Logical inferences from visual and auditory information in ruffed lemurs and sifakas

Inference by exclusion, or the ability to select a correct course of
action by systematically excluding other potential alternatives, is a form
of logical inference that allow individuals to solve problems without
complete information. Current comparative research shows that several
bird, mammal, and primate species can find hidden food through inference
by exclusion. Yet there is also wide variation in how successful different
species are, as well kinds of sensory information they can use to do so.
An important question is therefore why some species are better at engaging
in logical inference than others. Here, we investigate the evolution of
logical reasoning abilities by comparing two strepsirrhine primate species
that vary in dietary ecology: frugivorous ruffed lemur (Varecia spp.) and
folivorous Coquerel’s sifakas (Propithecus coquereli). Across two studies,
we examined their abilities to locate food using direct information versus
inference from exclusion and using both visual and auditory information.
In Study 1, we assessed whether these lemurs could make inferences when
full visual and auditory information about the two potential locations of
food were provided. In Study 2, we then compared their ability to make
direct inferences versus inferences by exclusion in both the visual and
auditory domains. We found that both lemur species can use visual
information to find food, but that only ruffed lemurs were also able to
use auditory cues, mirroring differences in the complexity of their wild
ecology. We further found that, unlike many anthropoid species tested to
date, both strepsirrhine species failed to make inferences by exclusion.
These results highlight the importance of natural history in understanding
the evolution of logical inference, and help reconstruct the deeper
phylogeny of primate cognition.