from a female in follicular phase were actually considered more pleasant and sexier by males than odours perceived from that female in the luteal phase. Researchers discovered that the persistence of the menstrual cycle-dependent olfactory identification was extensive. However, the possibility of odours in the living environment overwhelming the menstrual cycle-dependent odours was not ruled out. Repeated exposure to the specific odours increases the threshold level of odour detection and therefore reduces the stimulation in humans. Additionally, it has been proposed through research that male in a relationship would be more sensitive to their partner's onset of ovulation-linked odour, and resultantly become desensitised after repeated exposure. On the other hand, non-paired males were continuously sensitive to the odour. This demonstrates the persistence of the odour as an evolutionary or adaptation process, which could ensure the offspring reproduction success of female by keeping constant exposure to males during their menstrual cycle. During this period, the most fertile females tend to have more extra-pair copulations. Males testosterone levels are found to be associated with a females’ reproduction fertility, such that males exposed to scents of females during the peak period of their fertility, had significantly higher testosterone levels than their baseline levels. Evolution suggests this is an adaptation due to the physical changes inside the male body, although more research must be conducted to ensure these high levels of testosterone lead to reproductive behaviours. Body odour and fluctuating asymmetry Fluctuating Asymmetry (FA) is a type of biological asymmetry, referring to the extent to which small random deviations occur from expected perfect symmetry in different populations of organisms. In humans, for example, FA can be demonstrated through the unequal sizes of bilateral features such as the eyes, ears and breasts. FA acts as an index for measuring developmental instability as it provides a clear indicator of the possible environmental and genetic stressors affecting development. It is thought that having a preference for a symmetrical face offers some adaptive value as such symmetry may signal an individual's ability to cope with environmental challenges. FA shares an inverse relationship with certain desired traits; a low FA is correlated with higher stress tolerance, larger body size in males, smaller body size in females, and higher facial attractiveness. FA is detectable through the olfactory senses and it has a measurable effect on sexual attraction. Significant cues may be found through body odour relating to a potential mate's