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Chimpanzee

Pan troglodytes

One of our two closest living relatives (along with the bonobo). It passes the mirror test.

Neurons: about 28 billion (predicted from scaling rules, not counted) estimate · whole brain
Herculano-Houzel S, Kaas JH (2011), prediction from primate scaling rules.
In the cerebral cortex (or bird pallium): 3.7 billion in one hemisphere (measured)
Collins CE et al. (2016). Cortical cell and neuron density estimates in one chimpanzee hemisphere. PNAS 113:740-745. doi:10.1073/pnas.1524208113.

Evidence

Chimpanzees pass the mirror mark test.
Gallup GG Jr (1970) Chimpanzees: self-recognition. Science 167(3914):86-87. doi:10.1126/science.167.3914.86.

The New York Declaration on Animal Consciousness (2024) finds strong scientific support for conscious experience in mammals and birds.

In the trees