Exceptions At times, nature defies the human desire for orderly taxonomy. For example, there are mammalian monotremes that lay eggs (e.g. echidnas), flowering plants that lack chlorophyll and cold-blooded naked mole rats.

The Psittacidae (parrots) appear to have some mammalian behaviours. They exhibit play, learning through modelling, interspecies bonding and kissing and certain species are highly regarded for their intelligence. Some humans reciprocally bond with them, carrying them on their shoulders. This is never done with other avian species.