Symmetrical women have higher potential fertility

Abstract

Fluctuating asymmetry (FA) is believed to reflect developmental stability and thus may serve as a marker of the biologic quality of individuals. To test the hypothesis that degree of FA is related to a woman's potential fertility, we measured finger length together with levels of estradiol in saliva samples collected daily for an entire menstrual cycle in 171 Polish urban and rural women. We show that women who are more symmetrical, as assessed by the degree of inequality in the fourth-digit length of their right and left hands, have 13% higher average levels of estradiol over the menstrual cycle than less symmetrical women (19.4 and 17.1 pmol/l, respectively; p=.0001). Among urban women, mid-cycle levels of estradiol were 28% higher in the symmetrical group than in the asymmetrical group. Because higher hormone levels in women may lead to a substantial rise in the probability of conception, low FA may therefore be associated with increased fertility.

Introduction

Evolutionary psychology suggests that human traits that are considered attractive function as reliable markers of the biologic condition and reproductive potential of individuals (Grammer et al., 2003, Symons, 1995) Women, at least in Western societies, are considered attractive when they have relatively narrow waists, large breasts, and symmetrical features (Henss, 2000, Singh, 1993). We have recently documented that waist-to-hip ratio and breast size are indeed indicators of reproductive potential in women because they are related to the levels of reproductive hormones (Jasienska, Ziomkiewicz, Ellison, Lipson, & Thune, 2004).

In humans, individuals of both sexes who are more symmetrical are considered more attractive (Gangestad et al., 1994, Grammer et al., 2003, Grammer & Thornhill, 1994, Thornhill & Gangestad, 1994, but see Kowner, 1996, Noor & Evans, 2003, Swaddle & Cuthill, 1995). The degree of asymmetry may be an important indicator of developmental stability and a biomarker of fitness (Jones et al., 2001). Random deviation from the perfect symmetry expected in bilateral structures of bilaterally symmetrical organisms is referred to as fluctuating asymmetry (FA) (Møller & Swaddle, 1997). FA may result from the influences of genetic and environmental stressors operating during fetal development (Møller & Swaddle, 1997). It has been linked to some aspects of human health (Manning, 1995, Manning et al., 1997, Shackelford & Larsen, 1997, Waynforth, 1998), although other studies failed to find significant associations (Hume & Montgomerie, 2001, Milne et al., 2003, Rhodes et al., 2001, Tomkinson & Olds, 2000).

Very few studies show that the degree of FA correlates with reproductive potential. In men, more symmetrical individuals have higher sperm number per ejaculate, greater sperm speed and migration ability (Manning, Scutt, & Lewis-Jones, 1998), and higher sperm motility (Soler et al., 2003). In women, studies have indicated a relationship between breast symmetry and fertility-related traits. Women with higher breast symmetry have a higher number of offspring (Møller, Soler, & Thornhill, 1995)—a finding confirmed by another study that, in addition, documented an earlier age at first birth for more symmetrical women (Manning, Scutt, Whitehouse, & Leinster, 1997). However, both the higher number of children and lower age at first birth may be caused indirectly by the increased attractiveness of symmetrical women. Attractive women may be able to attract more partners and choose those with better socioeconomic status, which, in turn, may lead to a higher number of children in such unions. Similarly, earlier age at first birth in more attractive women may result from an earlier age at marriage, which is a strong predictor of first reproductive event (Wood, 1994). Therefore, the above data do not provide conclusive evidence of a causal biologic link between FA and fertility. To prove that such a link really exists, one needs to show a relationship between asymmetry and a woman's physiological reproductive potential. Such potential can be assessed by measurements of ovarian steroid hormones, which are indicators of the probability of conception and successful pregnancy (Baird et al., 1997, Baird et al., 1999, Gorkemli et al., 2004, Lipson & Ellison, 1996, Lukaszuk et al., 2005, Sharara & McClamrock, 1999).

We hypothesize that in women, FA will be related to reproductive potential. More specifically, we expect that more symmetrical women would have higher levels of estradiol, which are predictors and determinants of successful pregnancy.