#oestrous

Prynhawn,

There are indeed different British English and American English spellings. Here’s a clarifier:

Estrus: recurring period of sexual receptivity and fertility in many female mammals; heat.

Estrous: American English spelling, the main reproductive cycle of other species females of non-primate vertebrates, for example rats, mice, horses, pig have this form of reproductive cycle. (Also do not confuse with “estrus”, which is a phase of the cycle.)

Oestrous: British English spelling of estrous.

For some reason after the revolutionary war America decided that it needed to spell things differently. Probably just to be difficult.