Quercus alba and Panthera leo: reading the epithet

Trees are feminine

Classical Latin tree names in -us are feminine, whatever their declension, so their adjectives end in -a:

The Romans made the tree feminine and its fruit neuter: malus, the apple tree, and malum, the apple; pirus, the pear tree, and pirum, the pear.

Not every tree: Acer, the maple, is neuter, so red maple is Acer rubrum.

Because the code fixes the genus's gender, an adjective epithet is rewritten when a species moves genus. The organism has not changed; the adjective has:

Nouns as epithets: apposition and the genitive

When the epithet is a noun, nothing agrees. It either stands beside the genus in the nominative, naming it again, or sits in the genitive, meaning "of".

Rex is a noun, so it never becomes regis or regem to suit the lizard. Henry Fairfield Osborn named it in 1905.

Vulpes vulpes is a tautonym: allowed in zoology, banned in botany. The queen of Strelitzia reginae is Charlotte of Mecklenburg-Strelitz, which explains the genus too.

Epithets worth knowing

Learn these and you can decode much of any field guide. First, where it lives:

Then what it is like:

Officinalis comes from officina, a workshop, and in medieval Latin the storeroom where medicines were kept: the epithet marks the species the apothecaries stocked. Ursus arctos horribilis, the grizzly, is a trinomial: genus, species, subspecies. George Ord coined horribilis in 1815 for a species of his own; it now ranks as a subspecies of the brown bear.