Fluctuating asymmetry (FA), is a form of biological asymmetry, along with anti-symmetry and direction asymmetry. Fluctuating asymmetry refers to small, random deviations away from perfect bilateral symmetry. This deviation from perfection is thought to reflect the genetic and environmental pressures experienced throughout development, with greater pressures resulting in higher levels of asymmetry. Examples of FA in the human body include unequal sizes (asymmetry) of bilateral features in the face and body, such as left and right eyes, ears, wrists, breasts, testicles, and thighs.
Research has exposed multiple factors that are associated with FA. As measuring FA can indicate developmental stability, it can also suggest the genetic fitness of an individual. This can further have an effect on mate attraction and sexual selection, as less asymmetry reflects greater developmental stability and subsequent fitness. Human physical health is also associated with FA. For example, young men with greater FA report more medical conditions than those with lower levels of FA. Multiple other factors can be linked to FA, such as intelligence and personality traits.
Measurement Fluctuating asymmetry (FA) can be measured by the equation: Mean FA = mean absolute value of left sides - mean absolute value of right sides.
The closer the mean value is to zero, the lower the levels of FA, indicating more symmetrical features. By taking many measurements of multiple traits per individual, this increases the accuracy in determining that individual's developmental stability. However, these traits must be chosen carefully, as different traits are affected by different selection pressures.
This equation can further be used to study the distribution of asymmetries at population levels, to distinguish between traits that show FA, directional asymmetry, and anti-symmetry. The distribution of FA around a mean point of zero suggests that FA is not an adaptive trait, where symmetry is ideal. Directional asymmetry of traits can be distinguished by showing significantly biased measurements towards traits being larger on either the left or right sides, for example, human testicles (where the right is more commonly larger), or handedness (85% are right handed, 15% are left handed). Anti-symmetry can be distinguished by the bimodal distributions, due to some adaptive functions.
Causes 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. Various factors causing developmental instability and FA include infections, mutations, and toxins.
Genetic factors Research on twins suggests that there are genetic influences on FA, and increased levels of mutations and perturbations is also linked to greater asymmetry.
FA may also result from a lack of genetic immunity to diseases, as those with higher FA show less effective immune responses. This is further supported by evidence showing an association between FA and the number of respiratory infections experienced by an individual, such that those with higher levels of FA experience more infections. Increased prevalence of parasites and diseases in an organism is also seen more in individuals with greater levels of FA. However, the research in this field is predominantly correlational, so caution must be taken when inferring causation. For example, rather than a lack of immunity causing FA, FA may weaken the immune responses of an organism, or there may be another factor involved.
There is some speculation that inbreeding contributes towards FA. One study on ants demonstrated that, although inbred individuals show more asymmetry in observed bilateral traits, the differences were not significant. Furthermore, ant colonies created by an inbreeding queen do not show significantly higher FA than those produced by a non-inbreeding queen.
Environmental factors Multiple sources provide information on environmental factors that are correlated with FA. A meta-analysis of related studies suggests that FA is an appropriate marker of environmental stress during development.
Some evidence suggests that poverty and lack of food during development may contribute to greater levels of FA. Infectious diseases can also lead to FA, as studies have repeatedly shown that those with higher FA report more infections. 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'). Fluctuating asymmetry in human males is also seen to positively correlate with levels of oxidative stress. This process occurs when an organism creates excess reactive oxygen species (ROS) compared to ROS-neutralising antioxidants. Oxidative stress may mediate the association seen between high FA and infection amounts during development.
Toxins and poisons are considered to increase FA. Pregnancy sickness is argued to be an adaptation for avoiding toxins during foetal development. Research has reported that when a mother has no sickness or a sickness that extends beyond week 12 of gestation, the offspring shows higher FA (as demonstrated by measuring thigh circumferences). 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. Greater exposure to pollution may also be a fundamental cause of FA. Research on skull characteristics of Baltic grey seals (Halichoerus grypus) demonstrated that those born after 1960 (marking an increase in environmental pollution) had increased levels of asymmetry. Also, shrews (Crocidura russula) from more polluted areas show higher levels of asymmetry. Radioactive contamination may also increase FA levels, as mice (Apodemus flavicollis) living closer to the failed Chernobyl reactor show greater asymmetry.
