Category: sexual selection

Are male and female brains absolutely identical?

January 20, 2017 • 12:30 pm

The Guardian has a review out of Cordelia Fine’s new book, Testosterone Rex: Myths of Sex, Science, and Society, which continues her critique of innate differences between male and female brains and behaviors. The Amazon summary includes this:

In Testosterone Rex, psychologist Cordelia Fine wittily explains why past and present sex roles are only serving suggestions for the future, revealing a much more dynamic situation through an entertaining and well-documented exploration of the latest research that draws on evolutionary science, psychology, neuroscience, endocrinology, and philosophy. She uses stories from daily life, scientific research, and common sense to break through the din of cultural assumptions. Testosterone, for instance, is not the potent hormonal essence of masculinity; the presumed, built-in preferences of each sex, from toys to financial risk taking, are turned on their heads.

Moving beyond the old “nature versus nurture” debates, Testosterone Rex disproves ingrained myths and calls for a more equal society based on both sexes’ full, human potential.

Now I can’t quibble with the last sentence, though I can say that we simply don’t know very much about evolutionarily-based differences in behavior between the sexes. I guess I’m an equity feminist, feeling strongly that members of both sexes (or of a spectrum of genders) must be offered equal opportunities and educations from the very outset: from birth.  But if there are innate differences between genders or sexes, that won’t necessarily guarantee equality of outcomes. All we can do is ensure that nobody is discriminated against based on their genitalia, their chromosomes, or their own perception of gender.

I read Fine’s previous book, Delusions of Gender, and thought it was pretty good in taking apart some poorly designed experiments that themselves seemed to reflect the researchers’ ideologically driven agenda of hard-wired sex differences. But I also thought that Fine herself was at least partly motivated by ideology (the view that there are absolutely no behavioral differences between the sexes that don’t arise from social conditioning), and so my opinion of the book was mixed. In the end, I agreed with Diane Halpern’s take in Science on that book (Halpern also reviewed Brain Storm by Rebecca M. Jordan-Young, a book I did not read):

Cleverly written with engaging prose, Delusions of Gender and Brain Storm contain enough citations and end notes to signal that they are also serious academic books. Fine and Jordan-Young ferret out exaggerated, unreplicated claims and other silliness regarding research on sex differences. The books are strongest in exposing research conclusions that are closer to fiction than science. They are weakest in failing to also point out differences that are supported by a body of carefully conducted and well-replicated research.

I haven’t read Fine’s latest book, and so will address only the Guardian‘s take on it, which I find bizarre. One caveat is that it may be misrepresenting Fine’s views. But the quoted part below, which I’ve put in bold, suggests that the whole paradigm of sexual selection, and attendant behavioral differences between males and females, should be thrown out because of one flawed experiment:

Here’s one example Fine offers of Testosterone Rex mangling the way we think about sex. In the 1940s, biologist Angus Bateman conducted a series of experiments on fruit flies that appeared to show conclusively that competition between males for “fertile female vessels” was the driving force of evolution. The hypothesis goes something like this: laying eggs is a more substantial physical investment than producing sperm. Therefore, to maximise reproductive success, females should be selective and cautious while males should be promiscuous and competitive; therefore, women are domestic and monogamous, while men are thrusting away both in the public sphere and in as many beds as possible.

It’s elegant, it’s intuitive, and it’s wrong. Bateman’s experiments were biased by design and by his unexplained exclusion of data that, when included in a recent reanalysis, actually showed that males and females both produced more offspring when they had more mates. But there are limits to promiscuity as a strategy: taking into account female fertility, a man has more chance of being hit by a meteor than fathering 100 children with 100 different women in a year. The player who says it’s in his genes is missing a vital part of the story.

It’s true that Bateman’s experiment, purportedly showing that males had a much higher variance in mating success than did females—a crucial assumption of sexual selection theory—was flawed. This was pointed out in a PNAS paper by Patricia Gowaty et al., who noted that the use of certain mutations as genetic markers biased the outcome towards the sexual-selection hypothesis: that males are more promiscuous in mating, and females pickier, because females make a greater reproductive investment than males. But they didn’t say Bateman was flat wrong in seeing males more promiscuous than females; they said his results were “inconclusive.”

