Category: Adaptations

“How extremely stupid not to have thought of that.”

January 7, 2011 • 9:09 am

The quote above is, of course, from Thomas Henry Huxley; it was his reaction after learning about Darwin’s simple but powerful idea of natural selection.  But I had exactly the same reaction after reading a new paper in The Proceedings of the National Academy of Sciences by Shevtsova et al. (reference below; free online access). The paper shows that insect wings, viewed under certain conditions of light, display stable patterns of iridescence that might be of profound behavioral (and evolutionary) significance.

I’ve spent 42 years looking at flies—ever since I was a sophomore in college—and I must have seen millions of them under the microscope.  Usually we look at them against a white background, which we use for inspecting the beasts for mutations or sorting them into males and females for mating.  Very occasionally, as when I dissected out female ovaries to count the number of ovarioles, I’d put them on a black background, which makes the white eggs more visible.

On those occasions, I could see an iridescent pattern in the wings, like what you see on the surface of a soap bubble or an oil slick on wet pavement.  I never paid that pattern any attention, and that was my mistake.

Unlike every other fly person who’s seen the patterns, the authors of the PNAS paper—Ekaterina Shevstova, two Swedish colleagues (C. Hansson and J. Kjaerandsen), and Dan Janzen, the well-known American naturalist and entomologist—didn’t ignore the colorful patterns. They found that these patterns (see examples below) are produced by interference between light reflected off the two surfaces of the chitinous wing membrane, that they are stable within an individual but often variable among sexes and among species, and that they’re found in most Hymenoptera (wasps and bees) and Diptera (flies).  They call the patterns “wing interference patterns” (WIPs), and discovered that they’re best seen against dark backgrounds, and when the light strikes at certain angles.

The paper is well explained by Ed Yong at Not Exactly Rocket Science, so I won’t go over the findings in extenso (plus, the paper is pretty easy to read).  First, look at a few of the WIPs.  Here’s Figure 1 of their paper (click to enlarge):

G, at the top right, is a hymenopteran (Closterocerus coffeellae), showing how the difference in background can suddenly reveal a hidden iridescent pattern.  H and I, below that, show the common fruit fly, Drosophila melanogaster (the one I’ve seen millions of), showing the same background-dependent pattern.  J and K are the wings of another species of Drosophila (D. guttifera), which has patches of pigment on the wings under “normal” illuniation but bright iridescence before a black background. The authors suggest that maybe the pigment spots are there not to be seen on their own, but to control the patterns of iridescence, which is what the other flies are really looking for.

A and B show two different but related species of hymenopterans, demonstrating that the WIPs can be highly visible when displayed in the right circumstances, and invisible otherwise.  C shows the wasp Neorileya: like many insects, it has a dark abdomen, and so the WIPs are visible when the wings are folded against the body.  D shows a sepsid fly displaying a bright pattern of color during a wing display (this may be a mating display; in many flies the males vibrate and waggle their wings when courting females).

Finally, E and F show the WIPs appearing in two fly species against different backgrounds, including the green of a leaf.

What are these patterns for?  The authors suggest that they may be of profound significance in insect communication. (See Ed Yong’s article for a precis).  As I suggested above, for example, they could be used in mating displays, with females preferring the patterns of males from their own species, since related species may differ in pattern.   The paper provides intriguing evidence that within a species of flies and wasps, the WIPs of males differ from those of females, making these patterns (like the colors or plumage of many birds) sexually dimorphic traits.

Whenever males and females differ in an ornamental trait without obvious adaptive significance, one hypothesis is that the trait evolved by sexual selection (see WEIT for an explanation).  This is particularly true if, in a group of related species, the males differ in pattern but the females don’t.  This invariance of a female trait coupled with strong variation in a male trait that could be related to mating is a strong signal of sexual selection.  And that’s the pattern we seem to see, at least in part of Fig. 3 from their paper:

In this figure, each column represents one of three related species of parasitoid wasp in the genus Achrysocharoides.  Two “replicate” males of the species are above the line, and the female is below the line.  So, for example, B and C are males of one species, and D is the female of that species.  F and G are males of a related species, H the female, and so on.

