A critical review of Prum’s “The Evolution of Beauty” in Evolution

November 25, 2018 • 2:00 pm

Richard Prum’s 2017 book on sexual selection, The Evolution of Beauty: How Darwin’s Forgotten Theory of Mate Choice Shapes the Animal World—and Us, has gotten a lot of popular press, and a fair number of positive reviews, but it hasn’t fared very well in the scientific community.  Prum’s thesis, which is that the “runaway” model of sexual selection, in which random and nonadaptive female preference drive male-specific traits like elaborate plumage, behavior, and calls, may well be correct, but we simply lack the evidence in favor of that theory as opposed to other theories for sexually dimorphic traits. (Prum considers the runaway to be the “null model” that, if not falsified, should be taken as correct).  I’ve highlighted the problems with Prum’s book in several posts (especially here) and called attention to two reviews in the scientific literature, one by Gerald Borgia and Gregory Ball, the other by Doug Futuyma, that highlight serious problem’s with Prum’s presentation.

Further, Prum failed to lay out the problems with his own favored theory (e.g., any selection on female preferences themselves makes the “runaway” harder to occur)—a rather tendentious and misleading way to present a theory to the public. Finally, as I wrote in my own mini-review, Prum falls victim to the naturalistic fallacy in his book, arguing that the supposed ubiquity of nonadaptive female choice buttresses “female sexual autonomy” in humans. Here’s what I said:

The tie to feminism is a particularly invidious way to sell his theory, as female choice in birds is a direct product of evolution, while human feminism is a rational conclusion our species draws to improve society by treating people equally. Feminism should not be buttressed by biology, as that makes it susceptible to further knowledge from biology. If you study other species, for example, you could draw other conclusions and support other forms of sexual behavior in humans. Many species, for instance, have “traumatic insemination,” in which the male simply bypasses female choice by either forced copulation or, in the case of bedbugs and some invertebrates, injects the sperm directly into the body cavity, bypassing her genitalia and often injuring or killing her. Here are two bedbugs going at it, with the smaller male sticking his genitals directly into the female body cavity; you can see the puncture wound he’s making. (The sperm somehow still find their way to the eggs.)

If you studied deer or elephant seals, you might find support for the existence of harems in humans, in which powerful males, by virtue of their status, are able to dominate and mate with many women, while the “losers” go childless. This underscores the big mistake of using your favorite brand of biology to support ideological or moral conclusions. (I note that biology can, however, inform some aspects of morality: learning about fetal pain may, for instance, affect one’s views about abortion.)

Prum further demonizes “good genes” models of sexual selection (i.e., females looking for traits indicating that males have good genes or are in good condition to provide for their offspring) by tying them to Nazi eugenics, urging us to embrace his “beauty happens” model as a palliative against racism and genocide!

All reviewers have, however, noted that Prum’s book has good parts, most notably his lively writing and the often mesmerizing descriptions of some of the bizarre male traits produced by sexual selection. But distorting a theory in a popular science book, which happens to be a way to bypass peer review, is not so kosher, and disturbed that such a flawed book was not only one of the New York Times‘s ten best books of 2017, but was even one of three finalists for the 2017 Pulitzer Prize in Nonfiction (it didn’t win). Sadly, none of the reviewers in the popular press picked out the scientific problems with Prum’s book, as they were journalists or writers and not scientists, leading me to suggest that the media should find scientists to review “trade books” on science. (There are plenty of scientists who can write an incisive but lively review.)

That’s a long introduction to what I want to point out: the appearance of another review of Prum’s book, and a long one, by three scientists who work on sexual selection. It’s in press in Evolution, the premier journal of evolutionary biology, but is already on the journal’s website. If you have the free and legal Unpaywall app, you can find it by clicking on the screenshot below (pdf is here):

The three authors are, respectively, from the University of California at Davis, the University of Nebraska, and the University of Maryland Baltimore County. Full disclosure: I looked at a draft of the piece for the authors and made a few suggestions, which didn’t affect the tone or substantial conclusions of the review.

Patricelli et al.’s review is quite critical. To be sure, the authors take care to point out the useful parts of Prum’s work. Their overall take is in the first paragraph:

We were eager to read Richard Prum’s recent book, The Evolution of Beauty: How Darwin’s Forgotten Theory of Mate Choice Shapes the Animal World—and Us (2017). As behavioral ecologists and evolutionary biologists studying animal mating behavior and communication, we appreciated the book’s focus on the aesthetics of mate choice, its engaging descriptions of the natural world, and its representation of a diverse group of scientists and their research. The book is beautifully written and accessible to nonscientists, and we recognize its value in engaging the public in the study of evolution. We disagree, however, with the book’s advocacy of a single evolutionary explanation for beauty in nature, and we were disappointed by its portrayal of modern sexual selection research, which was strikingly out of step with our own research programs and those of our colleagues.

As with other scientists who have reviewed the book, Patricelli et al. find the angels in the prose but the devil in the scientific details. You can read this long review for yourself if you want to see the scientific problems and tendentious nature of the book, as I regard this as the definitive scientifically informed response to The Evolution of Beauty.

I note that the three authors are all women, and so one can’t accuse them of sexism when they fault Prum for his naturalistic fallacy about sexual autonomy in animals —> humans:

Finally, we offer a critical reminder that the behavior of nonhuman animals, sexual or otherwise, cannot and should not be used as a moral compass for our own lives. The book explicitly uses “sexual autonomy” in birds to support feminism in humans (see pp. 177–178). As previously pointed out by Borgia and Ball (2018), not only does this argument commit the naturalistic fallacy, but it opens the door to the justification and rationalization of other common, arguably undesirable, behavior. Lions and coots, for example, are notorious child killers; cuckoldry is relatively common in birds; male bed bugs traumatically inseminate females by injecting sperm into their abdomens; and numerous insects and arachnids engage in coercive mating or sexual cannibalism. Indeed, male self‐sacrifice and subsequent sexual cannibalism has evolved in multiple spider species, but we suspect no man would use this as a justification to encourage such behavior in humans.

