The Second to Fourth Digit Ratio and Age at First Marriage in Semi-Nomadic People from Namibia

Abstract

The second to fourth digit ratio (2D:4D) is used as a potential marker for prenatal androgen exposure. It is associated with many behavioral and biological variables, including fertility and sexual behavior. However, direct association between 2D:4D and reproductive success—in populations where no contraceptives are used—has not been investigated. Here, we present a study conducted among the semi-nomad Himba population living in northern Namibia. 2D:4D ratios were calculated for a sample of this population (N = 99; 60 women, 39 men), and the results were correlated with age, marital status, age at first marriage, number of children, and number of marriages. As found in the majority of previous studies, males had lower 2D:4D ratios than females. The 2D:4D ratio did not correlate with number of children. Females and males with a more masculine 2D:4D were married earlier and were more likely to have a husband or wife. We suggest that mating preferences for females with masculine 2D:4D are related to masculinity of phenotypic and personality traits of such women, which are beneficial in harsh environmental conditions and/or higher facial masculinity, which influences the perceived age of an individual. At the same time, masculine (physically strong, dominant, and hardworking) males might gather resources necessary to marry their first wife earlier.

Women with more feminine digit ratio (2D:4D) have higher reproductive success

“…In our study, we did not find a difference in mean age at first birth between groups of more feminine and more masculine 2D:4D women, contrary to, for example, Manning and Fink (). This is interesting because other studies have reported that women with more feminine 2D:4D tend to marry at an earlier age (Manning and Fink, ; but see: Sorokowski et al, ), prefer marriage at an earlier age (Manning and Fink, ) and have earlier menarche (Matchock, , but see: Helle, ), what might indicate that these women could start their reproduction earlier. Further, similarly to Helle (), we did not find differences in mean interbirth intervals between groups of more masculine and more feminine 2D:4D women, but we did find a difference between these groups in age at last birth and reproductive lifespan.…”

Digit ratio (2D:4D) as an indicator of body size, testosterone concentration and number of children in human males

“…Variables associated with male fertility such as sperm number (Manning et al, 1998), sex drive, level of sexual excitement (Manning & Fink, 2008), number of sexual partners per individual (Honekopp et al, 2006) and age at first marriage (Sorokowski et al, 2012) were all related to 2D:4D. These results might suggest that digit ratio might be widely associated with traits related to male reproductive strategy.…”

“…number of children) since it is related to mating and reproductive preferences and sexual behaviour (i.e. Manning & Fink, 2008), although not all studies have confirmed such associations Sorokowski et al, 2012).…”

Digit ratio (2D:4D) does not correlate with daily 17β‐estradiol and progesterone concentrations in healthy women of reproductive age

“…It has also been suggested that 2D:4D is strongly heritable through male lines (Voracek and Dressler, 2009) and that the fetal hormonal environment may result from the action of sexually antagonistic genes (which exert their effects prenatally) (Manning et al, 2000). Fetal androgen concentration (reflected in 2D:4D) is associated with adult testosterone levels (i.e., Klimek et al, 2014;Manning et al, 2004) and several reproductive parameters, including sex drive, level of sexual excitement (Manning and Fink, 2008), number of sexual partners per individual (H€ onekopp et al, 2006), and age at first marriage (Sorokowski et al, 2012). These traits, related to 2D:4D, might lead to higher reproductive success.…”