Two recent studies have shown that mules can outdo horses and donkeys at cognitive tasks, an intriguing example of "hybrid vigor."
"Hybrid vigor" refers to the tendency of hybrids-- or individuals that are half one species or breed, and half another-- to be stronger, larger, healthier, and biologically "better" than either of their parent species. (This occurs due to a combination of genetics and epigenetics: read more here). Common well-known examples are: "mutts," mixed-breed dogs that do not suffer from the congenital birth defects common in many inbred purebred lines; corn, rice, and other crops, which are crossed to increase yields and resilience; and mules, bred for their strength, docility, and long life span (useful attributes for farm animals). It is worth noting that unlike cross-bred plants or dogs, most mules are infertile because they are a cross between vertebrate species: this leaves a distinctive genetic stamp. Mules have 63 chromosomes (32 from their horse mother, and 31 from their donkey father); since 63 is an odd number, the chromosomes cannot pair up properly during meiosis.
Until recently, there was no evidence about the effect of hybridization on cognition; however, there have always been anecdotal tales of street dogs being smarter than purebred ancestors*. A 2009 study by Proops et al. found that mules outdid ponies and donkeys at a visual discrimination task wherein the subject had to choose one of a pair of images for four pairs, learning over time which was the right choice. In a 2013 study by Osthaus et al., mules solved a spatial problem more quickly than horses and donkeys. (I came across this second study after reading an excellent post in the animal cognition blog Bird Brains and Monkey Minds).
Thus, it seems likely that hybrid vigor extends to cognitive traits as well as physical.
The science fiction fans among you may be reminded of one of the greatest and most fearsome characters in all of sci-fi: the Mule, a conqueror and warlord in Asimov's famous Foundation series. He chose the name for himself for a different reason (one that I will not disclose to those of you who unfortunately have not yet read Foundation et al.), but it could easily have referred to how much smarter he was than everyone else.
Darwin may have said it best: "The mule always appears to me a most surprising animal. That a hybrid should possess more reason, memory, obstinacy, social affection, powers of muscular endurance, and length of life, than either of its parents, seems to indicate that art has here outdone nature."
*This is, of course, biased by the selection process: street dogs struggle to survive, so the smarter and stronger flourish while the weak die out.
This study compares the behaviour of the mule (Equus asinus × Equus caballus) with that of its parent species to assess the effects of hybridization on cognition. Six mules, six ponies (E. caballus) and six donkeys (E. asinus) were given a two choice visual discrimination learning task. Each session consisted of 12 trials and pass level was reached when subjects chose the correct stimulus for at least 9 out of the 12 trials in three consecutive sessions. A record was made of how many pairs each subject learnt over 25 sessions. The mules’ performance was significantly better than that of either of the parent species (Kruskal-Wallis: Hx = 8.11, P = 0.017). They were also the only group to learn enough pairs to be able to show a successive reduction in the number of sessions required to reach criterion level. This study provides the first empirical evidence that the improved characteristics of mules may be extended from physical attributes to cognitive function.