The examples of bird sexual autonomy highlighted throughout the book are evolved phenomena resulting from millions of years of selection on individuals. By comparison, the choice to treat men and women equally is a rational, moral decision for the good of individuals and society. Resting our politics and morality on what we see other animals do is the real “dangerous idea” here. Despite the book’s narrow taxonomic worldview (restricted to birds), animals and their sexual behavior are tremendously diverse, each with their own unique evolutionary history. Indeed, it is this diversity that personally drew each of us to the study of sexual selection. Within this diversity, humans could find justification for almost any behavior. Using nonhuman animals as our moral compass would be not only devastating to our society but would also bias our scientific approach to the study of nature.

But in the end these women are scientists, and their scientific judgment is summed up in the review’s conclusions:

If the book’s main goal is to explain the evolutionary origin of “beauty,” as it seems to be, it falls short in numerous ways. The book’s central argument is that indicator models [JAC: i.e., those models in which male traits indicate good genes, good health, or other signs that they could foster better offspring or produce more grandchildren] have been accepted without evidence and that Fisherian selection has been defined out of existence. Ironically, it proposes that we instead accept Fisherian selection without evidence and redefine sexual selection to explicitly exclude adaptive mate choice. This does not advance our understanding of mate choice or aesthetics. Nor does creating a false dichotomy between adaptation and aesthetics, assuming that beauty only arises from coevolving female preferences, and dismissing or ignoring decades of research on receiver psychology. The book also unfairly caricatures most sexual selection researchers as not only failing to acknowledge the subjective experiences of nonhuman animals, but also of supporting human eugenics and antifeminist politics. Moreover, by arguing that beauty is “irrational,” “unpredictable,” and simply “happens,” the book seems to simultaneously argue for the scientific study of beauty while setting it outside the bounds of scientific understanding.

It’s a good review of a readable but flawed book.

Saturday duck report: A mallard fight at Botany Pond; sadness ensues

October 6, 2018 • 8:00 am

The soap opera continues at Botany Pond, culminating yesterday in a terrible duck fight between James and an invading drake, and then the apparent displacement of James from Honey’s affections by the new interloper duck.  That incident also included the interloper getting trapped in a window well, and me having to rescue him.

But let’s start at the beginning. On Tuesday all was well: Honey and James were resting happily at the pond, and Anna and I fed them. Here’s Anna tossing them corn from a can (you can see it in mid-air):

Honey heard a noise above (her hearing is far better than mine), and did her cute cocked-head pose to look up. Because their eyes are on the sides, ducks have to turn their head sideways to see above them. (This photo is out of focus because the light was low and the shutter speed slow.)

The trouble began yesterday morning. The ducks were gone on both Wednesday and Thursday, and I thought for sure they’d left for good on their Great Migration. I was a bit sad but also happy that Honey and James had departed together. On both days I still made regular visits to the pond with food, just in case.

When I went downstairs on Friday morning, I saw one of the office staff and said, “I’m going to feed the ducks, but I guess they’re still gone.” The lady replied, “Well, I saw two males in the pond a while back.” Two males! I couldn’t believe it, but if it were true it meant Big Trouble. But when I got to the pond, only Honey and James were there. I fed them, but they were very skittish and didn’t eat much.

When I returned for another feeding a few hours later, there were indeed interloper drakes in the pond: two of them, making a total of three including James! James and Honey were cruising around the pond, with James particularly intent on driving away one of the Devil Ducks (that’s what I call them). He was after only one of them, whom I’ve now named Billzebub. Here are Honey and James cruising around, with James ultimately taking out after Billzebub. James didn’t hurt him or attack him, for James is a gentle duck and only wanted to warn the interloper. Needless to say, they weren’t interested in food.

The Devil Ducks have white-speckled heads, which I suspect means they’re molting. You can easily tell them apart from James:

The troubles mounted.  I decided to drive away the interlopers with my Super Soaker squirt gun, but it only made them fly to another part of the pond. Finally, James decided to investigate where Billzebub was ( he had gone to the narrow part of the pond), and when they encountered each other they got into a terrible fracas, which I filmed with trembling hands. Before I took the video below I did everything I could to break up the fight, including squirting them, shouting, and waving a branch at them. It was to no avail; they were in it well and truly.

Note below that Billzebub is the aggressor; James seems to want to escape the fracas but the Devil Duck wants to peck him. You can also hear Honey quacking: she was nearby watching the fracas—perhaps to see which duck was the victor.  Eventually I got them apart with the squirt gun, but they just started fighting again in the main part of the pond. It’s all about sex, of course, and perhaps territory.

Finally, I managed to squirt the fighting pair again, and Billzebub flew from the pond to the nearby sidewalk—accompanied by Honey! (It’s the first time I’ve seen her fly.) Together they waddled into the bushes, and I decided to pursue Billzebub further, hoping that I could get him to leave the pond permanently by squirting him. When I did that, however, Honey flew back into the pond but Billzebub flew to a basement window, somehow managing to enter the window well by flying underneath a set of narrow bars.

Billzebub was then trapped in the window well, trying to get out by hurling himself against the bars and flapping his wings. I had no choice: I had to free him.

