noun
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.
猫の発情期は春と秋に多い。
Cats' mating season is often in spring and autumn.
鹿の発情期にはオス同士の争いが見られる。
During the deer's rut, fights between males can be observed.
Compound of 発情 (hatsujō, 'sexual excitement, estrus') + 期 (ki, 'period, season'). The exact historical derivation is straightforward.