Any farmer will tell you that to get the strength of a donkey with the temperament of a horse, mate them to get a mule. This genetic swapping between species has been orchestrated by humans for centuries to create hybrids with desirable traits. Nature also sometimes confuses the reproductive rules. Wild populations of hybrids often persist alongside their parent species and many even look like a cross between the two. Unfortunately, this cut-and-paste approach to speciation has some downsides: just ask the mule, which is, of course, sterile. Although we know a lot about the reproductive disadvantages of inter-species relationships, it is unclear whether hybrids face everyday challenges as well. This knowledge gap piqued the interest of researchers from the University of California Davis, Texas A&M University and Towson University in the USA, who set out to measure exactly how much one hybrid sucks.
Matthew McGee and colleagues focused on a common cross between the bluegill sunfish, Leopomis macrochirus, and the green sunfish, L. cyanellus. Each of these parent species has a body and mouth specially adapted for feeding on certain food types. Bluegills are deep bodied with small, powerful mouths used to suck invertebrates off rocks. Conversely, green sunfish are slender and prefer to gulp fast-moving prey with their large mouths. The hybrid has an intermediate body and mouth shape, but just how this translates into feeding performance is unknown. The researchers used high-speed cameras to film live bluegills, green sunfish and crosses while feeding on mobile prey. The videos were then used to measure the timing and movement of the fish's mouth during an attack. From this information, they were able to estimate each fish's capacity to generate suction pressure. This suction index was then used to simulate an individual's feeding performance on either attached or free-swimming model targets.
Not surprisingly, the overall mouth kinematics and speed of the feeding hybrids fell right in between that of their parents. And performance simulations suggest that hybrids are not as good at bluegills at dislodging attached prey, nor as able as green sunfish to suck mobile food out of the water. In fact, with each type of simulated prey, the hybrids performed about as well as their worst performing parent. In other words, intermediate morphology does not equal intermediate performance.
The fact that hybrids inherit a master-of-none morphology that translates into poor performance is surprising given what we assume about body form and function. Interestingly, these results suggest that the relationship between performance and morphology is non-linear, meaning that it can be hard to infer the actual function of a body part from its shape alone. The mule might be a jack-of-all-trades, but it looks like the performance of most hybrids is not even stuck somewhere in the middle. It just sucks.