To call someone “monkey-brained” may not be all that much of an insult after all, as a new study in the journal PeerJ indicates that the size of a primate’s brain isn’t necessarily indicative of its intelligence. According to the researchers’ findings, lemurs can outperform great apes on certain cognitive tasks, despite having brains that are 200 times smaller.
The relationship between brain size and smarts has been a source of debate for some time, with the so-called General Intelligence hypothesis stating that larger brains allow for greater memory capacity and faster learning. This argument has been used to explain why humans, which have big brains relative to their body size, are so clever.
The great apes and other primates are our closest living evolutionary relatives and are all proud owners of large brains. Yet which of these species is the most intelligent is not something that can easily be determined.
In an attempt to identify the brainiest of the bunch, scientists have conducted a number of experiments using the Primate Cognition Test Battery (PCTB), which consists of a number of tasks that are designed to assess various aspects of intelligence. The scientific literature already contains a relative wealth of data regarding the PCTB scores of haplorhine primates – which includes great apes such as chimpanzees, orangutans, and gorillas, as well as monkeys like baboons and macaques.
Yet the authors of this latest study decided to apply the PCTB to lemurs, which split from the main primate lineage some 60 million years ago and therefore retain many ancestral primate traits. As such, they provide a living model for the origins of modern primates’ cognitive capacities.
In a major blow for the General Intelligence hypothesis, the three lemur species that the researchers put to the test all achieved scores comparable with the larger-brained haplorhines. Most intriguingly, the lemurs outperformed great apes on the theory of mind scale, which is evaluated using tests that require subjects to follow another individual’s gaze and figure out their intentions.
Lemurs perform a range of physical domain challenges as part of the Primate Cognition Test Battery at the German Primate Center (DPZ).
Even mouse lemurs, which have brains 200 times smaller than chimpanzees and orangutans, achieved scores comparable to other primates on the majority of tests within the social domain. These include the social learning scale, in which animals must use the information provided by a human in order to solve a puzzle, and the communication scale, which requires primates to understand signals given by humans.
Certain other primates did outperform the lemurs on physical domain tasks, though, with chimps proving to be the most adept at challenges that involved figuring out quantities, while apes and baboons were better than lemurs at spatial tasks.
“With our study we show that cognitive abilities cannot be generalized, but that species instead differ in domain-specific cognitive skills,” explained study author Claudia Fichtel. “Accordingly, the relationship between brain size and cognitive abilities cannot be generalized.”
Technically, then, this may mean that not all humans can claim to be smarter than monkeys just because they have bigger brains.