It was not easy. He was flapping around, kicking, and making little quacks. I knew I had to secure his wings so he wouldn’t break them, and so I reached underneath the bars, through the rather small gap, and managed to grab his body, holding his wings closed against it. Then I carefully worked his head underneath the bars. When his head was out, I pulled his body out very gently and carefully. It was not easy, as the gap was just a tad bigger than a compressed duck body. (I don’t know how he flew in there so quickly!) I prized him free and immediately put him down (he was wet and also heavy.).  He walked away quickly, and seemed all right.

Yes, he was all right, because an hour later I found Billzebub and Honey sitting cozily on the duck island, while James was huddled disconsolately on the edge of the pond, gazing at his former girlfriend and her new lover. It was heartbreaking, I tell you.

An hour later, James was still there and the new lovers were cozily ensconced on the island. I couldn’t bring myself to feed them. I haven’t gone downstairs this morning, as it’s still dark, but I hope everyone is gone.

It was and is saddening to me. First, James was attacked, and didn’t fight back because he’s a kindly drake. But that probably caused him to lose favor in Honey’s eyes, as she wants a mate who can defend her. It seems, then, that the fight caused Honey to switch affection from James to Billzebub. That’s sad because I thought James and Honey were a good pair. But you can’t obviate female choice in nature.

And now I’m not only sad, but feel guilty on three counts. First, I feel that I’ve lost affection for Honey because she switched mates so readily. I know that’s a dumb feeling, as nature will be nature and Honey chose the drake she found most appealing. Still, the sight of James huddled on the shore, looking sad and defeated, was heartbreaking.

I also feel guilty about this: I could have killed Billzebub when he was trapped in the window well, and then perhaps Honey would have stayed with James. Now I am not at all capable of killing an animal, much less hurting one, so this wasn’t in the cards, but someone to whom I told this story got me thinking about this when she she said, “Hmm. . . that duck could have made a nice dinner.” That was said in jest, but I started thinking. . .   And now I feel guilty for even entertaining those thoughts. No wild animal deserves death just because a stupid professor thinks he’s not a fit mate for his favorite mallard hen.

Finally, I could not bring myself to feed Honey yesterday, as that would also have involved feeding Billzebub as well, and I couldn’t bear to reward him. But that is stupid too, for ducks must have their noms, and I want Honey to be nice and plump before she migrates. So I will go downstairs with food in an hour, and feed every duck in the pond.

Nature must do what it will, and at some point I have to let go. I guess the time is now. But I’ve bonded with these wild animals over the summer, and it’s hard.

The only upside of this, and it’s not a big one, is that I finally got to hold a wild mallard. Sadly, it wasn’t the way I wanted. But I did rescue the duck, even though it was from a situation that I myself created.

I’m kind of hoping that all the ducks will have gone, so that I’ll see no more fights and not have to feed Billzebub. Then I can simply wait to see if Honey returns next year. But it’s not a good way to end Duck Season.

***************

I just went downstairs with food, as it’s getting light. It’s raining, and all the ducks were gone. I suspect they’re gone for good. This isn’t the way I wanted it to end, but at least Honey is fine and fledged eight healthy offspring. I hope she has a good winter down south and comes back to me in the Spring.

A new review (and critique) of Richard Prum’s book on sexual selection

August 30, 2018 • 9:01 am

I’ve now read Richard O. Prum’s new book, The Evolution of Beauty: How Darwin’s Forgotten Theory of Mate Choice Shapes the Animal World—and Us (I’d highlighted the work earlier in my critique of his NY Times article about the book). Click on the screenshot to go to the book’s Amazon site. (Prum is a professor of ornithology, ecology, and evolution at Yale.)

The review I want to highlight was published in the journal Animal Behaviour, but I’ll get to that in a minute. First, I want to give my own brief take on the book.

The Evolution of Beauty is in fact a mixed bag, but for me the problems outweigh the good parts. First, though, what’s good, and a bit of background.

Prum’s purpose is to explicate a theory of sexual selection that was suggested by geneticist Ronald Fisher eighty years ago and was elaborated much more recently by my colleagues Russell Lande and Mark Kirkpatrick. (Prum also draws a line of descent to this theory from Darwin’s own views of sexual selection published in 1871.)

This theory—the “runaway model”, called by Prum the “Beauty Happens” model—tries to explain why males are ornamented and show displays, while females choose among those males. This is sexual selection, which Prum (mistakenly, in my view) says is qualitatively different from natural selection (see my earlier post for my take on this pseudo-distinction). Prum’s view is that the runaway process, in which females evince randomly generated “aesthetic” preferences for male traits, have been the “ubiquitous” cause of sexual ornamentation and displays in animals. (His examples are drawn largely from birds, but he applies them to humans in the book’s final chapters.) The male traits and female preferences get exaggerated because they are genetically correlated: those females with preferences mate with males having the preferred trait, and the offspring carry genes for both the preference and the male trait. This causes a snowballing process that exaggerates both trait and preference to the extremes that we often see in nature.

Prum contrasts the runaway process with other forms of sexual selection in which females choose males not because they’re “beautiful” (to Prum that word that simply means “what appeals to females”), but because the offspring of females get benefits from her choice: either genetic benefits (“good genes” models) or “direct benefits” (healthy males can better take care of offspring or don’t pass parasites onto the mother and hence onto her young). These models, in which male traits and behaviors indicate the health or genetic quality of a male, are known as “indicator models”.

There are also other models that are either well known and substantiated (“male-male competition”, which causes sexual dimorphism and ornamentation, including antlers in deer, larger body size in competing male elephant seals, and so on), or more speculative but still credible models (“sensory exploitation,” in which female preferences are a byproduct of behaviors that existed for other reason, like preferring colors that resemble berries). There is in fact evidence for nearly every model of sexual selection, but it’s so hard to do this kind of work that we simply cannot say, as does Prum, that one scenario is “ubiquitous.” It’s early days here, and Prum’s suggestion that we know the answer is premature, though in the end he may be right. Or other models may be right. Or the answer may be that different models contribute to sexual dimorphism in varying degrees, or act together. We just don’t know.

