Latitude, digit ratios, and Allen's and Bergmann's rules: a comment on Loehlin, McFadden, Medland, and Martin (2006)

Ontogenetic Development of Sexual Dimorphism in Body Mass of Wild Black-and-White Snub-Nosed Monkey (Rhinopithecus bieti)

Sexual dimorphism exists widely in animals, manifesting in different forms, such as body size, color, shape, unique characteristics, behavior, and sound. Of these, body mass dimorphism is the most obvious. Studies of evolutionary and ontogenetic development and adaptation mechanisms of animals' sexual dimorphism in body mass (SDBM), allow us to understand how environment, social group size, diet, and other external factors have driven the selection of sexual dimorphism. There are fewer reports of the ontogenetic development of sexual dimorphism in body mass in Rhinopithecus. This study explores the ontogenetic development pattern of SDBM in wild black-and-white snub-nosed monkeys (R. bieti), and the causes resulting in extreme sexual dimorphism compared to other colobines. A significant dimorphism with a ratio of 1.27 (p < 0.001) appears when females enter the reproductive period around six years old, reaching a peak (1.85, p < 0.001) when males become sexually mature. After the age of eight, the SDBM falls to 1.78, but is still significant (p < 0.001). The results also indicate that males had a longer body mass growth period than females (8 years vs. 5 years); females in larger breeding units had a significantly higher SDBM than those in smaller ones (2.12 vs. 1.93, p < 0.01). A comparative analysis with other colobines further clarifies that Rhinopithecus and Nasalis, which both have multilevel social organization, have the highest degree of SDBM among all colobines. The large SDBM in R. bieti can be explained through Bergman's and Rensch's rules. Overall, environmental adaptation, a distinctive alimentary system, and a complex social structure contribute to R. bieti having such a remarkable SDBM compared to other colobines. In addition, we found that females' choice for males may not be significantly related to the development of SDBM.

Effects of caste, birth season, and family income on digit ratios

OBJECTIVES

The contributions of latitude and ethnicity in the determination of cross-society differences in digit ratios are unclear. In India, different castes (endogamous groups) have been living in the same areas (villages or towns) for the last 1500 years and, therefore, these groups may have different gene pools without a latitude-related difference component. Thus, in the present study, we studied the effect of caste on digit ratios. We also studied the effects of sex, birth season, and family income on digit ratios.

METHODS

We selected a sample of 301 college students (age: M = 19.9 years, SD = 2.63) in Muzaffarnagar city of western Uttar Pradesh, India, and asked participants for information regarding their birth month, religion, caste, and monthly family income. We measured participants' dorsal and palmar digit lengths (of all fingers, except the thumb, in both hands) using vernier calipers of 0.01 mm accuracy.

RESULTS

Other backward castes (intermediate castes) had longer digit lengths than general castes (upper castes), scheduled castes (lower castes), and Muslims. However, there was no difference in digit ratios of caste groups (scheduled castes vs. other backward castes vs. general castes vs. Muslims) or specific castes (Chamar-Jatav vs. Jat vs. Pandit-Tyagi). Winter-born women had lower left dorsal 2D:4D and 3D:4D ratios than summer-born women. Family income was related to higher dorsal 2D:4D and 3D:4D ratios among women. Moreover, in dorsal digit ratios, sex difference (men < women) occurred in digit ratios constituting digit 5, whereas, in palmar digit ratios, sex difference occurred in digit ratios constituting digit 2.

CONCLUSIONS

The present study suggests that endogamy-led genetic difference in ethnic/caste groups is not a determinant, whereas birth season (i.e., the exposure to sunlight) and family income might be determinants of digit ratios. In addition, compared to palmar digit ratios, dorsal digit ratios are better markers of sexual dimorphism.

Digital biomarker 2D:4D as a predictor of cancer: A systematic review

BACKGROUND

The digital ratio between the second and fourth digits of the hands, known as 2D:4D, is sexually dimorphic. It has been suggested that a low 2D:4D indicates high exposure to prenatal testosterone and low estrogen exposure while a high 2D:4D indicates the inverse. The 2D:4D may be predictive of cancer susceptibility, and this may be particularly true in cancers that show differences between sexes in their occurrence.

