Putting a Finger on Sexy Neanderthals

Posted on November 4, 2010


Digit ratios sounds like something students in primary school have to learn.

But according to research published today in the Proceedings of the Royal Society B (Biological Sciences), the proportions of the fingers of the hominin hand are tell tale signs of relative promiscuity, at least for Ardipithecus, early modern humans, and Neanderthals.

“The fingers weren’t found in compromising positions, although that would’ve made for some interesting photos”, Discovery News almost wistfully notes. Leave it to the French to go to the heart of the story: AFP titles its version, Neanderthals had a naughty sex life. The British media were equally quick to leap on the story: the Mirror starts out, Caveman were randier than people today, “because they had long ring fingers”.

The research itself is highly technical, highly inferential, and represents a classic case of demonstrating correlation, with only a loose argument for causation. Here’s the original summary from the abstract of the published article:

the second-to-fourth digit ratio (2D : 4D), a putative biomarker for prenatal androgen effects (PAEs), covaries with intra-sexual competition and social systems across haplorrhines; non-pair-bonded polygynous taxa have significantly lower 2D : 4D ratios (high PAE) than pair-bonded monogamous species. Here, we use proximal phalanx ratios of extant and fossil specimens to reconstruct the social systems of extinct hominoids.

Translation: when infants in the womb are exposed to high levels of androgens– steroid hormones that contribute to formation of male sexual anatomy– the lengths of fingers are affected, so that finger length ratios is a proposed (“putative”) sign of this exposure. Since we have no access to actual measures of androgen levels in the bodies of ancient human ancestors (let alone to their fetal levels), the durable finger bones provide an indirect (hence “putative”) measure of relative levels of androgen exposure. In studies of living monkeys and apes (haplorrhines), the ratio of forefinger (2D) to ring finger (4D) is correlated with forms of sexual pairing: species that form durable bonds between a single male and female have higher 2nd to 4th digit ratios as a result of having lower exposure to androgens in the uterus. The reverse is true of species where males mate with multiple females: they have lower 2nd to 4th digit ratios, indicating they were exposed to higher androgen levels.

Or, to simplify: androgens affect the fetal body in ways written in the bone, ways that also lead to different sexual behavioral patterns in adulthood.

(Turns out there is a large body of research about finger length ratios, some of it weirder than I am prepared to cover right now; but for a sample, try this blog post about Casanova’s supposed long ring finger and its significance.)

What fascinates me about the coverage, in addition to the complete misunderstanding of what the study actually says, is the way press coverage narrows the focus to the poor benighted Neanderthal. The British Telegraph announces that Neanderthals really were sex-obsessed thugs, saying that

Scientists examining fossils have discovered that Neanderthals were exposed to more testosterone during development which is likely to make them more unreconstructed in their behaviour.

That means they were more likely to start fights over mates and hierarchy in the group and more likely to have multiple partners.

The multiple partners part is indeed what the research proposes, but the idea that exposure to higher levels of androgens made Neanderthals “more unreconstructed” is almost unintelligible, except in terms of modern ideologies of male sexual behavior as something that naturally is aggressive and held in check only by culture.

Meanwhile, the fact that early modern humans– the direct ancestor of our species– had a similar ratio of finger lengths, with all that might imply, seems to be of almost no interest to the press.

The storyline that is compelling for journalists is the one about how far we have progressed from what are pictured as brutish predecessors: most reports manage to contrast the “randy” “sex-obsessed brutes” of the past with contemporary humans, whose fingers suggest should be monogamous.

Not that early humans escape without comment– although clearly, journalists are somewhat confused about what “early modern humans” means. Where the article’s authors commented on specific human remains from Qafzeh cave in Israel, the Irish Times is not alone in associating the story with a generalized  “Stone Age“, and cave man references were rife.

MSNBC’s Cosmic Log comes closest to accurately portraying the research results, and is almost unique in asking the question, Can fingers point to sex habits?. The author writes that

the big question is whether there’s anything substantial to this analysis. Nelson acknowledged that the fossil record was sparse, and that more fossils were needed for study.

Citing University of Wisconsin anthropologist John Hawks, the Cosmic Log manages to raise the key questions that other news organizations, in their delight at having stereotypes of cave men reinforced, ignore:

He said the index-to-ring ratio “may be correlated with mating system in primates, but that doesn’t mean it’s a good predictor of mating system. … As fossil hominins go, I wouldn’t expect the story to go any further — there just aren’t many hands, so there’s never going to be a significantly predictive result.”…

“If you were going to do this study right, you would look far beyond the apes to take in many kinds of primates with different social systems. Then you could see whether closely related species have 2D:4D ratios that track their mating systems.”

Until then, brace yourself for more gleeful coverage of the supposed promiscuity of poor Neanderthals: mirror for all our worst imaginings about our own species and its past.