The declensions and the endings of words in general do not determine gender. In the first declension, nouns end in -a in the nominative sigular and are in the main feminine, but not so poeta, agricola 'farmer', and nauta 'sailor'. Poeta in Greek was also masculine. Likewise, not all nouns ending in -us are masculine: some in the third declension are neuter corpus and some in the 4th declension are feminine anus 'old woman' (not to be confused with the masculine 2nd declension word anus) and tribus. Also grammatical gender should be considered separately from natural gender.