Developmental stability Developmental stability is achieved when an organism is able to withstand genetic and environmental stress, to display the bilaterally symmetrical traits determined by its developmentally programmed phenotype. To measure an individual's developmental stability, the FA measurements of 10 traits are added together, including ear width, elbows, ankles, wrists, feet, length of ears and fingers. This is achieved by: (L - R)trait 1 + (L - R)trait 2 + ......(L - R)trait 10. This provides a good overall measure of body FA, as every individual has some features that are not perfectly symmetrical.
Common environmental pressures leading to lower developmental stability include exposure to toxins, poison and infectious diseases, low food quality and malnutrition. Genetic pressures include spontaneous new mutations, and "bad genes" (genes that once had adaptive functions, but are being removed through evolutionary selection). 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. The energy required for bilateral symmetry development is extremely high, making fully perfect bilateral symmetry functionally nonexistent in natural organic creatures. 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.Graham, J. H., D. C. Freeman and J. M. Emlen. 1993. Developmental stability: A sensitive indicator of populations under stress, pp. 1366158. /)I W. G. Landis, J. S. Hughes and M. A. Lewis (Eds.), Environmental Toxicology and Risk Assessment. American Society for Testing and Materials, Philadelphia, PA.Moller, A. P. 1994. Sexual Selection and the Barn Swallow. Oxford University Press. Oxford.
Health
Susceptibility to diseases
Research has linked higher levels of fluctuating asymmetry (FA) to poorer outcomes in some domains of physical health in humans. For example, one study found that individuals with higher levels of FA report a higher number of medical conditions than those with lower levels of FA. However, they did not experience worse outcomes in areas such as systolic blood pressure or cholesterol levels. Higher levels of FA have also been linked to higher body mass index (BMI) in women, and lower BMI in men. Research has shown that both men and women with higher levels of FA, both facial and bodily, report a higher number of respiratory infections and a higher number of days ill, compared to men and women with lower levels of FA. In men, higher levels of FA have been linked to lower levels of physical attractiveness and higher levels of oxidative stress, regardless of smoking or levels of toxin exposure. There is no gender difference in the susceptibility of diseases depending on body FA."Gangsted2006">
A large-scale review of the human and non-human literature by Møller found that higher levels of fluctuating asymmetry were linked to increased vulnerability to parasites, and also to lower levels of immunity to disease. 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. 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.
Health-risk behaviours
It has been suggested that individuals with lower levels of FA may engage in more biologically costly behaviours such as recreational drug use and risky body modifications such as piercings and tattoos. 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. The relationship between FA and behaviours with high health risks has received mixed support. Individuals with body piercings and tattoos (which increase risk of blood-borne infections) have been shown to have lower levels of FA, but individuals with lower FA do not engage in any more recreational drug use than those with higher FA levels.
Mental health in humans
Higher levels of FA have been linked to higher levels of some mental health difficulties. For instance, it has been shown that, among university students, higher FA is associated with higher levels of schizotypy. Depression scores have been found to be higher in men, but not women, with higher levels of FA. One study by Shackelford and Larsen found that men and women with higher facial asymmetry reported more physiological complaints than those with lower facial asymmetry, and that both men and women with higher asymmetry experienced higher levels of psychological distress overall. For example, men with higher facial asymmetry experienced higher levels of depression compared to men with lower facial asymmetry. Fluctuating asymmetry has also been studied in relation to psychopathy. One study looking at offenders and non-offenders found that, although offenders had higher levels of FA overall, psychopathic offenders had lower levels of FA compared to offenders who did not meet the criteria for psychopathy. Additionally, offenders with the highest levels of psychopathy were found to have similar levels of FA to non-offenders.
Other health issues in humans