Season of birth and fluctuating asymmetry

Anthropometric fluctuating asymmetries in living humans through the eyes of an anthropologist

There are many environmental and genetic factors that disrupt the stable structure of development in organisms. Although the strength of these vary, they leave certain signs in the body structure. Fluctuating asymmetry is a widely used population-level index of developmental instability, developmental noise, and robustness. Many bilateral traits are used in fluctuating asymmetry studies in humans. These traits include dermatoglyphics, limb lengths and widths, bilateral facial characters, and teeth. In this review, I evaluate the measurement methods of many bilateral anthropometric characters, taken from the bodies of living individuals with classical digital calipers.

Season of birth and biomarkers of early-life environment

OBJECTIVE

Early-life conditions play an important role in human development, affecting health status and survival. Conditions in utero partly depend on the external environment and thus vary in relation to the season of birth. The aim of this study was to investigate if people born in different seasons of the year differ in values of biomarkers that reflect conditions during fetal development.

METHODS

The study was conducted among Polish rural women recruited at the Mogielica Human Ecology Study Site. The participants were 234 women aged 45 to 92 (mean = 60.2; SD = 10.44). The indicators of early-life environment analyzed in the study were: Absolute Finger Ridge Count (AFRC), the difference between mean number of ridge counts in both thumbs and both little fingers (Md15), overall facial fluctuating asymmetry (OFA), central facial asymmetry (CFA), right and left hand 2D:4D.

RESULTS

Values of biomarkers of fetal development did not vary among groups of women born in different seasons of the year.

CONCLUSIONS

Lack of differences in values of biomarkers according to birth season may indicate that: (a) season of birth is not a good indicator of early-life conditions; (b) tested biomarkers do not reliably reflect the prenatal environment; (c) season of birth does not fully overlap with the sensitive periods of biomarker development and thus fails to capture differences in developmental conditions.

Analysing the seasonality of births in mainland China

The seasonality of human births varies in different countries and regions. Explanations for this variation have been divided into biological and behavioural factors. This paper documents birth seasonality in mainland China using data for a large sample from China's Fifth National Population Census (FNPC) conducted in 2000. The main method used was the decomposition of monthly time series birth data into annual, seasonal and random trends. The results show large seasonal birth fluctuations, with a salient peak of October births. The study hypothesis is that this seasonal birth pattern is partially due to a home-bound wave of movement of people after the annual Spring Festival. Subsequent analysis of the calculated de-trended monthly births provided supportive evidence for this hypothesis. Further in-depth analysis showed that the magnitude of births varied with location and family characteristics. This result should inform researchers in the field of economics, where seasonality of births has been previously regarded as exogenous.

Low temperatures during ontogeny increase fluctuating asymmetry and reduce maternal aggression in the house mouse, Mus musculus

Maternal aggression is behavior displayed by post-partum lactating female mice toward unfamiliar conspecifics, presumably as a defense against infanticide. A variety of perinatal stressors can impair maternal care in adulthood. Previous studies on associations between developmental perturbations and maternal aggression have produced mixed results. To address this issue, we employed a proxy for developmental instability, fluctuating asymmetry (FA) to further elucidate the relationship between low temperature stress and maternal aggression. FA, small, random deviations from perfect symmetry in bilateral characters is used as a quantitative measure of stress during ontogeny. Dams were either maintained in standard laboratory temperatures (21 ± 2 °C), or cold temperatures (8 ± 2 °C) during gestation. During lactation, their progeny either remained in the temperature condition in which they were gestated or were transferred to the other temperature condition. Four individual measures of FA, a composite of these measures, and three measures of maternal aggression were assessed in the female progeny in adulthood. Exposure to low temperatures during both pre- and early post-natal development increased composite FA and reduced all three measures of maternal aggression compared to controls. Exposure to low temperatures during the pre- or postnatal period alone did not induce either high FA or altered maternal aggression. Certain measures of FA and nest defense were negatively correlated. Our results suggest that low temperatures experienced during gestation and lactation may have important fitness costs. Low maternal aggression towards infanticidal conspecifics is likely to limit the number of offspring surviving into adulthood. Overall, FA appears to be a reliable indicator of chronic developmental stress with implications for fitness.

More symmetrical children have faster and more consistent choice reaction times

Greater cognitive ability in childhood is associated with increased longevity, and speedier reaction time (RT) might account for much of this linkage. Greater bodily symmetry is linked to both higher cognitive test scores and faster RTs. It is possible, then, that differences in bodily system integrity indexed by symmetry may underlie the associations of RT and intelligence with increased longevity. However, RT and symmetry have seldom been examined in the same study, and never in children. Here, in 2 large samples aged 4 to 15 (combined n = 856), we found that more symmetrical children had significantly faster mean choice RT and less variability in RT. These associations of faster and less variable RT with greater symmetry early in life raise the possibility that the determinants of longevity in part originate in processes influencing bodily system integrity early in the life-course.