What you see is that while the WIPs of males vary among the species (but don’t vary much within a species), the females of different species (D, H, and L) are all pretty much the same.  To me, this implies sexual selection: the males diverge by sexual selection driven (perhaps) by female choice, while there’s no selection on the females to diverge.

This is precisely the pattern seen in insect genitalia: very often closely related species show big differences in male genital shape, while the females of those species are all the same.  As William Eberhart has pointed out often, this suggests sexual selection involving female choice. (See his wonderful and underappreciated book, Sexual Selection and Animal Genitalia.)

WIPs have other evolutionary significance.  They might be used not to recognize individuals of the same species for mating, but to discriminate against individuals of different species that have different WIPs.  That is, they could constitute a reproductive isolating barrier that contributes to speciation.

The authors (probably Dan Janzen, who studies fig wasps) point out another possible “use” of WIPs.  Female fig wasps pollinate the figs (and lay their eggs) by squeezing through the tiny hole at the bottom of the fig (the ostiole).  To get through, they break off their wings and leave them on the outside of the fig, at the same time secreting a fluid from their abdomen that glues the wings in a “protruding and visible position” on the fig.  This might serve as a signal to other females to leave that fig alone, since it’s already occupied with the reproductive output of a previous female.

Here’s a female about to enter the ostiole of a fig.  After she enters, and lays her eggs, she dies inside. Every time you eat a fig, you’re eating at least one dead wasp.

The WIPs are useful taxonomically, too: the authors have used them to diagnose “sibling species” of wasps: species that are almost morphologically identical.  It turns out that despite their similarity, the species have diagnostic wing patterns.  Once you separate species based on those patterns, you can begin to see subtler differences in morphology that you might have missed.

This paper doesn’t cure cancer or anything, but it’s a really nice presentation of a ubiquitous pattern in nature that may have important evolutionary explanations.  And how stupid of all of us drosophilists (and other entomologists) not to have paid attention to it!

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Shevtsova, E., C. Hansson, D. H. Janzen, and J. Kjaerandsen. 2010. Stable structural color patterns displayed on transparent insect wings.  Proc. Nat. Acad. Sci. USA Early edition. doi/10.1073/pnas.1017393108.

Dipteran of the week

December 15, 2010 • 8:24 am

In our continuing series on weird flies, here’s a corker.  At about 1.5 inches long, it’s Africa’s largest fly:  Gyrostigma rhinocerontis, the rhinoceros bot fly.  It’s highly specialized, laying its eggs only on the head of the white and black rhino. The larvae then burrow into the flesh, where they develop in the rhino’s stomach.  When they’re ready for the next life stage, they exit through the rhino’s anus and pupate in the soil.

According to London’s Natural History Museum site,

Despite their large size, adults live only for a few days (3–5 days in captivity) because they have very reduced non-functional mouthparts and do not feed. [You can see the lack of mouthparts in the photo below.]

Within that short life span, female flies have to locate a male, mate and then find a new rhinoceros host for their eggs.

As rhinoceros numbers decline so do the numbers of these flies, and should rhinos become extinct, the flies would probably disappear too, providing an example of co-extinction.

Because the adults are so ephemeral, it’s rare to find one of them, and they’re prized by collectors.  To me they look like wasp mimics, which may afford them some protection:

Photograph Copyright by the Natural History Museum, London

 

Here’s the parasitic larva:

There’s a lot more on this beast in David Barraclough’s article from Natural History in 2006. e.g.:

In 1847 the French naturalist and explorer Adulphe Delegorgue described large numbers of bots in the stomach of a black rhinoceros from northeastern South Africa. He published this vivid description of them in his Voyage dans l’Afrique australe (“Travels in Southern Africa”):

The Rhinoceros Africanus bicornis could well claim the title of foster father of bots. The imagination boggles at the quantity contained in his stomach; they could be shoveled out in bushels…. I am much inclined to think that the viciousness and ill-humor which characterize the Rhinoceros Africanus bicornis are due simply to the presence of thousands of these parasites and can be compared with the irritability of a man infested with tapeworm. However, in spite of their numbers, which sometimes seem to exceed all natural limits, bots do not, as far as I know, cause the death of indigenous animals.

h/t: Matthew Cobb

A whistling caterpillar

December 14, 2010 • 8:35 am

Well, it may not be smoking, but it’s whistling.  According to MSNBC science news, researchers have found that the walnut sphinx moth caterpillar (Amorpha juglandis), can make whistling noises by forcing air through its external breathing holes (“spiracles”).  The paper, by Bura et al., is in the December issue of the Journal of Experimental Biology.

