How many letters does the top answer for “of birds and mammals; having constant and relatively high body temperature” have?

The most common answer is homeothermic (12 letters).

Crossword clue: of birds and mammals; having constant and relatively high body temperature

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

homeothermic 12 letters wn-def

of birds and mammals; having constant and relatively high body temperature

homoiothermic 13 letters wn-def

of birds and mammals; having constant and relatively high body temperature