The second to fourth digit ratio and asymmetry

BACKGROUND

Deviations of physical characteristics from bilateral symmetry, in otherwise symmetric individuals, are supposed to result from environmental perturbations during development. One cause of such perturbations may be sex steroids such as testosterone and oestrogen.

AIM

The study examined the relationship between second to fourth digit ratio (2D:4D), a putative negative correlate with prenatal testosterone and a positive correlate with prenatal oestrogen, and asymmetry.

METHODS

Eleven traits (including the second and fourth finger lengths) were measured in a sample of 680 English children aged 2-18 years, and second to fifth finger lengths in samples of 120 Austrian and English undergraduate students aged from 17 to 30 years and 213 Polish adults aged from 26 to 90 years.

RESULTS

Significant U-shaped curvilinear associations between 2D:4D and all 11 traits were found in English children with the strongest associations between 2D:4D and composite asymmetry of second plus fourth digit, and second to fifth digits. Further investigation of the relation between 2D:4D and digit asymmetries in the sample of Austrian and English undergraduates and the Polish adults confirmed significant U-shaped relationships between 2D:4D and finger asymmetries.

CONCLUSION

Our data show that both low 2D:4D (a marker of high prenatal testosterone) and high 2D:4D (a marker of high prenatal oestrogen) are associated with elevated levels of asymmetry and this relationship applies particularly to finger asymmetry.

Fluctuating asymmetry as an animal welfare indicator -- a review of methodology and validity

It has been suggested that fluctuating asymmetry (FA) reflects an animal's ability to cope with the sum of challenges during its growing period and, thus, is a potential welfare indicator. In this review we investigate the evidence of associations between FA and other welfare indicators measured at the level of the individual and of effects of welfare-relevant environmental conditions on FA in populations of captive birds and mammals including humans. As the question of validity cannot be treated independently from the quality of the available data, first a checklist for the proper measurement and analysis of FA is drafted and used to evaluate the methodological quality of the various studies. We recommend this checklist to be used as a standard for future FA studies. We found 17 relevant studies on associations between FA and other welfare indicators, and 36 studies on effects of welfare-relevant factors on FA. Frequent methodological shortcomings or insufficient methodological information allow for only cautious conclusions. The proportion of significant results supporting the link between higher FA and poorer welfare is only moderately high. Independent from statistical significance, almost all studies found the relationship between FA and welfare to be prevailingly in the expected direction. FA is a promising measure of animal welfare, despite a great number of open questions, e.g. relating to the ontogeny of FA or its sensitivity to various stressors. The considerable potential of FA as a welfare indicator makes it worthwhile to pursue more intensely validation studies as well as applied studies. These studies should pay particular attention to an appropriate methodological approach.

Genetics of anthropometric asymmetry in an Indian endogamous population—Vaidyas

To understand the genetics of Fluctuating Asymmetry (FA) and Directional Asymmetry (DA), the present study comprised 14 bilateral morphometric traits from 200 Vaidya families including 824 individuals (of two generations) from North 24 Parganas, West Bengal. The statistical analysis included: Regression analysis to remove the age effect, Familial correlation, Heritability estimation, Principal Component Analysis and Segregation Analysis (SA) using genetic model test. The obtained results revealed little effect of genetic factor and considerable amount of environmental influence on anthropometric asymmetry. The results support the idea postulated by several previous authors that FA provides a measure of developmental instability in man. The contribution of heredity on these asymmetric variables is not unimportant but that of the common environment is very substantial. The magnitude of heritability of DA traits is slightly higher than that of FA traits. Five principal factors were detected from these asymmetric traits (three factors are on asymmetry on length, head, and breadth; while last two factors represent the asymmetry of diameters). SA did not suggest any evidence of major gene contribution. But the involvement of minor genes or polygenes could not be discarded. As the study on SA of asymmetry in man is limited, similar other studies are needed to confirm the result of the present study.

Season of birth contributes to variation in university examination outcomes

Epidemiological studies show that birth season influences a wide range of biological parameters such as growth, reproduction, mental illnesses, dyslexia, personality, and success in school. The present study is aimed at examining birth season's relationship to examination marks achieved at a university in a very large contemporary sample of male and female undergraduate students. We find that female university students born in spring and summer achieve better marks than those born in autumn and winter. Male students born in spring receive worse marks than those born in other seasons of the year. Furthermore, we find a birth-week periodicity in examination results of female students, with highest examination results for those born in May. We suppose that biological mechanisms might explain part of the observed effects.