The good aspects of Prum’s book include a historical overview of the difference between Darwin and A. R. Wallace in their views of sexual selection, and a critique of the many modern behavioral ecologists who uncritically adopt “good genes” models as the default explanation for sexual dimorphism. Further, Prum’s descriptions of male traits and behaviors, most seen by him in the field, are superbly written and often mesmerizing.

The book’s problem is that it is tendentious. Prum doesn’t describe the issues with his favored runaway model; he mistakenly regards it as a “null model” against which other models must be tested since, he wrongly claims, it makes no assumptions (he also claims that his null model can neither be proven nor disproven, which makes it non-scientific); he neglects other forms of sexual selection; he does not recognize that various models can work together and likely do work together; he ties “good genes” models to eugenics and even Nazi eugenics, unfairly tarring sexual selection theory with the residue of an unsavory past; and he claims that female choice of mates, which he calls “sexual autonomy”, somehow vindicates feminism in our own species.

The tie to feminism is a particularly invidious way to sell his theory, as female choice in birds is a direct product of evolution, while human feminism is a rational conclusion our species draws to improve society by treating people equally. Feminism should not be buttressed by biology, as that makes it susceptible to further knowledge from biology. If you study other species, for example, you could draw other conclusions and support other forms of sexual behavior in humans. Many species, for instance, have “traumatic insemination,” in which the male simply bypasses female choice by either forced copulation or, in the case of bedbugs and some invertebrates, injects the sperm directly into the body cavity, bypassing her genitalia and often injuring or killing her. Here are two bedbugs going at it, with the smaller male sticking his genitals directly into the female body cavity; you can see the puncture wound he’s making. (The sperm somehow still find their way to the eggs.)

If you studied deer or elephant seals, you might find support for the existence of harems in humans, in which powerful males, by virtue of their status, are able to dominate and mate with many women, while the “losers” go childless. This underscores the big mistake of using your favorite brand of biology to support ideological or moral conclusions. (I note that biology can, however, inform some aspects of morality: learning about fetal pain may, for instance, affect one’s views about abortion.)

Nevertheless, the problems with Prum’s book are so serious that it distresses me that it’s gotten such good and uncritical reviews. The New York Times, which gave it a glowing and uncritical review, selected it as one of the Ten Best Books of 2017.  It was also one of three finalists for the 2018 Pulitzer Prize in General Nonfiction (it didn’t win), which convinces me that those who choose these candidates don’t know much about the books’ subjects. I’m starting to realize that magazines and websites should choose scientists, or at least people who deeply understand the science at issue, to review trade books about science.

But enough; I will likely give a fuller explication of my views in the future. For now I want to highlight a newly published critique of Prum’s book by Gerald Borgia and Gregory Ball, two researchers from the University of Maryland who work on mate choice in bowerbirds. (Borgia is my former colleague.) Their review of The Evolution of Beauty occupies a page and a half in the latest issue of Animal Behaviour. The reference with link (also here) is below, and the pdf is here; both are free to me with Unpaywall, but if you can’t download them, a judicious inquiry might yield you a pdf.

Borgia and Ball’s criticisms are similar to mine, but as they are experts, they go far beyond what I’ve written above. If you know about sexual selection it may be an easier read, but if you’re interested in sexual selection I suggest you give it a try. At any rate, at the end of their review, Borgia and Ball raise the feminism and eugenics angles, and the excerpt below should be comprehensible to everyone. I’ve put in bold a particularly misguided quote from Prum:

In the last chapter of Beauty, Prum reveals the political view that has shaped the core arguments in this book. Prum’s style and message is a rerun of Gould and Lewontin’s divisive writings from 40 years ago, in which they unfairly associated adaptationism with genetic determinism, eugenics and Nazi atrocities (Allen et al.,1975, p. 186). Prum applies similar arguments to sexual selection, associating good genes and adaptive mate choice hypotheses with eugenics and Nazi violence. He states: To permanently disconnect evolutionary biology from our eugenic roots, we need to embrace Darwin’s aesthetic view of life and fully incorporate the possibility of nonadaptive, arbitrary aesthetic evolution by sexual selection. “…Accordingly, evolutionary biology should adopt the nonadaptive, Beauty Happens null model of the evolution of mating preferences and display traits by sexual selection”(page 331).

That’s incredibly dumb. What it says is that we should adopt one scientific theory over others (surprise!—it’s Prum’s favored theory) not for scientific and evidential reasons, but simply because that theory can’t be tied to discredited views of eugenics. Borgia and Ball continue:

Prum’s conflation of eugenics and genocide with adaptive mate choice is a disservice to his readers and colleagues for several reasons. First, this argument is wrong. Mate choice represents individual, and especially female, reproductive freedom, whereas eugenics and genocide represent a restriction by the state on an individual’s ability to live and reproduce. It is not more moral to choose a mate  for arbitrary reasons, as Prum suggests, than to choose one to enhance the success of one’s offspring. Consistently across societies, parents support the interests of their offspring, suggesting that adaptive mate choice may be viewed as morally superior. Second, Prum’s mistaken association of eugenics with adaptive/good genes mate choice represents a threat that can limit free scientific discussion of important issues, and this should be resisted. Third, science validates or rejects hypotheses based on evidence, not on potential or contrived historical associations, altruistic intent, or political belief.

