Small neotropical mammals were studied in relation to factors determining the onset of the breeding season. The first studies were based on data from specimens housed at the Museu Nacional, Rio de Janeiro, and were followed by field and laboratory studies. Two modes of reproduction were first proposed. Later, it was found that marsupials have the onset of the breeding season determined by the variation of photoperiod. The Sigmodontini rodents in Northeastern Brazil have their reproduction set by the beginning of the rainy season. Therefore, the marsupial mode of reproduction is markedly climatically seasonal, being somewhat independent of environmental conditions, solstices and equinoxes functioning as proximal factors. The Sigmodontini mode of reproduction has the onset of the breeding season determined by the storage of reserves by the females being linked directly to primary factors.
