Psychologist Robert Deaner of Grand Valley State University in Michigan scoured hundreds of cognition studies to pin down which nonhuman primate species is the smartest.
To avoid methodological differences, he identified nine types of task intelligence, like navigating through mazes or deciphering a tangle of patterned strings.
After ranking 24 evolutionarily diverse primate genera, he found that very few of them did well on one task and particularly badly on another. Some genera just prove more capable overall.
The great apes, not unexpectedly, cluster at the top, while groups that evolved earlier, like lemurs, cluster at the bottom.
Deaner says his findings support the hypothesis that absolute brain size matters more than brain size adjusted for body size. What's more, he adds, the longer you live, the smarter you're likely to be.
The Hierarchy of Primate Intelligence