Monday Pets: Dumb Guinea Pigs? (The I Just Got Back From APS Edition)

Zen recently wrote mentioned this study on his blog, so I thought it was time to dredge it out of the archives. Also, I've just returned from APS (see my daily recaps here here and here), and I am TIRED.

Domestic animals and their wild counterparts can be different in big ways; there can be differences in morphology (physical characteristics), physiology, and behavior. These changes may depend on spontaneous adaptations to captivity or to artificial selection pressures arising from the motivation for domesticating the animal in the first place.

One change that is often observed as a result of domestication is a general reduction in relative brain size. This reduction has been observed in all species that have been investigated with only one exception: the house mouse (mus musculus). Importantly, this reduction is not simply a result of increased body size. Despite the fact that it has not been scientifically explored, many assume that the reductions in brain size are reflected in a similar reduction in brain function. In other words: smaller brains (relative to body size) and dumber animals.

Behold the domesticated guinea pig (Cavia aperea f. porcellus):

A Brief History of the Domesticated Guinea Pig (isn't this what you always wanted to know?)
Guinea pigs were first domesticated about 4500 years ago in South America, providing the native South Americans with meat as well as critters for ritual sacrifice. Apparently, guinea pigs are still an important source of meat in Peru. (They taste like rabbit, supposedly. Follow the link. I dare you. But don't if you have a pet guinea pig. You'll be sorry, and I will have told you so.) In the 16th century, domesticated guinea pigs were brought to Europe where they were subjected to further selective breeding, resulting in the common form we know and love today as pets and laboratory animals.

Their wild ancestors, the wild cavy (cavia aperea) remains one of the most common rodents in South America. Their natural habitat consists of open areas used for brief feeding periods, as well as areas of dense vegetation where they spend most of their time.

Not much is known about the cognitive or memory skills of wild cavies, though they differ in many ways from domestic guinea pigs. Guinea pigs are less aggressive, more "friendly," display more male courtship behavior, and have reduced stress reactivity. In other words, they don't get stressed out as easily as their wild cousins. Does this sound familiar? Also, the brain of the domesticated guinea pig is about 13% smaller than that of the wild cavy.

Dumb Guinea Pigs; Intelligent Cavies?
A group of behavioral biologists and neuroscientists from the University of Munster (Germany) wondered about the spatial navigation abilities of domestic guinea pigs and wild cavies. Despite their reduced brain size, would the guinea pigs outperform the wild cavies at the Morris water maze?

The Morris water maze is an apparatus commonly used to study spatial and navigational abilities in mice and rats. The animal is put into the round tank and made to swim. In one spot, there is a submerged platform which will allow the animal to comfortably stand without needing to swim or tread water to keep afloat. This would be pretty easy in clear water, so the tank is filled with an opaque milky liquid. The walls of the tank have various geometric markings on them in different places. The question is: after being introduced to the maze and finding the location of the platform during training, will the animal be able to use the navigational cues given by the different geometric markers to swim directly towards the platform during testing? Importantly, the tank is circular, so corners and edges can not be used to aid in navigation - the only cues available are the geometric markings.

During the testing phase, you put the animal in its bath, and it swims around until it finds the platform. If, after a given amount of time (45 seconds in this experiment), it had not found the platform, the experimenter would manually move the guinea pig to the platform. This was repeated up to three times, until, after being moved, the guinea pig stayed on the platform for at least fifteen seconds. If s/he found the platform by him- or herself, s/he was left there for fifteen seconds. After the training period ended, there were five days in which the guinea pigs were not placed into the water maze.

Then, they were placed back into the maze for 60 seconds, but the platform was removed. Would they spend more time in the previously-correct quadrant of the pool? If so, then they had accurately used spatial cues to learn the task. If they spent equal amounts of time in all four quadrants, then they did not learn the task.

Was it generalized problem solving, or simply a conditioned response? Finally, they were put into a pool in which the submerged platform was located in a different quadrant. Could they re-learn the location of the platform? If so, this suggested that they could generalize the task. (This is a similar to the experiment in which Rio (the sea lion) generalized her categorization of arbitrary stimuli.)

Results

So domesticated guinea pigs did somewhat better than the wild cavies at learning the task (figure 4), though the cavies did eventually learn the task and perform above chance (figure 6).

What can account for the relatively poorer performance of the wild cavies in terms of finding the most direct path (figure 4), while also considering their equal performance on the amount of time it took to find the platform (figure 5), and the fact that all groups were actually above chance on learning the task overall (figure 6)?

It might have something to do with swimming speed. Male domestic guinea pigs swam at a speed of 1.12 km/h on average and females at an average speed of 1.19 km/h. In the group of wild cavies, average swimming speeds were 1.18 km/h for males and 1.35 km/h for females. Females were significantly faster than males, and wild cavies were significantly faster than domesticated guinea pigs. Despite taking more circuitous routes to the platform (demonstrating that they weren't taking advantage of the spatial cues), their faster swimming speed meant that they found the platform in roughly the same amount of time. It is therefore likely that the wild cavies were using some other sort of problem-solving strategy, and the data indicate that it was sufficiently successful. It is unclear, however, what their strategy was.

And why were the wild cavies faster swimmers than the domesticated guinea pigs? Remember how domesticated animals tend to respond better to stressful situations? In addition to being able to use the spatial cues, it is possible that the domesticated guinea pigs were less stressed out by being in the water maze (maybe they were less stressed out BECAUSE they could use the spatial cues). It is possible that higher levels of physiological stress in the wild cavies led to the faster, more panicked, swimming.

Why is this important?
The authors say it pretty well, so I'll let their words do the speaking:

Thus, guinea pigs' domestication as an artificial selection for human-desired traits did not led to a degeneration of cognitive capabilities but rather to an adaptation to a man-made environment that allows solving the task even more efficiently.

As with other studies of animal cognition, this gives us important knowledge about the evolution and architecture of spatial navigation and generalized problem-solving. It reminds us that bigger brains do not always equal better problem-solving. It also gives us some clues as to the evolutionary adaptations that may have allowed humans to develop increasing spatial and navigational skills.

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