In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle, even when they are not about to ovulate. Humans and some other primates, do not have any obvious external signs to signal when ovulation occurs (concealed ovulation). Recent research suggests that women tend to have more sexual thoughts and are more prone to sexual activity right before ovulation (estrus). Nevertheless, this is nothing as obvious as the frenzy which other mammals show during 'heat'.