To confirm their idea, researcher Veronica Bura at Carleton University gently applied latex over all eight pairs of the caterpillars’ abdominal spiracles and then uncovered each pair systematically while pinching the larva. The whistles definitely came from the eighth pair, generating trains of whistles lasting up to four seconds each, and spanning frequencies that ranged from those audible to birds and humans up to ultrasound.

Why do they do this?  The video below gives one clue, showing that they whistle when they’re attacked (they also thrash about):

Video of whistling caterpillar.

To test this theory, Bura et al. exposed caterpillars to a bird predator, a trio of yellow warblers (Dendroica petechia).  When attacked, the caterpillars produced sounds (and thrashed), and the birds were scared off, even when they attacked for a second time.  None of the test caterpillars were injured.  The authors conclude that the sound is a key part of the caterpillar’s anti-predator defenses.  I find this intriguing but unproven, since the authors apparently didn’t do the required controls in which birds attacked caterpillars whose whistle had been silenced by occluding their eighth spiracles (as another control, they could just occlude the seventh pair of spiracles, which also help breathe but can’t whistle).  The sound, then, may play some role or no role (it could, after all, be the thrashing that scares off birds).   More work is needed here!

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Bura, V. L., V. G. Rohwer, P. R. Martin, and J. E. Yack. 2010.  Whistling in caterpillars (Amorpha juglandis, Bombycoidea): sound-producing mechanism and function.  J. Exp. Biol. 214:30-37.

Behe’s new paper

December 12, 2010 • 12:37 pm

The latest issue of The Quarterly Review of Biology has a paper by intelligent-design advocate Michael Behe, “Experimental evolution, loss-of-function mutations, and “the first rule of adaptive evolution.” It’s a review of several decades’ worth of experimental evolution in microbes (viruses and bacteria), with an eye toward revealing exactly what kinds of mutations have occurred in these studies.  He concludes that microbial evolution in the lab has been based largely on mutations that either 1) degrade or destroy functional elements like genes and promoter sequences, or 2) “modify” the function of pre-existing genetic elements so they do something slightly but not qualitatively different. What Behe does not observe is the evolution of “new functional elements” (see below): new genes, new coding sequences, new promoter regions, and the like.

When Behe produces a paper like this, it’s hard to resist imputing a motivation for the work.  After all, the man has a long history of promoting ID, and has written two books (which I’ve reviewed—negatively—here and here [click “God in the details” at the bottom of the page]) purporting to show that a celestial hand was necessary in evolution.  I’ve also caught Behe deliberately misquoting me in the service of his creationist views.  I believe—and think that time will prove me right —that his intention is to show that evolution cannot provide new structures or new “information” (e.g., genes), but can only either tinker with ones already present or degrade them.   Thus, to explain the evolution of truly new genetic information, one must invoke the intervention of an intelligent designer.

I think that while Behe’s summary of the results of these short-term lab experiments is generally accurate, one would be completely off the mark to extend his conclusions to evolution in general—that is, evolution as it has occurred in nature, be it in microbes or eukaryotes.

To make a long paper short, let me give the definitions Behe uses to reach his conclusion.

These are the things Behe doesn’t see arising in lab experiments:

Functional Coded elemenTs (FCTs): “An FCT is a discrete but not necessarily contiguous region of a gene that, by means of its nucleotide sequence, influences the production, processing, or biological activity of a particular nucleic acid or protein, or its specific binding to another molecule. Examples of FCTs are: promoters; enhancers; insulators; Shine-Dalgarno sequences; tRNA genes; miRNA genes; protein coding sequences; organellar targeting- or localization-signals; intron/extron splice sites; codons specifying the binding site of a protein for another molecule (such as its substrate, another protein, or a small allosteric regulator); codons specifying a processing site of a protein (such as a cleavage, myristoylation, or phosphorylation site); polyadenylation signals; and transcription and translation termination signals.”