We offer two key takeaways from this book. First, contrary to Prum’s claims, LK [“Lande-Kirkpatrick”] runaway selection has enjoyed a privileged position, including being prominently presented in evolution and behaviour textbooks despite a lack of supporting evidence. Perhaps the most significant implication of Beauty comes from Prum’s inability to make a credible case for LK runaway sexual selection in this book’s 448 pages, suggesting that it may be time to shift focus to other, better-supported models. Second, Prum’s (and other, earlier) efforts to inject politics into science commonly distort the science to justify political goals. We should have all learned by now that science is about understanding what nature is, not what we want it to be. The arguments in Beauty that suggest otherwise should be rejected.

That’s a fine ending of a good review—and by that I mean a thoughtful review.

Here’s a 7-minute Big Think video in which Prum explains his views. His drawing a connection between eugenics and adaptive theories of mate choice begins about 2:10. He continues, tying the theories he opposes to eugenics and racism (and to his view that sexual selection differs from natural selection), at about 4:50.

h/t: Nilou, Tami Mendelson

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Borgia, G. and G. F. Ball. 2018. Review of The evolution of beauty: how Darwin’s forgotten theory of mate choice shapes the animal world—and us.  Animal Behaviour 137:187-188.

 

 

Male pipefish who get pregnant reduce investment in their offspring if they see a sexy female

August 22, 2018 • 11:45 am

This new paper in the Proceedings of the Royal Society B caught my eye because of its title (abstract here but paper behind a paywall; judicious inquiry might yield you a pdf).  Check out the title and the abstract, and I’ll explain a bit of the results below (I read the paper quickly):

First, a bit of biology. Many species of pipefish and seahorses have a reproductive system in which males become pregnant. What happens is that females produce their eggs and stick them into the brood pouches of males, who fertilize them and then protect them, control the salinity, and even nourish the eggs using materials from the male’s body.  Since there is a shortage of available male brood pouches compared to the number of females who want their eggs fertilized, there’s competition between females for male pouches—the reverse of the usual competition by males for access to female eggs when females get pregnant or lay eggs. (Don’t believe Richard Prum’s new book about this, in which he implies that there really isn’t male competition by males for females, even in humans where the data on male profligacy vs. female choosiness are clear.)

Since sexual selection is reversed in these species, you find reverse ornamentation as well: it is the females who are ornate compared to the drabber males. And males in tests prefer both larger and more ornate females above smaller or less showy females. Here is the difference in seahorse anatomy and a male giving birth:

Look at all those babies!

In the new paper, authors M. Cunha et al did three sets of studies:

  1. They gave pregnant males of black-striped pipefish the sight (in a divided aquarium) of new females, some larger, some smaller, and some about the same size as females who had already “impregnanted” the males. In the first study, those males who got to see larger females produced smaller offspring. This may be because they’re reducing their investment in past pregnancies in hopes of remating to a “fitter” or more attractive female and producing more and better offspring.
  2. The authors measured the incidence of aborted embryos in pregnant male pipefish, and showed that the presence of very large females on the other side of the divider increased the percentage of aborted embryos in the male (they didn’t test females of different size, but used large females vs no females). This suggests that males can somehow reduce their investment when seeing another attractive female, though they didn’t use females of different size (attractiveness), which would have been a marked improvement of the design.
  3. They looked at wild pipefish males whose embryos were at different stages of development, and found that the heterogeneity of embryo size within a male decreased as pregnancy proceeded, and this was correlated with marked reduction in brood size. This was not due to random culling of embryos, but to culling of either the smallest or largest embryos (they couldn’t determine which were culled), which the authors see as a a form of selective abortion.

Overall, although the data aren’t terribly strong, this suggests a form of post-fertilization sexual selection, in which males decide to abort or reduce investment in embryos if they see a better mating opportunity. To quote from the paper:

Male pipefish, triggered to invest less in ongoing pregnancies by the sight of a very large, sexy female, hence produced smaller and more heterogeneous offspring, while also reducing brood size. As in pipefish resources derived from aborted embryos are captured by the father, instead of being directly distributed among developing embryos [14], males stricken by a sexier female not only save energy reserves for a presumably more auspicious future reproductive event, but they also potentially seize resources from a less rewarding brood.

As I lacked the time to read this paper as closely as usual, readers may find problems with the paper or errors in my summary.

_______

Cunha, M., A. Berglund, S. Mendes, and N. Monteiro. 2018. The ‘Woman in Red’ effect: pipefish males curb pregnancies at the sight of an attractive female. Proc. Roy. Soc. B. 285:20181335. Published online.

Hybrid speciation in Amazonian manakins?

January 14, 2018 • 11:15 am

Rather than give a long introduction to hybrid speciation, I refer you to a recent post I did on diploid hybrid speciation in the Galápagos finches; just have a look at the introduction, which talks about the commonness of hybrid speciation in plants (via polyploidy) and its rarity in animals.  The Galápagos finches may be a case of diploid speciation following hybridization, but it’s not clear, for they don’t get a chance to mate with one of the two ancestral species (i.e., we don’t know if it’s reproductively isolated from one parent), and it’s also likely, I think, that this small-population hybrid species won’t last very long, but will be “mated to death” by its one sympatric parent.

But there’s a new report of a hybrid species in manakins from the Amazon, and this one looks pretty good. Published in PNAS (reference below, free access, and pdf here), its first author is Anfredo Barerra-Guzmán, and the last author is Jason Weir, a postdoc in Trevor Price’s lab who was next door to me for a few years.

The paper is long and complex, with some genetic details that aren’t necessary for general readers, but I’ll summarize the results as succinctly as I can.

