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Rat

Rattus norvegicus

About 200 million neurons. Rats release trapped cagemates.

Neurons: 200 million measured · whole brain (isotropic fractionator)
Herculano-Houzel S, Mota B, Lent R (2006). Cellular scaling rules for rodent brains. PNAS 103:12138-12143. doi:10.1073/pnas.0604911103.
In the cerebral cortex (or bird pallium): 31 million
Herculano-Houzel S, Lent R (2005). Isotropic fractionator. J Neurosci 25:2518-2521. doi:10.1523/JNEUROSCI.4526-04.2005.

Evidence

Rats free trapped cagemates. This has been interpreted as empathy-driven helping.
Ben-Ami Bartal I, Decety J, Mason P (2011). Empathy and pro-social behavior in rats. Science 334:1427-1430. doi:10.1126/science.1210789.

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