Domestication is by far man’s greatest genetic experiment, and we’ve been at it for well over 10,000 years. While domestication can produce wild variation (see my post on dogs, for example), a few changes seem to be universal. These include behavioral changes, like reduced fear of humans and friendliness, as well as physiological ones, like floppy ears (they develop in domesticated foxes, too). One of the most well-documented differences between domesticated animals and their wild counterparts is their brains: across every species that has been looked at, the brains of domesticated animals are smaller. Why does domestication lead to shrinking brains? And what does this mean in terms of intelligence?
Allometry is the study of the relationship between size and shape of things, and how these relate to their functions. Allometric methods are often applied to understanding brains and intelligence and how they may differ between animals, especially when looking at non-living creatures that can’t be put through behavioral tests. Of course, you can’t just look at the size of a brain all by itself. After all, by weight, elephant brains are far larger than ours, and sperm whale brains are even larger than theirs! But that doesn’t tell us anything about intelligence, since brains are bound to be bigger in larger animals in general. Instead, scientists look at the ratio of brain size to body size, not just the weight or width of the brain itself. The amount of brain mass exceeding what would be expected by an animal’s total body mass is termed it’s degree of ‘Encephalization’, or its ‘Encephalization Index’, which is thought to relate directly to intelligence. We have a higher degree of encephalization than apes, for example, and they in turn have a higher degree than most other animals.
The table on the right (Kruska, 2005) shows the Encephalization Indicies (EI) and the percent decrease in relative brain size (DV, %) between wild and domestic forms of different species. A wide variety of domesticated animals, from horses to rats, all show decreases in relative brain size (FYI this is true for domesticated birds, like geese and ducks, too). The trend is stronger in species that had higher degrees of encephalization begin with, like pigs. Since encephalization is supposed to correlate directly with intelligence, this would indicate that our domesticated animals are much dumber than their wild relatives, particularly those that were smart to begin with. Furthermore, studies have supported that these differences are indeed genetic rather than environmental in nature, as crosses between wild and domesticated versions have brains in the middle of the two. Also, captive-bred wild animals do not show this marked decrease in brain size, so it’s not likely the captivity itself alters the size of animal brains, and domestic animals that become feral don’t show an increase in brain size over time.
Why might domestication have bred stupidity? Well, there is the obvious reason: intellect isn’t a trait we selected for first. Horse breeders were likely more consumed with whether their studs ran fast than if they could learn to push a certain shaped button for food. Similarly, people wanted cute guinea pigs that were easy to hold, not ones that could memorize a maze. Indeed, there are potential downsides to a smart pet: smarter animals might be more able to get out of enclosures or be otherwise devious, making them harder to keep. Furthermore, it’s possible that intelligence is positively correlated with a trait we really don’t want, like aggression, so that by breeding for other desirable traits, we lowered intellect accidentally. It’s also possible that we started out with dumber stock – after all, the smartest geese might have been those we couldn’t catch.
But is this really true? Are dogs and guinea pigs stupider than wolves and cavies?
In many cases, the answer appears to be yes. Dogs, for example, appear to be a few crayons short of the box when compared to wolves. A study in 1985 found that wolves vastly outperformed malamutes in getting a food dish from a series of complex puzzle boxes. And while one study had found that dogs are better able to pick up human social cues than wolves, they were criticized for using wild wolves that had little interaction with people. Indeed, when another set of researchers did the same kind of study using wolves that had been raised by humans, the wolves beat the dogs hands down.
But not everyone buys into the smaller-brains-dumber-animals hypothesis. In their review of a number of brain size studies, Healy & Rowe concluded that whole brain size, by itself, was not a useful variable “when considering the role of selection in the relationship between brain and behaviour.” They cite numerous studies of primates and birds which fail to find a relationship between whole brain size and indicators of intelligence like social complexity.
They’re backed up by studies which have found domesticated animals, despite their smaller brains, fare just as well as wild ones. Rodents, in general, don’t seem to have suffered from domestication. Wild rodents don’t outperform domesticated ones on a number of tests, and often, it’s the lab rats that do better than their undomesticated cousins. For example, experiments using a number of different rat strains found that inbred and albino rats showed decreased learning abilities compared to wild ones, but the non-inbred domestic strains did just as well. Furthermore, a study just published online first for Frontiers in Zoology found that domestic guinea pigs found an underwater platform much faster than wild cavies. They conclude that in the case of guinea pigs, “artificial selection for human desired traits did not led to a degeneration of cognitive capabilities.”
Of course, all of this is confused by the fact that “intelligence” is an amorphous and often hard to quantify concept, particularly in anything that isn’t human. While we can give people SATs or IQ tests (which are controversial enough as measures of intellect), pinning down whether an animal is ‘smart’ is much, much harder. Do we define intelligence as social complexity and their interactive memory of their peers? Ability to learn behaviors when trained? Problem solving? Self-recognition? The battery of tests for ‘intelligence’ in animals is almost endless, and few of them have been given to wild and domesticated species in a comparable setting.
In the end, it’s hard to say if domesticated animals are in truth dumber than their wild relatives without further, more rigorous studies. If they are, though, would it really be that shocking? We don’t prize intelligence all that highly in our own society – just look at the popularity of celebrities like Paris Hilton. Why should our animals be any different?
Kruska, D. (2005). On the Evolutionary Significance of Encephalization in Some Eutherian Mammals: Effects of Adaptive Radiation, Domestication, and Feralization Brain, Behavior and Evolution, 65 (2), 73-108 DOI: 10.1159/000082979
Frank, H., & Frank, M. (1985). Comparative manipulation-test performance in ten-week-old wolves (Canis lupus) and Alaskan malamutes (Canis familiaris): A Piagetian interpretation. Journal of Comparative Psychology, 99 (3), 266-274 DOI: 10.1037/0735-7036.99.3.266
UDELL, M., DOREY, N., & WYNNE, C. (2008). Wolves outperform dogs in following human social cues Animal Behaviour, 76 (6), 1767-1773 DOI: 10.1016/j.anbehav.2008.07.028
Healy, S., & Rowe, C. (2007). A critique of comparative studies of brain size Proceedings of the Royal Society B: Biological Sciences, 274 (1609), 453-464 DOI: 10.1098/rspb.2006.3748
Harker KT, & Whishaw IQ (2002). Place and matching-to-place spatial learning affected by rat inbreeding (Dark-Agouti, Fischer 344) and albinism (Wistar, Sprague-Dawley) but not domestication (wild rat vs. Long-Evans, Fischer-Norway). Behavioural brain research, 134 (1-2), 467-77 PMID: 12191833
Lewejohann L, Pickel T, Sachser N, & Kaiser S (2010). Wild genius – domestic fool? Spatial learning abilities of wild and domestic guinea pigs. Frontiers in zoology, 7 (1) PMID: 20334697