In the Brazilian Amazon, at the headwaters of the Cururu-ri River in Pará state, exist three species of manakins: Lepidothrix vilasboasi, L. nattereri, and L. iris, which has two subspecies.  L. vilasbosi, which I’ll call LV, lives in an area intermediate between L. nattereri (“LN”) and L. iris (“LI”); all are separated by rivers, which for many Amazonian birds constitute geographic barriers because birds simply don’t like to fly over water.  Here are their geographic distributions from the paper:

You can see that LV, the putative “hybrid species” lies in between its two parents LN and LI (two subspecies). When LV was first found in 1957, was thought to be simply a “hybrid zone”: an area of hybridization between the two parental species.

The three species differ most markedly in the color of their head crowns. As the paper notes:

The crown patch in L. iris is iridescent and varies from brilliant white (its usual look, which is very similar to Lnattereri) to blue or purple, depending on the angle of light. Males of the two subspecies of Liris distributed on either side of the Xingu River are almost identical in plumage, with Liris iris possessing a thin green strip between the upper mandible and the crown patch and with the crown patch extending all the way to the mandible in Liris eucephala. Females (not shown) appear like males but lack the contrasting crown and rump patches and do not differ appreciably among species.

In contrast, LV has a yellow crown patch, very distinct from that of the two parental species (see below).

Here’s LV (the “golden-crowned manakin”) with its yellow crown:

And LN (the “snow-capped manakin”) with its white crown:

Well, what is the evidence that LV was formed after hybridization between LN and LI? That rests on genetics. The genome of LV is a mixture of genes from the two parental species, and attempts to reconstruct the ancestry of these birds, using both standard phylogenetics and “coalescent” simulations, show that it’s far more likely that LV came from mixed genes from LN and LI than that it branched off from one or the other species (the conventional “branching” scenario for speciation). Between 15% and 38% of the LV genome comes from LN and 62%-85% of the genome from LI, depending on what method is used to do the calculation.

One problem is that LV could still represent an isolated hybrid population, or a hybrid zone, between the two species. The authors say this is unlikely because pure F1 hybrids of the two species (that is, first-generation offspring), would have 50% of the genes from each species rather than this skewed distribution.  But that would be the case only if the F1 hybrids are sterile so that they couldn’t form a “hybrid swarm” by further reproduction. It’s still possible that we have here a hybrid zone or swarm in which members are not reproductively isolated from the two parental species, so that LV is not really a “species”. The crucial evidence is whether LV is reproductively isolated from LN and LI (note: it’s already geographically isolated, but that doesn’t count as a reproductive barrier in the “biological species concept”, for in that case any geographically isolated population would be a species).

So is LV reproductively isolated from the two parents? We don’t know for sure, for they don’t coexist in the same area. Barrera-Guzmán et al. suspect that LV would be isolated—by its differently colored crown. Crown colors are used by birds as ways to identify mates from their of own species, and to discriminate against the wrong ones, so the authors suppose that LV males would not be recognized as appropriate mates by females from LN and LI because of LV’s novel yellow crown. Likewise, it’s possible that LV females wouldn’t recognize the iridescent crowns of its parental species, so there would be reproductive isolation on all fronts.

The interesting thing about the LV crown is its novel yellow color. The authors found that the iridescences of the LN and LI crowns is caused by different structural properties of their feathers, and that the LV crowns are intermediate between these two sets of properties. The interesting thing is that if you make a pure F1 hybrid between LN and LI, you get a dull crown with the same intermediate structural properties of the putative hybrid LV, but it’s not yellow!

The authors then speculate—reasonably in my view—that what happened in this scenario was that the two parental species hybridized, producing a population that lives in the forest (where LV does) but had a dull crown. Sexual and/or natural selection then caused carotenoid-retaining structures to evolve in the hybrid population so that males would be visible to females in the forest. (Female preferences for yellow would, by sexual selection, go along with this.) Thus, hybridization itself wasn’t sufficient for speciation: the reproductive isolation would have to have involved some post-hybridization selection.

The upshot: I think the authors have a good case for having uncovered a rare diploid hybrid species of bird. It’s not an absolutely compelling case, as it’s still possible we have a hybrid swarm that is not a new species and is not reproductively isolated from the parents. If they could demonstrate strong mate discrimination among species based on crown color (or anything else), the data would be more convincing. But I still think they’ve uncovered a possible (even probable) case, which, with some follow-up work, could represent one of just a handful of diploid hybrid species in animals.

Some people may say, “well, this is a new evolutionary paradigm, for speciation is supposed to occur by branching of lineages, not by anastomosis (fusion) of separate branches into one.” And yes, if this is the way species normally formed, it would mandate a pretty big revision of Darwinian theory. But the evidence is that this is not the way most diploid species form, for if it did, phylogenetic analysis would not resolve any bits of the tree of life: we’d just get a spaghetti-like mess. In Drosophila, the group I know best, this is clearly not the case: branching rather than fusion is the rule. Although there is more leakage of genes between animal species than we suspected two decades ago, that is “introgresssion”, not speciation. We still see little evidence that the formation of new species in animals occurs by the fusion of already-existing branches (species).

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Barrera-Guzmán, A. O., A. Aleixo, M. D. Shawkey, and J. T. Weir. 2018. Hybrid speciation leads to novel male secondary sexual ornamentation of an Amazonian bird. Proc. Nat. Acad. Sci. USA 115:E218-225. Published ahead of print December 26, 2017doi:10.1073/pnas.1717319115

A bright blue tarantula

September 20, 2017 • 2:00 pm

Have a look at this lovely blue tarantula from Brazil. The species is Pterinopelma sasimai, named after Dr. Ivan Sazima, a Brazilian zoologist who discovered the species in 1971 but didn’t formally name it. He had a blue female which died during molting. Here are two videos:

The species wasn’t in fact formally named until 2011, in a paper by Rogério Bertani et al. in Zootaxa (link and free access below).  Finding it was hard work; as the first link above notes:

The 3 researchers were only able to rediscover the species after 5 years of intensive work, due to the fact the area she lives in is very harsh. All species were found on high altitude with poor vegetation and extreme weather conditions (10-35°C), experiencing periods of extreme precipitation and drought.