AIM

To conduct a systematic review of published epidemiological literature examining the association between 2D:4D and cancer.

METHOD

This review was carried out according to criteria recommended for the systematic review of Statement PRISMA. We enrolled 25 papers involving eleven cancer topographies with 4,569 cases and 19,416 controls from Europe, America, Asia and Australia.

RESULTS

We noticed variations and discrepancies in the results of the association between 2D:4D and cancer among the studies, either in those that had evaluated the same or different types of cancer, or in the same or distinct lateralities. However, a high 2D:4D was considered a likely predictor of cancer risk in 11 of 25 studies, a low 2D:4D was suggested as a predictor of risk in eight papers, and five of the twenty-five studies did not demonstrate any association.

CONCLUSIONS

Although this biomarker has the advantage of being easy to measure, it is noted that its relationship with sex hormone levels at specific stages of life has not yet been well quantified, and it has still been questioned. Hence, it is suggested that those findings from studies involving 2D:4D as a proxy for foetal hormone exposure should be interpreted with caution, as well as those studies which claim its association with cancer. Thus, this review shows the need for a greater number of epidemiological studies using more homogeneous methodology and techniques to better investigate the strength of the findings.

No sex difference in digit ratios (2D:4D) in the traditional Yali of Papua and its meaning for the previous hypotheses on the inter-populational variability in 2D:4D

OBJECTIVES

The ratio between the 2nd and 4th digit (2D:4D) is considered to be a sexually dimorphic trait in humans, with males having on average lower 2D:4D values than females. However, significant inter- and intra-sexual differences in digit ratios across populations have been reported. In order to further explore factors influencing the variability of 2D:4D, we performed a study among the Yali, a traditional population from Papua.

METHODS

We measured digit ratios of 79 adults (47 males) from the traditional Yali society, a polygamous group who inhabit a harsh high-mountain environment almost directly on the equator.

RESULTS

Statistical analysis of the data show no significant inter-sexual difference in digit ratios in our sample.

CONCLUSIONS

We discuss the results in light of various factors that putatively shape the differences in digit ratios. We conclude that the results of our study contribute to the existing evidence suggesting that digit ratio might not be universally sexually dimorphic in humans.

The Relationship Between Digit Ratio (2D:4D) and Sexual Orientation in Men from China

We examined the relationship between 2D:4D digit ratio and sexual orientation in men from China and analyzed the influences of the components used to assess sexual orientation and the criteria used to classify individuals as homosexual on this relationship. A total of 309 male and 110 female participants took part in a web-based survey. Our results showed that heterosexual men had a significantly lower 2D:4D than heterosexual women and exclusively homosexual men had a significantly higher left 2D:4D than heterosexual men whereas only exclusively homosexual men had a significantly higher right 2D:4D than heterosexual men when sexual orientation was assessed via sexual attraction. The left 2D:4D showed a significant positive correlation with sexual identity, sexual attraction, and sexual behavior, and the right 2D:4D showed a significant positive correlation with sexual attraction. The effect sizes for differences in 2D:4D between homosexual and heterosexual men varied according to criteria used to classify individuals as homosexual and sexual orientation components; the more stringent the criteria (scores closer to the homosexual category), the larger the effect sizes; further, sexual attraction yielded the largest effect size. There were no significant effects of age and latitude on Chinese 2D:4D. This study contributes to the current understanding of the relationship between 2D:4D and male sexual orientation.

Hypothermia: effects on platelet function and hemostasis

Mild therapeutic hypothermia is considered standard care in the treatment of patients resuscitated from cardiac arrest. With increasingly more frequent concomitant use of platelet-inhibiting drugs, clinicians must be cognizant of the ramifications of hypothermia on platelet function as part of hemostasis. The effects of hypothermia on platelet function have been studied for more than 50 years, but the results are inconsistent and may be related to the circumstances during which hypothermia is achieved. This review summarizes current knowledge of platelet function during hypothermia and the impact on hemostasis.

