The web is full of images of love gone wrong, from rabbits mounting chickens to elephants getting amorous with rhinos. Besides the humor, serious questions remain about why this happens. From an evolutionary point of view, interspecies mating is almost always a dead end. Now, a new study on two species of squash bugs (Anasa tristis and Anasa andresii) adds to this growing body of evidence, while also backing the well-known song by Meghan Trainor.
Currently, the A. tristis' Florida range is being invaded by its relative A. andresii. “Male A. andresii are commonly found copulating with the larger female A. tristis in the field,” Dr. Christine Miller of the University of Florida told the Behaviour 2015 conference.
Miller investigated and found that male A. andresii like a little more booty to hold at night, chasing the largest females in their own species. Sharing a habitat with a larger relative apparently feels like entering a harem and the A. andresii males take full advantage.
“We used to think that these kinds of situations were rare but they are becoming increasingly common as we move species around the planet,” Miller told IFLScience. “We know almost nothing about them. They're an excellent chance to see evolution in action.”
For both Anasa species, sex is a serious business. Mating can last for hours or even days, an astonishing feat considering both species live for only months in the lab and weeks in the wild. Although the females lay eggs after interspecies encounters, Miller said few hatch and none survive to adulthood.
Moreover, such epic shags carry a heavy price. Coupling bugs are vulnerable to being eaten by predators and having eggs laid on them by parasitic flies, the main causes of death for both species in the area. Moreover, their private parts lock so strongly that disentangling can do serious damage to both parties.
It might be expected that all this pointless mating would severely impact each species, but Miller said she was surprised that females who engaged in interspecies bouncy-bouncy did not suffer any penalty in terms of total offspring. “We found surprisingly little evidence of fecundity costs of across-species mating and living in communities that contain members of both species,” Miller told the conference.
While this continues to puzzle Miller, she told IFLScience that part of the explanation lies in the fact that A. tristis females learn to stick to their own kind. None mate with A. andresii more than once, although she has no idea how they do this. A. andresii males, however, never move along, continuing to chase the biggest females they can get.
The male's preference should create a selective pressure on each species to evolve to larger sizes, but Miller noted that increased visibility to predators and resource costs create a balancing effect. The fact that the two species have balanced these pressures at different sizes could teach us how these evolutionary forces work, she added.
Miller told IFLScience that male preferences for larger females probably drive many examples of interspecies mating, but that she was aware of only one previous observation, when similar findings in frogs were reported as a minor sideline to another study.