The Guardian‘s canard about the unlikelihood of a male fathering 100 children with 100 different women is simply misdirection: the question is whether some males get a lot of offspring compared to others (higher variance in reproductive success) while the variance among females is smaller. If that is the case, then there will be male-male competition—either direct or through display, ornaments, and so on—to woo discriminatory females.

In fact, Bateman’s experiment has been repeated properly in other species, with exactly the predicted finding of higher male variance and of males competing to fertilize scarce uninseminated females. To throw out the whole edifice of sexual selection (and I’m not using it to claim that “women are domestic and monogamous, while men are thrusting away both in the public sphere and in as many beds as possible”) because of one flawed experiment is to neglect the pervasive evidence from many areas that males are indeed evolutionarily adapted to try to mate as often as possible, while females are adapted to be more choosy. We don’t jettison an entire body of consilient evidence because one guy did a bad experiment.

Data supporting sexual selection, and a greater promiscuity of males rather than females, include the following:

  • In human, primate, and many other animal species, males do indeed have a higher variance in reproductive success than do females (it’s been measured). It would be extraordinary if that was just a coincidence based on “social conditioning” in humans but evolution in all the other species that don’t have social conditioning.
  • The theory of sexual selection is well worked out, and precisely explains this difference in sex-specific behavior.
  • In species in which males make a greater reproductive investment than females, like seahorses and pipefishes (the males get “pregnant,” holding the eggs and young in pouches), we see the exact opposite of what we normally see. The males are choosy, while females, who produce eggs faster than males can accept them, are promiscuous. In fact, in those groups it is the females who are brightly colored and ornamented while males are drabber: the opposite of the normal situation, but exactly as sexual selection theory predicts.
  • The difference in body size and strength between human males and females implies an evolutionary basis, almost certainly having something to do with male-male competition, as it does in many mammals, insects, and other groups (see my posts here and here). Holly Dunsworth, whose theories I’ve criticized, has never responded to my comments.
  • Replicated experiments in both humans and other animals show a strong difference in promiscuity (in humans it’s done using experiments in which attractive strangers proposition people of the opposite sex). Again, it would be extraordinary if the parallel between human and animal behavior were purely coincidental.
  • There is no convincing way to explain the pervasive existence of bright coloration, elaborate plumage (maladaptive for survival), calling and displays, and other “look-at-me” features of males versus females other than sexual selection.  How that selection works may be enigmatic (do the male traits show good genes? good phenotypes? appeal to some innate preferences of females?)—but all of it supports the action of sexual selection.
  • Bonobos (“pygmy chimps”), which may behaviorally more similar to humans than are “regular chimps”, have a fairly matriarchal society with more promiscuous mating of females than do other chimps, but still show a 25% greater body weight in males than females. Is that a holdover from an ancestor, or a byproduct of males competing for females? (After all, bonobo females are still saddled with pregnancy and child-rearing, and thus have far fewer potential offspring over their lives than do males.)
  • Finally, insofar as the morphological traits are connected with differences in sexual behavior and proclivities of males versus females, it shows some genetic differences affecting behavior between the sexes—and differences that may rest largely in brain wiring. Now that needn’t reflect a difference in male versus female brain structure, as it could simply represent how brains that are identical produce different responses when affected by different hormones produced outside the brain. (Testosterone, for example, may trigger “promiscuous mating” genes that reside in both male and female brains but are activated only by male hormones.)

As I said, I haven’t read Fine’s latest book; what I’m reacting to here are the two bolded paragraphs in the Guardian summary—paragraphs implying (based on the flawed study of Bateman) that sexual selection simply doesn’t exist: it’s all social conditioning and the Patriarchy. But there are simply too many biological facts (first adduced by Darwin) to support that conclusion, not least the number of animals lacking a “patriarchy” who show strong evidence for sexual selection and sexual behavior resembling those of humans.

