As species diverged from their relatives, male and female genitalia evolved together. But this evolution was primarily a change in shape. Size, for the most part, stayed the same.
Another way of understanding this result is that size does matter. Deviation from a standard size, perhaps because it leads to processes not working the way the organs' owners expect them to, is discouraged by evolution. Outliers don't pass on their genes. But when it comes to the shape of genitalia, evolution encourages experimentation. Male and female beetles that develop a new lock-and-key fit between their organs can quickly distinguish themselves from related populations. If other beetles' equipment is no longer compatible with yours, you've become a new species--and kicked others out of your gene pool.
What's impressive is the speed at which beetle populations can diverge like this. The O. taurus populations in the study have only been separated since the 1960s, when the dung beetles were introduced to new parts of the world. And even in the beetle species that still live in overlapping habitats, evolution is driving their genitalia in different directions. The quickest way to form a new species and exclude all your relatives, it turns out, may be to change the locks.
Images: Onthophagus bonasus Fabricus (not one of the species used in the study), Flickr/urjsa; bottom figure, Macagno et al.
Macagno, A., Pizzo, A., Parzer, H., Palestrini, C., Rolando, A., & Moczek, A. (2011). Shape - but Not Size - Codivergence between Male and Female Copulatory Structures in Onthophagus Beetles PLoS ONE, 6 (12) DOI: 10.1371/journal.pone.0028893