The intriguing thing about this gorgeous creature (how many blue spiders can you think of?) is that it is the females who are blue while the males are a drab brown. You can see this from the figure below, taken from the Zootaxa paper (see caption):

(From paper): FIGURES 14─16. Pterinopelma sazimai sp. nov. (14) Female, holotype. (15) Male, paratype. (16) Juvenile. Photos: 14, 16, C. S. Fukushima; 15, R. Bertani.

Here are its habitat and range:

What’s intriguing about the paper on this species, and several descriptions, is that there is no speculation I’ve seen about the big difference in color. (The sexes described as so different in color weren’t really different species, for you can see them mating, as in the video below.) Usually when one sex is brightly colored or ornamented in animals, while the other is less conspicuous, it’s almost always the males who are colored or ornamented. That, of course, is because of sexual selection, which results from a differential parental investment that makes females a scarce resource for which males must compete. So why is the female blue here? Your guess is as good as mine.

Here is a female molting. When tarantulas do this, and they do it throughout their lives, they simply pop the top of their cephalothorax and emerge from it, as if from a tank turret, withdrawing their legs from the old appendages like someone taking off their pajamas. The newly molted animal, which remains quiescent till it hardens, is all shiny and pretty.

When I kept tarantulas at Harvard (I had more than half a dozen, and kept them well fed with crickets and clean cockroaches), I’d see this all the time, and it was mesmerizing.

 

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Bertani, R., H. Nagahama, and C. S. Fukushima. 2007. Revalidation of Pterinopelma Pocock 1901 with description of a new species and the female of Pterinopelma vitiosum (Keyserling 1891) (Araneae: Theraphosidae: Theraphosinae). Zootaxa 2418: 1-18.

New peacock spiders

September 8, 2017 • 8:00 am

Peacock spiders are not only beautiful, but great examples of sexual selection, for the males show both amazing colors and fascinating display behaviors that they use in their attempts to attract females. “Attempt”, of course, doesn’t mean they’re behaving with conscious intent, but just showing the results of sexual selection.

The only reason peacock spiders don’t get as much attention as, say, their avian counterparts—the birds of paradise—is that they’re tiny, like this (all photos by Jurgen Otto):

Like all salticids, peacock spiders are also lightning fast, as you’ll see in some of the videos below.

I’ve written about these arthropod jewels before (here and here); they are, as I said, salticids, or jumping spiders, and peacock spiders fall in the genus Maratus. All but one of the 50-odd species (there may be 60 or more) are found in Australia. Their primary popularizer and discover is Dr. Jürgen Otto, who has a video site devoted to them as well as a Facebook page and a Flickr page.

As is typical of sexually-selected species, only the males show bright colors and displays, which ultimately result from reproductively competent females being a scarce resource that must be attracted.The video below shows the amazing variety of behaviors of Maratus spiders (51 in this clip). Males frantically wave their legs and abdomens (all strikingly marked) to get a female’s attention.

Otto has a mildly disturbing habit of adding dance music to the spider videos; I prefer to turn the sound off. One can appreciate these creatures without anthropomorphizing them:

Here’s one species, Maratus volans, discovered, filmed, and narrated by Otto (at 5:21 you see an unsuccessful male eaten by a female):

Two years ago Otto described seven new species of Maratus; here’s a video showing those:

Now, according to several sources, including the Australian Broadcasting Corporation, Otto’s found four more species, as well as a new subspecies. Annoyingly, none of the articles reporting this discovery cite or link to the scientific paper with formal descriptions (this is a flaw in a lot of science journalism). I finally found Otto’s paper in an obscure journal, Peckhamia, with a citation and free link at the bottom of this post.

If you click on the screenshot just below, you’ll go to an ABC video that shows all of the new species:

And here are the new species. First, the abstract of Otto’s paper:

Four new species of the genus Maratus are described from Western Australia: M. cristatus, M. electricus, M. gemmifer, and M. trigonus. M. electricus is compared to the closely related M. linnaei Waldock 2008. A new subspecies of M. melindae Waldock 2013, M. melindae corus, is reported from a new locality east of Cervantes. The courtship display of all six species is also documented.

Go to the paper for a lot more photos, as well as pictures of the females, which are similar and much less colorful than the males shown below:

Maratus electricus:

Maratus cristatus:

Maratus gemmifer:

Maratus trigonus:

M. melindae corus (it has not escaped my notice that this species looks very similar to M. gemmifer pictured above, but it’s identified in Otto’s paper as a subspecies of a different species, differentiated by color markings).

h/t: Phil D.

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Otto, J. C. and D. E. Hill.  2017.  Five new peacock spiders from Western Australia (Araneae: Salticidae: Euophryini: Maratus Karsch 1878).  Peckhamia 152.1:  1-97

The bizarre mating dance of the hooded grebe

August 6, 2017 • 2:30 pm

Reader Charleen sent me the tweet below, which shows the courtship ritual of the hooded grebe (Podiceps gallardoi), a rare and critically endangered species (fewer than 1000 individuals) that lives in isolated Patagonian lakes. Have a look at this craziness, and ask yourself “Why the hell are they doing this?” or “What selective advantage is there in testing each other this way?” And don’t ask me, because I have no idea! It’s clearly a bonding ritual, but may be a form of mutual sexual selection in which potential mates size each other up in some way:

A photo:

That in turn led me to a new article in Audubon magazine that identifies the clip as coming from a new movie about the species, “Tango in the Wind.” Fortunately, the half-hour documentary is free on Vimeo, and I’ve put it below. Grebe species are known for some really fancy footwork during courtship (Google “courtship grebe”), but the Hooded takes the cake.  And, as Audubon notes,

But the Hooded Grebe’s courtship hasn’t been nearly as well documented [as that of other grebes], thanks to its limited range in the harsh and isolated barrens of Patagonia, near the tip of South America. In fact, the species wasn’t even known to science until 1974. “There aren’t many people who know much about Hooded Grebe courtship,” says Audubon field editor Kenn Kaufman. “The people who made this video probably know as much about the bird as anyone does.”

