The mammalian genome is coding for about 1000 different types of olfactory receptors, but there seems to be a reduction in the proportion of functional olfactory receptor genes in some species. Humans, for example, have only about 390 functional olfactory receptor genes, with the rest being pseudo genes which are assumed not to be transcribed into proteins whereas Old World primates, such as macaques, have about 700 and New World primates such as squirrel monkeys have about 900 functional olfactory receptor genes. Other species that have been investigated in this respect are mice, which have about 1190 functional receptor genes, rats have about 1280, dogs have about 870 functional receptor genes and honey bees, which have only about 160 functional olfactory receptor genes.

The large between-species differences in the number of functional olfactory receptor genes raises the question whether these affect threshold sensitivity and/or discrimination performance.