While some of those whom Fine has criticized may have distorted their science in the name of ideology, I worry that Fine is doing the same thing. I will find out when I read her book. But certainly the Guardian has engaged in scientific distortion in its article about Testosterone Rex. 

The evolution of sexual dimorphism in humans: Part 2

December 21, 2016 • 11:00 am

In a post one week ago, “The ideological opposition to biological truth,” I argued that sexual dimorphism for body size (difference between men and women) in humans is most likely explained by sexual selection, and that it also reflects behavioral differences between males and females: males compete for females, and greater size and strength give males an advantage. That competition results from females—in many species, not just ours—being a “scarce” resource for males, since the number of males capable of breeding far exceeds the number of females who cannot breed because they’re tending offspring or in gestation. This disparity can be categorized in two ways:

  • The behavioral operational sex ratio: the ratio of sexually active males to fertilizable females at a given time. This is about 11.7 in humans!
  • The physiological operational sex ratio, the same ratio but for all individuals capable of reproducing (rather than those actually engaged in mate-hunting). This is about 8.6 in humans.

The ratios are greater in some primates (gorillas have values of about 84!), but if they’re greater than 1, there’s room for sexual selection, since there are more males seeking females than there are females available as mates. This itself is one bit of evidence for the operation of sexual selection in humans.

Now how the sexual selection actually operated in our ancestors is not perfectly clear. Some of it, as the data suggest, involves male-male competition: fights between males to control females, as we witness in gorillas, deer, and elephant seals. Females are more or less constrained to mate with the winning males. Or females may prefer to mate with the biggest and strongest males, for those males may protect their offspring—and hence the female’s genes—better than do smaller, weaker males. (This gives an evolutionary advantage to those females who can discern and choose the best males.)

Both of these factors can, of course, work at the same time, and there are other more arcane forms of sexual selection I won’t mention, including other signs in males of “good genes”. But any sexual-selection scenario goes along with a difference in sexual behavior, explaining why, even today, males are more promiscuous and willing to mate than are the choosier females.

A further possibility is that there could be an ecological distinction between males and females, with males hunting, and thus needing size and strength, while females do gathering (presumably females don’t have time to hunt because they’re rearing children). That doesn’t involve sexual selection, but it also fails to explain all the data, like the correlation between sexual dimorphism and polygyny within humans, and the fact that in our primate relatives there’s not only the same correlation among species, but no palpable division of labor among males and females. It also doesn’t explain the existence of traits like beards, lower voices, or same-sex aggression among human males but not females. Nevertheless, there’s no reason why several forces couldn’t work together to cause men to have evolved larger body size and increased musculature (as well as other features) in our ancestors. But surely sexual selection is one, for the evidence below fits no other hypothesis.

As I noted, these relatively uncontroversial ideas about sexual selection (not mine, actually; they’re the conventional wisdom among evolutionists beginning with Darwin), was challenged by Holly Dunsworth, an associate professor of anthropology at the University of Rhode Island, on her website. Dunsworth, who called my theory (supported by lots of data cited in my original post) a “story”, offers her own speculations, which really are a story because they lack empirical support and don’t explain a lot of observations. Here’s what she said:

It’s not that Jerry Coyne’s facts aren’t necessarily facts, or whatever. It’s that this point of view is too simple and is obviously biased toward some stories, ignoring others. And this particular one he shares in this post has been the same old story for a long long time.

What about the other side of the body size sexual dimorphism story?

What about the women?

Selection could well be the reason they stop growing before men and why they end up having smaller bodies than men, on average.

Perhaps men can make babies while growing, but perhaps women can’t. Energetically, metabolically. So reproduction wins over growth. We reach sexual maturity and stop growing. Is that just a coincidence?

