Since the summer of 2002 we have been studying cognition in lemurs at the Duke Lemur Center using a variety of methods. Prosimians (lemurs) are a suborder of primates that split from the common ancestor of monkeys, apes, and humans approximately 47-54 million years ago and are our best living model of the earliest primate mind.
The Duke Lemur Center has the largest living collection of prosimian primates and therefore offers unparalleled opportunities for comparative cognition. Thus far we have only trained and tested a handful of species. Many of our studies require training lemurs to respond via touch sensitive screens hooked up to computers and pellet delivery systems. Unexpectedly many of the lemurs prefer to respond with their noses rather than their hands!
Research over the last 25 years has demonstrated that animals are able to organize sequences in memory and retrieve ordered sequences without language. Qualitative differences have been found between the serial organization of behavior in pigeons and monkeys. Here we test serial ordering abilities in ring-tailed lemurs, a prosimian primate whose ancestral lineage diverged from that of higher primates, including monkeys and apes, over 50 million years ago. Lemurs’ accuracy and response times were similar to monkeys, thus suggesting that the serial organization of memory may date to a common primate ancestor.
Transitive Inference is a form of deductive reasoning that has been suggested as one cognitive mechanism by which animals could learn the many relationships within their group’s dominance hierarchy. This process thus bears relevance to the social intelligence hypothesis which posits evolutionary links between various forms of social and nonsocial cognition. Recent evidence corroborates the link between social complexity and transitive inference and indicates that highly social animals may show superior transitive reasoning even in nonsocial contexts. We examined the relationship between social complexity and transitive inference in two species of prosimians, a group of primates that diverged from the common ancestor of monkeys, apes, and humans over 50 million years ago.
In Experiment 1, highly social ring-tailed lemurs, Lemur catta , outperformed the less social mongoose lemurs, Eulemur mongoz , in tests of transitive inference and showed more robust representations of the underlying ordinal relationships between the stimuli. In Experiment 2, after training under a correction procedure that emphasized the underlying linear dimension of the series, both species showed similar transitive inference. This finding suggests that the two lemur species differ not in their fundamental ability to make transitive inferences, but rather in their predisposition to mentally organize information along a common underlying dimension. Together, these results support the hypothesis that social complexity is an important selective pressure for the evolution of cognitive abilities relevant to transitive reasoning.
We are measuring Weber fractions for numerical discrimination in a variety of lemurs. In an initial study we found that ringtailed lemurs (Lemur catta) represent the ordinal relations between numerosities much like monkeys. Currently we are comparing numerical acuity in mongoose lemurs, ring-tailed lemurs, and blue-eyed black lemurs.
In addition to training lemurs with touch-screens, we’ve employed a different kind of task to examine their numerical capacities. Mongoose lemurs observed as an experimenter sequentially placed grapes into an opaque bucket. On half of the trials the experimenter placed a subset of the grapes into a false bottom such that they were inaccessible to the lemur. The critical question was whether lemurs would spend more time searching the bucket when food should have remained in the bucket compared to when they had retrieved all of the food. We found that the amount of time lemurs spent searching was indicative of whether grapes should have remained in the bucket and furthermore that lemur search time reliably differentiated numerosities that differed by a 1:2 ratio but not those that differed by a 2:3 or 3:4 ratio. These results suggest that mongoose lemurs have numerical representations that are modulated by Weber’s Law.
We are currently examining uncertainty judgments and decision making in a variety of lemurs. Preliminary data suggests that ringtailed and mongoose lemurs are extremely conservative decision makers! They almost always prefer a safe bet over a variable option even when the two options yield the same average reward.