In my very first post, I talked about how dogs can understand human social cues such as pointing to find hidden food. An interesting study published this month investigates whether domesticated pigs can understand human social cues, too.
The leading hypothesis of how dogs gained the ability to understand human social cues suggests that the ability may develop along with domestication (which I discussed here). It therefore seems reasonable that domesticated pigs may also possess the ability to understand human social cues.
Nawroth et al. (2013) conducted the study investigating the ability of young domesticated pigs to understand human social cues. Something I particularly like about this study is that they really tested the limitations of this ability in pigs by examining many factors, including the distance of the experimenter to the food and the length of time the experimenter pointed to the food.
The researchers used the same task throughout their entire experiment: an experimenter positioned between two bowls (only one of which contains food) makes some sort of cue towards the baited bowl, and then the pig chooses a bowl.
The first part of the study investigated two factors using what’s called a 2 x 2 factorial design. This type of study looks at two factors (independent variables), where each factor has two levels (a 2 x 2 x 2 factorial design would look at three factors with 2 levels each; a 3 x 3 factorial design would look at two factors with 3 levels each).
The first factor was the position of the experimenter: kneeling or standing. The second factor was the length of time the experimenter pointed to the food: one second (momentary) or until the pig chose a bowl (sustained). In a 2 x 2 factorial design, there will be four experimental conditions comprising all possible combinations of the factors; in this case, the conditions were: standing with momentary pointing, standing with sustained pointing, kneeling with momentary pointing, and kneeling with sustained pointing. The benefit to using a factorial design study is that it makes it easy to compare the effects of different factors (and combinations of factors) on the results (in this case, the pigs’ performance).
Nawroth et al. found that the pigs performed better than chance when the experimenter was kneeling, regardless of how long the experimenter pointed at the baited bowl. When the experimenter stood, however, performance was no better than chance. The researchers suggested two reasons why the pigs understood the kneeling experimenter better than the standing experimenter: one, the experimenter’s pointing hand was closer to the food in the kneeling condition than in the standing condition, so the pigs could rely on a close proximity between the experimenter’s hand and the bowl to make their choice. Alternatively, since pigs spend much of their time with their heads close to the ground, foraging, the standing experimenter may be outside the pigs’ field of attention (the pigs may not pay attention to things that far above the ground).
To investigate these two possibilities, Nawroth et al. tested the pigs on the two kneeling conditions from the first part of the experiment (momentary and sustained pointing), but this time, the bowls were farther apart from each other. Thus, in this part of the study, the distance between the kneeling experimenter’s pointing hand and the bowl was the same as the distance between the standing experimenter’s pointing hand and the bowl in the first part of the study.
The pigs performed above chance in both conditions, indicating that the height at which the social cue was presented, and not necessarily the distance between the experimenter’s hand and the bowl, affected the pigs’ performance.
Nawroth et al. further investigated the effect of the experimenter’s proximity to a bowl by testing the pigs with the experimenter either kneeling behind the baited bowl, or kneeling behind the empty bowl and pointing to the other (baited) bowl. They found that the pigs performed better than chance when the experimenter kneeled behind the baited bowl, but not when the experimenter kneeled behind the empty bowl and pointed to the baited bowl. So, while the pigs were unaffected by the distance between the experimenter’s pointing hand and the bowl in the second part of the experiment, the much closer distance between the experimenter and the bowl in this part of the experiment served as a strong cue for the pigs.
Nawroth et al. tested a few more factors with additional experiments – I don’t have space to detail them here, but you can read about them in the original paper, if you’re interested.
The gist of their study, though, is that pigs can understand human social cues including pointing (and also head and body orientation – part of the study I didn’t discuss here). There are some limitations to this ability (very close human proximity trumps the pointing gesture as a cue). But the fact that pigs are generally able to use these cues lends support to the hypothesis that this ability develops through domestication.
(As Nawroth et al. discuss in the introduction and discussion sections of their paper, the ability to understand pointing cues has also been found in other domesticated species, including cats, goats, and horses. However, none of these previous studies investigated head and body orientation cues.)
Nawroth, Christian, Mirjam Ebersbach, and Eberhard von Borell. “Juvenile domestic pigs (Sus scrofa domestica) use human-given cues in an object choice task.” Animal Cognition (2013): 1-13.