Why doesn’t this (and other tales) fit alongside the big-aggressive-males-take-all explanation for sexual dimorphism? #evolution

But as I noted in the piece she criticized, selection on females—through either evolution of female preference or on differential ecological roles between the sexes—could affect sexual dimorphism. But Dunsworth conveniently ignored that bit. Her criticisms were echoed by an article by Jesse Singal in New York Magazine, which claimed, as did Dunsworth, that I was offering mere “stories”—unevidenced speculation. Singal said this:

In Dunsworth’s view, all she is asking for is some nuance and, well, skepticism. “People love to boil complex processes down to their preferred (intentional or not) story,” she wrote, “with some in leading roles and others completely absent, and we don’t have to take that anymore.” Her tweeted example about growth nicely captures this: It could be that Coyne’s aggressiveness story leaves out important details about why men are bigger than women, or fails to explain certain aspects about that differences. Overall, it certainly seems like people are quicker to latch onto evo-psych stories that reinforce certain views of men and women.

That last sentence is a veiled accusation that my piece was sexist. I reject that completely.

As I noted in part I of this response, neither Singal nor Dunsworth appreciated that I have a long published history of criticizing “just-so” stories in evolutionary psychology. I don’t like unevidenced speculation when it’s promoted as truth. But the sexual selection theory for human sexual dimorphism is supported by a lot of evidence. It is manifestly not a mere “just-so” story.  In my original piece I adduced this evidence (revised slightly):

  • In human societies studied by Richard Alexander, those societies that are more polygynous (in which males compete more intensively for females) show greater sexual size dimorphism than societies that are more monogamous. This was a prediction made before the data were acquired—a prediction derived from sexual selection theory. And it was fulfilled. UPDATE: I see now that Alexander’s finding wasn’t reproduced in another experiment, so consider this conclusion questionable.
  • Among species of primates, there’s a good correlation between the polygyny of a species and sexual dimorphism: those species in which males have a higher variance in offspring number, and in which males thus compete more intensely for females, also show a greater ratio of male/female body size, even when corrected for phylogeny. (Too, in primate species in which males fight each other over females, the relative size of the canine teeth, used in battle, is larger than in species showing less direct male-male competition.)
  • In humans, as in many other species in which males compete for females, the sex ratio at birth favors males. They then die off at a higher rate due to higher risk-taking and exploratory behavior, and also senesce faster, which is why among older humans there are so many more females than males. (Check out any Gray Line tourbus.) This is predicted by sexual selction theory.
  • In line with the above, in humans and other primates, males show from the outset great exploratory and risk-taking behaviors, and as adults show many other behaviors that differ from those of females, such as greater dispersal. Is this due to the Primate Patriarchy? Probably not, given that these differences in behavior are shown in many species besides ours and make evolutionary sense.

There’s more evidence, too, which I’ll mention shortly.

But what’s the evidence for Dunsworth’s theory? As far as I can see, there isn’t any. Her theory claims that 1) females can’t reproduce while growing, while males can. 2) There’s a tradeoff between growth and reproduction, so if you stop growing as a female, you can start reproducing earlier. Conclusion: females stop growing before males because reproduction is all-important, and therefore they’re smaller than males as adults.

But the data don’t even support her theory. Puberty begins in females at about ages 10 and 11, and in males between 11 and 12.  (The age of both appears to be decreasing in recent years.) Yet males keep growing this whole period and well beyond, as do females. There’s no indication that females stop growing when they become reproductively competent. Here are growth curves (stature and weight) for both males and females. Stature begins tapering off at about ages 14-15 in both sexes (a slower taper in males), but both sexes continue to grow until age 20.

Females:

growth-2-20-girls

Males:growth-2-20-boys

Now we don’t know about body sizes and ages of puberty in our ancestors, which is the really important information, and I doubt we’ll have that given that it’s virtually impossible to ascertain the age of puberty in fossils. But clearly there’s no support in any data for Dunsworth’s hypothesis that “perhaps men can make babies while growing, but perhaps women can’t. Energetically, metabolically.” Both men and women can make babies while they’re still growing. But men continue to grow not only faster but also bit longer than do women (see above), something which explains sexual dimorphism. But since men are reproductively competent when they hit puberty, why do they keep getting bigger? Dunsworth doesn’t tell us, but sexual selection theory does. Men achieve greater stature and muscle mass by both growing faster than females, and tapering off a bit later.

So Dunsworth’s hypothesis is not only unsupported by data, but fails to explain the growth data that do exist.

