...and what can word-learning in dogs teach us about the evolution of language in humans?
What is involved in the learning of a single new word? Consider the word "tiger", being learned by a child with already a modest vocabulary, at least for animal words. First the child must make a new entry in the mental lexicon - that "tiger" is a word in the first place. He has to categorize it as a noun. It has to be categorized under "animal" (a supernym) and related to its hyponyms, like "Sumatran tiger." Then, of course, the child has to learn what actual *thing* the word "tiger" refers to. Now, various conceptual categories likely have to be restructured. Before, the child might have referred to tigers as "cats," but now the child has must conceptually distinguish cats from tigers. Sometimes, the child has to accomplish all of this without explicit instruction; he or she may be exposed to a word casually, or in the course of conversation. Early research showed that children indeed were able to learn new words after just a single casual exposure. As you can see, learning only one new word involves learning a considerable amount of new information.
The process by which a child learns a new word after only one exposure is called "fast mapping." And kids "fast map", well, fast. And often. From 2 years of age, typical English-speaking children add about ten new words a day to their vocabulary until they reach an average vocabulary size of 60,000 words by high school graduation.
Is the ability to fast map unique to language learning, or does it reflect more general cognitive learning skills that may be shared with other animals? Meet Rico.
Rico, a border collie, was born in December 1994. At the time of this research, he was about 10 years old. He was first introduced to the fetching of items at 10 months of age, when his owners would place various items around the room and ask the dog to retrieve one of them. He was rewarded with food or attention if he retrieved the correct object. Over the years, he was introduced to more and more items, and by the time of this research, his owners reported that he knew the labels of over 200 individual items, mostly toys and balls, which he could correctly retrieve upon request. It was unclear, however, whether Rico was simply responding to subtle behavioral cues of his owners, as opposed to actually knowing the words for the objects. Was Rico a modern-day canine version of Clever Hans?
Experiment 1 was designed to assess Rico's ability to retrieve various items under precisely controlled conditions, to ensure that his owners weren't unintentionally cuing him to the proper answer. The owner waited with Rico in one room, while the experimenter arranged a set of eight of Rico's items in a second room, before joining the owner and dog. Then, the owner asked the dog to bring two randomly chosen items (from the set of possible items set out) from the next room. This ensured that while Rico searched for the proper item, he could not see the owner or the experimenter. He retrieved a total of 37 out of 40 items correctly, indicating that that he indeed knew the labels of those objects. Amazingly, Rico's vocabulary was larger than any other dog studied, and was comparable to the vocabularies of language-trained monkeys, dolphins, sea lions, and parrots.
Experiment 2 addressed Rico's potential ability to fast map. The setup was the same, but this time one new item that Rico had never seen before (and for which he did not know the name) was included together with seven familiar items. In each of 10 sessions, a different novel item was introduced each time, for a total of 10 new items. There were 8 trials per session (one trial per item, per set), for a total of 80 trials. Of course Rico took breaks between each session.
In this video, Rico is first instructed to bring two familiar items: first the "tyrex" (the blue dinosaur) and next the "weihnachtsmann" (the little red doll). Subsequently, a novel word ("sirikid") is used to ask for the novel item, the white bunny. Perhaps now it would make sense to mention that Rico and his owners lived in Germany. Rico successfully retrieved the correct new item (the white bunny) in the first session, and continued to do so in 7 out of 10 sessions for additional new items. Amazing! The experimenters reasoned that he was able to link the novel word with the unfamiliar item based on exclusion learning - because he already knew the familiar items names' or because they were not unfamiliar.
Experiment 3 was conducted four weeks later, and was designed to assess Rico's retention of the relationship between the novel words and the novel items. Three of the novel items from experiment 2 were used again in this experiment. Again, the procedure was the same as in experiments 1 and 2. The target item, along with four additional novel items, and four familiar items were laid out in the room, for a total of nine items. As before, Rico was first asked to bring one or two familiar items before being asked for the target item. Rico correctly retrieved the target item in three out of six sessions. It is worth noting that in the failed trials, Rico never brought a familiar item when asked for the target item; he always brought one of the other unfamiliar items. Fifty percent success doesn't seem like much, but consider that his success rate was comparable to three year old humans, in similar tasks.
Overall, these experiments indicate that Rico consistently learns to associate arbitrary acoustic patterns (i.e. human words) with specific objects. His extensive experience with acquiring object names has allowed him, apparently, to learn that objects have names. Further, he has been able to learn the association between a new word and a new object based on exclusion. He is also able to store this association in memory, since he was able to correctly retrieve the target item from a set of familiar and unfamiliar items both immediately as well as one month later.
It may be the case that Rico's amazing word-learning ability is the result of an "exceptional mind" or his extensive experience with learning word-object relations. Even if that is the case, as for any other cognitive capability, Rico's performance can be broken into its constituent building blocks:
(1) the principle that objects have labels,
(2) a general learning mechanism, driven by the exclusion principle, and
(3) the ability to store that knowledge in long-term memory.
Young children, of course, have a much broader understanding about words and their functions than Rico does. Children (even babies) can readily distinguish among nouns and verbs, for example, based on their acoustic structure. Children are also able to use newly-learned words productively in speech, while dogs, obviously, do not. However, this research suggests that the ability to attach meaning to specific sounds may have emerged earlier than, and evolved independently from, the ability to flexibly produce those sounds. Some of the perceptual and cognitive building blocks that underlie speech comprehension may have been in place before humans began to talk. What appears to be a complex linguistic skill used by human children to learn new words may depend on simpler cognitive building blocks also present in at least one other species, the domesticated dog.
Kaminski, J., Call, J., & Fischer, J. (2004). Word Learning in a Domestic Dog: Evidence for "Fast Mapping" Science, 304 (5677), 1682-1683. DOI: 10.1126/science.1097859