These are important in naturally occurring triesters (lipids).
The unsaturated acids found inn these esters have the cis-configuration at the double bond. This means that the molecules have relatively uncompact structures, so that intermolecular forces between unsaturated triglyceride molecules are weaker than those in the corresponding saturated triglycerides, meaning that vegetable and fish oils, which contain a high proportion of unsaturated triglycerides, are liquid at room temperature, whilst the saturated triglycerides found in animals tend to be solids at room temperature (fats, such as lard).
Scientists have investigated an ester of an unsaturated acid called ethyl oleate. The body makes this from oleic acid, a natural fatty acid, and the ethanol ("alcohol") from drinks; fatty acid ethyl esters are the major metabolites of ethanol in neural tissues. They found that ethyl oleate speeds up the release of potassium ions from brain cells, which in turn slows down the release of neurotransmitters that the body uses to pass on messages. This results in symptoms like slurred speech and slow reflexes. Sound familiar?
A recent discovery concerning a simple unsaturated monoester is that female Asian elephants release (Z)-7-dodecenyl acetate in their urine to signal that they are ready to mate. It uses the same pheromone as the turnip looper, the cabbage looper and over a hundred other species of butterflies and moths.