More important, her theory doesn’t explain the four points given above—points that are well explained by sexual selection theory.  She and New York Magazine fail to realize that the sexual-selection explanation for human sexual dimorphism is not a “story”, but makes supported predictions and clarifies previously obscure observations. How irritating to see these people distort what we know about evolutionary theory and human biology!

As I mentioned in earlier posts, I think Dunsworth is blinkered by her ideology, because she thinks that sexual selection theory ignores females. Well, straight male-male competition without female choice does involve evolution mainly in males, but there are forms of sexual selection that involve female choice, too, and that has surely happened in species like birds and fish. In those groups, and others, males show ornaments and colors not useful in male-male competition, but are the object of female choice. And some of that process may have happened in our own lineage. The competing theories are not zero-sum, so that only one can be right. All these processes can work together. But surely one is sexual selection.

Regardless, sexual selection as an explanation implies that there are also sexual differences in behavior: differences we see in modern experiments and are probably not purely cultural because a. they’re predicted by the differences in body size and b. we see the same difference in mate choosiness in many other species—and not just primates. It’s an ineluctable consequence of the difference in reproductive investment between males and females.

I’ll now list some other observations about human mating and morphology that are explained by sexual selection theory but not explained at all by Dunsworth’s theory. Some of these come from the references given at the bottom of the post.

  • In other sexually dimorphic primates, including chimpanzees and gorillas, direct contests between males can be observed, and probably existed in our ancestors since paleoanthropological data show that many more males were killed by violence than females, possibly reflecting inter-group battles, which in modern hunter-gatherer societies are often over females. Many societies also show “bride theft”, capture of females by bands of males—common in Amazonian hunter-gatherer societies.
  • Male humans have more robust skulls than do females, including mandibles and brow ridges. This may reflect evolution to withstand blows to the head. (Males also have a higher tolerance for pain.)
  • Men are not only taller and heavier than women, but are stronger, particularly in the upper body. While size differences are about 8%, and body mass about 15-20%, women’s bodies have a higher percentage of fat, so that when you look at fat-free body mass, men are 40% heavier, have 60% more lean muscle mass, 80% greater arm muscle mass, 75% more upper-body muscle mass, and 50% more lower body mass. This difference in relative amount of muscle mass cannot be explained by Dunsworth’s theory, which is purely about growth, but is explained by male-male competition under sexual selection—and perhaps by female preference as well. This is reflected in differential athletic performance, and is why men and women usually compete separately in athletics. Even for men and women of equal sizes, men are far stronger; as Hill et al. note, “the average man is stronger than 99.9% of women (some of this, of course, may be because men work out; I haven’t checked the references.)
  • In every society studied, men are physically more aggressive than women, both in play as kids and as adults. The vast majority of murderers are men, and this aggressive activity peaks during men’s peak reproductive years, when they would be competing for mates most strongly. These data do not include killings in war.
  • Traits like beards and lower voices in men (men’s vocal folds are 60% longer than women’s, giving them lower voices) have been shown to act as indicators of dominance; both are evolved morphological traits. (The evidence supporting all these claims can be found in the papers cited below.) Women also prefer larger men and deeper voices, so there may have been an element of female choice in sexual selection, though of course the observations we make are on modern rather than ancient hominins.
  • Sexual dimorphism is also seen in our ancestors like Australopithecus and H. erectus, implying that it’s been acting on our lineage a long time. But there’s also some evidence, cited by Plavcan, that the degree of sexual dimorphism has waxed and waned as females got either bigger or smaller over time, implying that there may have been some separate natural selection in females that could increase or decrease sexual dimorphism (but never effaced it).
  • Finally, Buss’s article and others not cited outline the psychological and behavioral differences between males and females that make sense under sexual selection. These not only include the greater promiscuity of males than females, but also the greater sexual jealousy of males toward women than vice versa (our male ancestors weren’t always sure who the father of their mate’s children was, while women were far more certain). There is also a big difference between males and females in their attitudes towards casual sexual experiences (guess in which direction), and in how exacting their standards are for a short-term mate (guess again). Men have lower psychological thresholds for risk-taking. And so on. As Buss wrote, “Large sex differences appear reliably for precisely the aspects of sexuality and mating predicted by evolutionary theories of sexual strategies.”

