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Cats reproduce when females begin estrus, more commonly known as “going into heat,” during which hormones and behavior changes signal she’s ready to mate.

But now they must wait until the female rhino is in estrus—the period when the animal is ready to mate—to implant the egg.

In the other, the whales weren’t pregnant during the first massive progesterone surge, but were instead experiencing a prolonged state of estrus or ovulating.

Otter moms will often go immediately into estrus if they lose a pup.

Instead, a bull spends his annual two- or three-month stint of erotic drive searching for willing females in estrus, or fighting with other males in musth, all the while eating relatively little.