. . . The Hooded Grebe’s courtship dance may look funny, but with fewer than 500 breeding pairs remaining, this is serious business. Its numbers have declined by about 80 percent in the past 25 years, largely due to the introduction of the non-native American mink and rainbow trout, according to BirdLife International. The International Union for the Conservation of Nature labeled the species critically endangered in 2016.

Researchers interviewed in Tango in the Wind say climate change further threatens Hooded Grebe’s habitat and survival, noting that it’s already drying up the lakes where they nest. While more abundant than their Patagonian cousins, Western Grebes and Clark’s Grebes both face significant losses of their summer range, thanks to climate change. They’re among the 314 climate-endangered North American bird species identified by Audubon.

Now, if you have half an hour, watch this lovely but sad movie—sad because this wonderful species is on the verge of extinction, probably due to global warming. And watch it on full screen; the link for that is here. (The courtship segment begins about ten minutes in and you can see a bit at the very end.)

 

The amazing display of the Standard-winged Nightjar

August 1, 2017 • 12:00 pm

by Matthew Cobb

About 20 months ago we discussed the amazing Peruvian lyre-tailed nightjar, with its fabulous streamer feathers. This morning a rather different, and extraordinary nightjar tailfeather adaptation popped into my inbox, thanks to the Nightjar News email newsletter. It included information about the Standard-winged Nightjar (Caprimulgus longipennis), which sounds kind of… standard, until I realised that by ‘standard’ they mean ‘flag’. Here’s a picture of a male, by Paul Cools.

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Those things behind him are his tailfeathers – they are what prompted the species name: longipennis (long feather). Nightjar News explains:

The Standard-winged Nightjar is one of the more impressive members of the Caprimulgid family. This true nightjar species breeds in dry savannah habitat of central Africa. During the breeding season, the male grows highly-specialized wing feathers up to 38 cm long, primarily of bare shaft with feather plumes on the end. The feathers are used as part of a flight display to attract female.

Here’s another picture, from Wikipedia, taken by Jan Steffen:

Mlongipennis

These long feathers look pretty sexy, and you can see why a gal might like them, but there’s more! Look at how extraordinary they are when he flies (there’s no sound, sadly) – video by Dermot Breen:

Sometimes, the display can look like the bird is followed by two bats:

Here’s another example, from the Gambia – at the end you can hear the male churring:

It seems fairly certain that this is an example of sexual selection – only the male has the feathers, and he has them only during the mating season.

However, this insight is only the beginning of wisdom, for it raises a whole set of questions:

• What exactly does the female see (or maybe hear)?

• Is it simply the length and shape of the feathers she likes, or is there something about the way he does the display that somehow reveals his ‘good genes’ (this is one of the key ideas about how sexual selection works, though in fact we generally know very little about what is ‘good’ about those genes)?

• What exactly is it about the display? Is it ‘supposed’ to look like two birds following him (why?), or is that just an aerodynamic consequence of having those long streamers?

• And here’s the real killer question – how on earth could we find the answers to these hypotheses?

Please chip in below…

[EDIT: On reflection, it seems like there is a difference in the behaviour we can see. In the first video, the male is flying very low, with the ‘standards’ upright, like flags. I wonder if this is a mating flight. In the second video, ‘three birds or one’, he’s flying like a hungry nightjar, with the feathers jiggling along behind him as he does his hunting flight, jinking around trying to catch insects. In the third video, he’s just mooching about and then starts his call… My guess is that the first behaviour is the key one, in terms of sexual selection.]

The superb Superb Bird of Paradise

July 19, 2017 • 2:30 pm

The Cornell Birds-of-Paradise Project is a great website that contains all kinds of information about the 39 species in this fantastic group. There are videos and information about the sexual dimorphism in plumage and behavior, and other aspects of the birds’ biology, information about their evolutionary history and the people who study them, and general information about evolution and sexual selection—even a video on speciation. It’s a remarkable and informative site: the best place to visit if you want to see what are the most stupendous examples of sexual selection—and I’m referring not just to the male behavior, but to the female choice that drives much of it. It’s a rich resource for those who teach evolution.

Below is a 4½-minute video of the famous Superb Bird of Paradise (Lophorina superba), whose Cornell page is here. I like this video because it’s not just a “gee whiz–look at this!” presentation (you can see Attenborough’s shorter video of this species on a previous post), but one that shows how at least four different groups of feathers have evolved, and conspire, to create the “smiley face” appearance of the displaying male. There are also several evolved changes in male behavior, including jumping around to stay in front of the female and raising his bill to bisect the blue crown feathers.

After an evolutionist has gotten over her amazement, the first question that then arises is, “Why is the male bird doing this?” That is, what, exactly, is the female looking for that makes her not only drive the evolution of this display, but makes some patterns and behaviors more acceptable than others? Good genes? Some pre-existing sensory bias in the female?

In fact we know almost nothing about what drives this genre of “female choice” sexual selection. This means that, for the time being, we can only marvel at it, and at the power of natural selection—of which sexual selection is a subset.

h/t: Taskin