I’ve adduced about a dozen pieces of evidence supporting the sexual selection explanation for human morphological and behavioral dimorphism—none of which can be explained by Dunsworth’s hypothesis. (And that hypothesis was dead in the water anyway, contradicted by the known data.) Since all hypotheses must, at bottom, be supported by the weight of accumulated scientific evidence, it is clear that sexual selection, and male-male contest competition in particular, is a compelling explanation for human sexual dimorphism. In contrast, Dunsworth’s hypothesis isn’t in the least compelling. That doesn’t mean we shouldn’t keep evaluating the evidence or suggesting new hypotheses, but simply that these should be supported by data rather than ideological preference.

I urge readers to look at the papers below, and use the data (and that from other papers) to evaluate theories about human behavioral and sexual dimorphism. I don’t propose to engage in a dialogue with Dr. Dunsworth about this, but I would like to know how her theory can explain the dozen-odd observations given above.

Dunsworth must have emitted something like twenty tweets about her piece, impugning me; and she even issued this over-the-top pronouncement:

Well, there’s fighting material above, but I’ve had my say. Still, I can’t believe that simply my writing a post on human sexual dimorphism and its implications would drive anybody away from studying human evolution. After all, the give-and-take of hypotheses, critical thinking, and data are the very meat of science, and if you disagree with somebody, you don’t simply walk away from a field. I sure as hell am not leaving evolutionary biology because Dunsworth and New York Magazine took out after me!

2028_4_16

UPDATE: Things are getting worse: Peter Boghossian is arguing with Dunsworth on Twitter (I’m not involved, as I avoid Twitter Wars), but now we’re getting lumped with some rather unsavory types (except for the “evolutionists”):

h/t: Steve, David

_______

Buss, D. M. 1995. Psychological sex differences. Amer. Psychologist 50:164-168.

Hill, A. K., D. H. Bailey, and D. A. Puts. 2017. Gorillas in our midst? Human sexual dimorphism and contest competition in men. pp. 235-249 in: On Human Nature: Biology, Psychology, Ethics, Politics, and Religion. in M. Tibayrencand F. J. Ayala (eds.) .On Human Nature, M.  Tibayrenc and F. J. Ayala, eds. Academic Press.

Puts, D. A. 2010. Beauty and the beast: mechanisms of sexual selection in humans. Evolution and Human Behavior 31:157-175.

Plavcan, J. M. 2012. Sexual size dimorphism, canine dimorphism, and male-male competition in primates. Where do humans fit in? Human Nature 23:45-67.

Spot the nightjar!

November 2, 2015 • 1:45 pm

JAC: Matthew got really excited when he found out about a species of nightjar that was new to him, and the following post reflects his enthusiasm. It is a cool bird.

by Matthew Cobb

Holly’s Tw*tter bio reads: “Conservation Biologist, Zoologist, working and studying in the remote Peruvian Amazon, previous Paraguay and Antarctica. Graduate of St Andrews University.” That’s quite a CV she has there!

Holly explained in another tw**t that she also caught the female partner of this lyre-tailed nightjar (no pic) – they had seen the pair flying together the night before.

The streamers are presumably used by females in sexual selection – there must be some link between their length and the male’s overall fitness. However, as with all such sexually-selected characters, there will come a point at which the fitness advantage that the male accrues via female mate preference is outweighed by the damage to his fitness (probably survival) caused by the sexual ornaments being too large. Without knowing anything about nightjar aerodynamics, these males look to be pretty much at the edge of what might be possible without losing the ability to fly so well.

