mating season; estrus; rut
Refers to the period when animals are sexually receptive and mate. Used for mammals in general; 発情期 can apply to both wild and domestic animals.
Cats' mating season is often in spring and autumn.
During the deer's rut, fights between males can be observed.
year-round mating season
Compound of 発情 (hatsujō, 'sexual excitement, estrus') + 期 (ki, 'period, season'). The exact historical derivation is straightforward.