Fluctuating asymmetry
This can further have an effect on mate attraction and sexual selection, as less asymmetry reflects greater developmental stability and subsequent fitness.
Fluctuating asymmetry (FA) is often considered to be the product of developmental stress and instability, caused by both genetic and environmental stressors.
The notion that FA is a result of genetic and environmental factors is supported by Waddington's notion of canalisation, which implies that FA is a measure of the genome's ability to successfully buffer development to achieve a normal phenotype under imperfect environmental conditions.
[3] Research on twins suggests that there are genetic influences on FA,[10] and increased levels of mutations and perturbations is also linked to greater asymmetry.
One study on ants demonstrated that, although inbred individuals show more asymmetry in observed bilateral traits, the differences were not significant.
[3][17] Alternatively, this association between levels of FA and infections may be due to a lack of immunity to diseases, as mentioned earlier (see 'Genetic factors').
This process occurs when an organism creates excess reactive oxygen species (ROS) compared to ROS-neutralising antioxidants.
This suggests that when a mother fails to expel environmental toxins, this creates stress and developmental instability for the foetus, later leading to increased asymmetry in that individual.
[21] Radioactive contamination may also increase FA levels, as mice (Apodemus flavicollis) living closer to the failed Chernobyl reactor show greater asymmetry.
Common environmental pressures leading to lower developmental stability include exposure to toxins, poison and infectious diseases, low food quality and malnutrition.
A large fluctuating asymmetry (FA) and a low developmental stability suggests that an organism is unable to develop according to the ideal state of bilateral symmetry.
Energy is invested into survival in spite of the genetic and environmental pressures, before making bilaterally symmetrical traits.
Research has also revealed links between FA and depression, genetic or environmental stress and measures of mate quality for sexual selection.
[24][25][26][27] Research has linked higher levels of fluctuating asymmetry (FA) to poorer outcomes in some domains of physical health in humans.
[32] A large-scale longitudinal study in Britain found that facial FA was not associated with poorer health over the course of childhood, which was interpreted as suggesting smaller effects of FA in Western societies with generally low levels of FA [33] A review of the relationship between various attractiveness features and health in Western societies produced similar results, finding that symmetry was not related to health in either sex, but was related to attractiveness in males.
[34] It has been suggested that individuals with lower levels of FA may engage in more biologically costly behaviours such as recreational drug use[35] and risky body modifications such as piercings and tattoos.
[36] These ideas have been proposed in the context of Zahavi's handicap principle, which argues that highly costly behaviours or traits serve as signals of an organism's genetic quality.
[45][46][47] In addition to general health and susceptibility to disease, research has also studied the link between FA and physical fitness.
Research has found that lower levels of lower-body FA is associated with faster running speeds in Jamaican sprinters,[48] and individuals with greater body asymmetry have been shown to move more asymmetrically while running, although do not experience higher metabolic costs than more symmetrical individuals.
[53] It has also been found that, among Iberian red deer (Cervus elaphus hispanicus), higher FA was slightly negatively related to both antler size and overall body mass (traits thought to indicate overall condition).
[55] While research on the relationship between FA and longevity is sparse in humans, some studies using non-human populations have suggested an association between the symmetry of an organism and its lifespan.
[63] However, research has failed to find changes in women's preferences for low FA across the menstrual cycle when assessing pictures of faces, as opposed to scents.
It was also found that sex-typical joint configurations were rated as more attractive and linked to lower FA in men, but not women.
[68] Research by Van Dongen also found FA to be unrelated to attractiveness, physical strength and level of masculinity in both men and women.
For instance, FA of male forewing length seem to have an important role in successful mating for many insect species, such as dark-wing damselflies and Japanese scorpionflies.
[71] Other animals also show similar patterns, for example, many species of butterfly, males with lower levels of FA tended to live longer and flew more actively, allowing them to have more reproductive success.
Therefore, the males with longer and more symmetrical tails show higher levels of reproductive success with more attractive females.
For instance, in the lekking black grouse and red junglefowl, no correlations were found between FA and mating success.
From a general view, one would expect someone who is more symmetrical (usually meaning greater attractiveness), to be high on agreeableness, conscientiousness, extraversion and openness, and low on neuroticism.
It is suggested that the stress introduced during early embryonic stages via certain factors (such as excess serotonin) may create developmental instability, causing phenotypic and behavioural variations (such as increased or decreased aggression).