Here’s some general biology from the Cornell neotropical birds site:

Lyre-tailed Nightjar (Uropsalis lyra) is unmistakable, with the males flaunting spectacular, pale-tipped tail streamers more than twice the bird’s body length. Uncommon and local in the Andes from Venezuela south to northwestern Argentina, this nightjar occupies gorges and most rocky cliffs, often near running water, at 2500-3000 m (and sometimes much lower). These birds roost on cliff faces and in caves, often concealed by hanging vegetation, using one roost for extended periods. Excluding the tail streamers, sexes are similar. A rufous collar extends across the nape, the scapulars are generally a pale, vermiculated gray, and the primaries solid black. Females are distinguished from female Swallow-tailed Nightjars (Uropsalis segmentata), which tend to be at higher elevations, by having a vermiculated black and gray crown (rather than a dark brown crown densely spotted with rufous) and a more prominent rufous nuchal collar. Male Lyre-tailed Nightjars forage and display nocturnally from the forest edge, with brief, fluttering sallies into the open, sometimes hovering.

Here’s an atmospheric video of a male swooping overhead in the Peruvian twilight. What a marvellous bird is the nightjar!

Here’s a male catching insects:

Here’s a male roosting during the day:

And here’s a very patient nesting female from Columbia:

And finally, to prove that it’s not all lyre-tailed nightjars out there in Peru, Holly tw**ted this stunning picture today:

JAC: I’ve encountered that species in the tropics; it’s spectacular!

Wonderful Life: The birds of paradise

September 20, 2014 • 12:02 pm

The Cornell Laboratory of Ornithology has completed its Birds-of-Paradise Project, a veritable paradise for bird aficionados, nature lovers, and especially professors like me who want to show students the most stunning examples of sexual selection on Earth. (Remember that sexual selection is just a form of natural selection: a subset of that phenomenon that rests on differential mate choice.)

You can read about the project’s aims here, but the site is huge and interactive: you can hear and see the birds, and read about their evolution and natural history (there are 39 species on New Guinea, the surrounding islands and [a few in] Australia). Best of all are the fantastic videos: a ton of them, and they’ve put them on YouTube. I’ll show just the introductory video, which displays many of the species, and then a few others. They’ve managed to film all of them, so that will whet your appetite for more.

Evolution educators: this is a site you shouldn’t miss.  Their words:

It took 8 years and 18 expeditions to New Guinea, Australia, and nearby islands, but Cornell Lab scientist Ed Scholes and National Geographic photographer Tim Laman succeeded in capturing images of all 39 species in the bird-of-paradise family for the first time ever. This video gives a sense of their monumental undertaking and the spectacular footage that resulted.

Here are all 39 species:

Carola’s Parotia (Parotia carolae), the “King of the Dance.” Don’t miss this one!

The dance of the Carola’s Parotia is the most complex of all birds-of-paradise. The male has to go through five introductory dance moves before starting the main event, called the “ballerina dance.” All the while, four or five females may be perched above him, examining every detail of his performance before deciding whether to mate.

Imagine sexual selection accumulating genes that make the male do this, with all components of that behavior contributing to the male’s chances of leaving those genes.

The Male Riflebird (Ptiloris victoriae), with a directional iridescence:

Iridescence can be seen only when light hits feathers at just the right angle. By adjusting where they are relative to their audience, males can “turn on” their bright colors. Magnificent Riflebirds seem to use this feature with particular precision, even choosing display sites that put their audience in exactly the right place to see the show in the best light.

Finally, one of my favorite, the King-of-Saxony bird of paradise (Pteridophora alberti), with its amazing cull and impressive head feathers:

Throughout their evolution, male birds-of-paradise have been under immense selective pressure to win the attentions of females. Even the King-of-Saxony’s extraordinary head wires aren’t quite enough. They’ve had to develop a display that includes waving the head plumes, rhythmically bouncing on a perch, and delivering an extraordinary screeching, buzzing, hissing call that sounds like anything but a bird.

Note that only the males have the elaborate colors, plumage, and behaviors: females are generally inconspicuous and dull-colored. That’s one of the observations, consistent among many animals, that led Darwin to propose the theory of sexual selection (1871). Sadly, Darwin never saw these species, as the H.M.S. Beagle didn’t visit New Guinea. Here’s the relevant part of the voyage:

1280px-Voyage_of_the_Beagle

There are many more videos. Knock yourself out!

As for why this particular group of birds was so prone to forming new species—and with speciation probably based on differential sexual selection—who knows?

h/t: Gunnar