serotonin

a phenolic amine neurotransmitter C10H12N2O that is a powerful vasoconstrictor and is found especially in the brain, blood serum, and gastric mucous membrane of mammals—called also 5-HT, 5-hydroxytryptamine

Serotonin is a neurotransmitter and is found in allbilateral animals, where it mediates gut movements and the animal's perceptions of resource availability.

In less complex animals, such as some invertebrates, resources simply mean food availability. In more complex animals, such as arthropods and vertebrates, resources also can mean social dominance. In response to the perceived abundance or scarcity of resources, an animal's growth, reproduction or mood may be elevated or lowered. This may somewhat depend on how much serotonin the organism has at its disposa