Why don't female cats bleed during their menstrual cycle?

Simply because female cats don't have menstrual cycles. In human females, menstruation occurs due to the disintegration of the endometrium, the mucous membrane that lines the uterus. If no egg is fertilised, the endometrial cells shed and are evacuated from the body in the form of menstrual blood, a characteristic shared by primates such as gorillas.

With other female mammals, the process is known as an oestrous cycle. Dogs, for example, bleed at the beginning of their cycle, not because of the elimination of endometrial matter but because of a strong rush of blood to their genitals when they go into heat.

For feline females, explains Dr Réjean Lefebvre, a reproductive specialist at the Faculty of Veterinary Medicine at the Université de Montréal, discharges at the beginning of the oestrous cycle go unnoticed.