Correlation of 2D:4D digit ratio and craniofacial shape in prepubertal children

OBJECTIVES

The 2D:4D ratio is sexually dimorphic and is considered a proxy of prenatal androgen levels, or, according to recent evidence, is related to genes involved in ocular and palate development. Our aim was to investigate correlation between the 2D:4D ratio and the shape of the craniofacial skeleton in a population of prepubertal children.

METHODS

We conducted a cross-sectional study in a group of 58 male and 59 female prepubertal children aged 7-12 years. Craniofacial shape was evaluated using 15 skeletal landmarks on lateral cephalometric radiographs and fingers were measured with a computer-assisted procedure that involved tracing the finger outline. Geometric morphometric analysis was applied to the craniofacial landmarks and multivariate regression between digit ratios and craniofacial shape was computed in shape space and form space.

RESULTS

The male 2D:4D ratio was smaller than the female ratio (Cohen's d: 0.275 left hand, 0.126 right hand), but the difference was not statistically significant. Craniofacial shape did not show sexual dimorphism, but males were larger than females. No correlation was found between digit ratio and craniofacial shape in prepubertal children, either for the whole sample or for any of the two sex groups.

CONCLUSIONS

As several factors might be involved in the development and growth of both the craniofacial complex and fingers, the 2D:4D ratio, a putative proxy for fetal sex-hormone levels, is probably unable to impose a measurable effect within the variation of a normal population. Future research needs to examine an adult sample for potential covariation arising after the pubertal growth spurt.

Scientometric analysis and bibliography of digit ratio (2D:4D) research, 1998-2008

A scientometric analysis of modern research on the second-to-fourth digit ratio (2D:4D), a widely studied putative marker for prenatal androgen action, is presented. In early 2009, this literature totalled more than 300 publications and, since its initiation in 1998, has grown at a rate slightly faster than linear. Key findings included evidence of publication bias and citation bias, incomplete coverage and outdatedness of existing reviews, and a dearth of meta-analyses in this field. 2D:4D research clusters noticeably in terms of researchers, institutions, countries, and journals involved. Although 2D:4D is an anthropometric trait, most of the research has been conducted at psychology departments, not anthropology departments. However, 2D:4D research has not been predominantly published in core and specialized journals of psychology, but rather in more broadly scoped journals of the behavioral sciences, biomedical social sciences, and neurosciences. Total citation numbers of 2D:4D papers for the most part were not larger than their citation counts within 2D:4D research, indicating that until now, only a few 2D:4D studies have attained broader interest outside this specific field. Comparative citation analyses show that 2D:4D research presently is commensurate in size and importance to evolutionary psychological jealousy research, but has grown faster than the latter field. In contrast, it is much smaller and has spread more slowly than research about the Implicit Association Test Fifteen conjectures about anticipated trends in 2D:4D research are outlined, appendixed by a first-time bibliography of the entirety of the published 2D:4D literature.

Digit ratio (2Dratio4D) differences between 20 strains of inbred mice

The second to fourth digit ratio (2D∶4D) is sexually differentiated in a variety of species, including humans, rats, birds, and lizards. In humans, this ratio tends to be lower in males than in females. Lower digit ratios are believed to indicate increased prenatal testosterone exposure, and are associated with more masculinized behavior across a range of traits. The story seems more complicated in laboratory mice. We have previously shown that there is no sex difference in the digit ratios of inbred mice, but found behavioral evidence to suggest that higher 2D∶4D is associated with more masculinized behaviors. Work examining intrauterine position effects show that neighbouring males raise pup digit ratio, suggesting again that higher digit ratios are associated with increased developmental androgens. Other work has suggested that masculinization is associated with lower digit ratios in lab mice. Here, we examine the fore- and hindlimb digit ratios of 20 inbred mouse strains. We find large inter-strain differences, but no sexual dimorphism. Digit ratios also did not correlate with mice behavioral traits. This result calls into question the use of this trait as a broadly applicable indicator for prenatal androgen exposure. We suggest that the inbred mice model presents an opportunity for researchers to investigate the genetic, and gene-environmental influence on the development of digit ratios.