Neonatal exposure to short days and low temperatures blunts stress response and yields low fluctuating asymmetry in Siberian hamsters

Fluctuating asymmetry (FA) refers to small, non-directional deviations from perfect bilateral symmetry in morphological characters. Individuals with low FA presumably either developed in a relatively stable environment and/or were better able to buffer against developmental stressors. The present study investigated the effects of seasonal factors measured by day length and ambient temperature manipulations on the development of bilateral characters and concomitant changes in stress responses. Siberian hamsters were exposed to either long days (16 h of light per day) or short days (8 h of light per day) combined with either standard temperatures (21+/-2 degrees C) or low temperatures (8+/-2 degrees C) on the day of birth until weaning. Cortisol concentrations at baseline and following acute restraint stress, and FA values were measured in adulthood. Females reared in winter-like conditions with short day lengths and low temperatures had low FA and low cortisol concentrations following restraint stress compared to other females. Females reared in long day lengths and standard temperatures had the highest rate of increase in cortisol concentrations after restraint among other female groups. No group effects were observed in males regarding day length and temperature manipulations. Baseline and post-restraint cortisol concentrations were higher in females than males for all groups except in animals reared in short day lengths and low temperatures. Our results suggest that winter-like conditions during neonatal period evoke hyposensitivity to stress in adult females and this blunted response to stress is a key factor in achieving ideal growth patterns.

Breast asymmetry and predisposition to breast cancer

INTRODUCTION

It has been shown in our previous work that breast asymmetry is related to several of the known risk factors for breast cancer, and that patients with diagnosed breast cancer have more breast volume asymmetry, as measured from mammograms, than age-matched healthy women.

METHODS

In the present study, we compared the breast asymmetry of women who were free of breast disease at time of mammography, but who had subsequently developed breast cancer, with that of age-matched healthy controls who had remained disease-free to time of the present study. The study group consisted of 252 asymptomatic women who had normal mammography, but went on to develop breast cancer. The control group were 252 age-matched healthy controls whose mammograms were also normal and who remained free of cancer during the study period. Breast volume was calculated from the cranio-caudal mammograms for each group, and the relationships between asymmetry, established risk factors and the presence or absence of breast cancer were explored.

RESULTS

The group who went on to develop breast cancer had higher breast asymmetry than controls (absolute asymmetry odds ratio 1.50 per 100 ml, confidence interval (CI) 1.10, 2.04; relative asymmetry 1.09, CI 1.01, 1.18), increased incidence of family history of breast cancer, lower age at menarche, later menopause, later first pregnancies and a higher frequency of high risk breast parenchyma types. Conditional logistic regression analysis showed that breast asymmetry, height, family history of breast cancer, age at menarche, parenchyma type and menopausal status were significant independent predictors of breast cancer. When age at menopause was included in the model for the subgroup of post-menopausal women, absolute breast fluctuating asymmetry (FA) and relative breast FA remained significant effects.

CONCLUSION

Breast asymmetry was greater in healthy women who later developed breast cancer than in women who did not.

Digit length ratios predict reactive aggression in women, but not in men

Considerable evidence suggests that digit length and dermatoglyphic asymmetry patterns in the hand are affected by early exposure to androgens. Because androgens play an important role in sexual differentiation of morphological and behavioral traits, digit length patterns often display sex differences. When present, sex differences in digit lengths are more pronounced on the right side as compared to the left side. Moreover, the ratio of the second to fourth digit length (2D:4D) in the right hand is inversely correlated with testosterone (T) in men. Because T is implicated in agonistic behavior, 2D:4D may be used as a marker of androgen exposure and subsequent behavioral variation in adulthood. Consequently, we investigated the relationships among 2D:4D, directional asymmetry of 2D:4D (left hand 2D:4D minus right hand 2D:4D) as a variant of T, and human reactive aggression. One hundred young men (n = 51) and women (n = 49) participated in our experiments (mean age = 20.1 years). Participants called two noncompliant confederates to solicit donations for a fictitious charity organization and selected follow-up letters after the calls. The force exerted when hanging up the phone and the "tone" of the follow-up letters were used to assess reactive aggression. High aggression scores were associated with high directional asymmetry of 2D:4D and masculinized (low) right hand 2D:4D, only in females and under high provocation. Directional asymmetry of 2D:4D was positively correlated with T in males (pooled data, n = 97). Taken together, these data confirm the predominantly right-sided influence of androgens on digit length and suggest that digit length ratios may be associated with female reactive aggression when sufficient provocation is present.