In other other words, FCTs are new genes or new parts of genes, including those genetic changes that produce proteins with qualitatively new functions, not just a change in protein amount (e.g., changes in splicing or phosphorylation sites).

According to Behe, there are three types of adaptive genetic mutations that can be seen as differing in their effects on FCTs:

1.  “Loss-of-FCT” mutations. Those are the changes that have adaptive effects by destroying or degrading an FCT.  These include frame-shift mutations that render a protein inactive, or mutations that destroy a gene’s ability to bind to a transcription factor.

2.  “Gain-of-FCT” mutations.  These are the ones that Behe doesn’t see.  He defines them as mutations “that produce a specific, new, functional coded element while adapting an organism to its environment. The construction by mutation of a new promoter, intron/exon splice site, or protein processing site are gain-of-FCT mutations. Also included in this category is the divergence by mutation of the activity of a previously duplicated coded element.” In other words, mutations in this category produce new genes, parts of genes, or confer drastic new capabilities on genes by adding new splicing sites.  Also note that because almost no bacteria or viruses have introns in their cellular genes, it’s impossible to even see one class of this mutation in lab experiments on these groups.

3.  “Modification-of-function” mutations. These include every adaptive mutation that doesn’t fall in the above two categories, including point mutations that affect protein structure or quantitatively affect protein quantity, gene duplications that occur without sequence divergence,  rearrangements of gene order, etc.  He calls these “modification of function” rather than “modification of FCT” because the functional change doesn’t have to occur by changing an FCT itself.

Now these categories are not cut and dried.  For example, the “sickle-cell” mutation that, when present in one copy, protects carriers against malaria, is a point mutation in the beta hemoglobin molecule, changing a glutamic acid residue to a valine. You’d think that this would fall under class 3 (“point mutations”), but Behe considers it an adaptive gain of an FCT because the mutation causes the mutant hemoglobins to stick to each other in blood cells, somehow inhibiting the growth of the malaria parasite.  And because the point mutation is thereby said to specify a “new protein binding site”, Behe puts it into class 2 (gain of FCT).  Unfortunately, a lot of the single-gene mutations that Behe reviews from the experimental microbial-evolution literature work in unknown ways, so he could be missing similar cases that really fall into class 2.

Anyway, Behe reviews the last four decades of work on experimental evolution in bacteria and viruses (phage), and finds that nearly all the adaptive mutations in these studies fall into classes 1 and 3.  We see very few “gain of FCT” mutations.  Although this is not my field, the review seems pretty thorough to me, and the conclusions, as far as they apply to lab studies of adaptation in viruses and bacteria, seem sound.  From this Behe formulates what he calls “The First Rule of Adaptive Evolution:

Break or blunt any functional coded element whose loss would yield a net fitness gain.

What this means is that if adaptation can be gained by losing gene or enzyme activity, it’s more likely to occur by a loss-of-FCT mutation than the appearance of a new FCT itself with altered and reduced function.  That’s not really a “law” but a generalization from these lab experiments.

My overall conclusion: Behe has provided a useful survey of mutations that cause adaptation in short-term lab experiments on microbes (note that at least one of these—Rich Lenski’s study— extends over several decades).  But his conclusions may be misleading when you extend them to bacterial or viral evolution in nature, and are certainly misleading if you extend them to eukaryotes (organisms with complex cells), for several reasons:

1.  In virtually none of the experiments summarized by Behe was there the possibility of adapting the way that many bacteria and viruses actually adapt in nature: by the uptake of DNA from other microbes.  Lenski’s studies of E. coli, for instance, and Bull’s work on phage evolution, deliberately preclude the presence of other species that could serve as vectors of DNA, and thus of new FCTs.   This is not an idle objection, since we know that adaptation in natural populations of microbes often arises by incorporating new FCTs from other species.  Pathogenicity and antibiotic resistance in bacteria, for example, arise in this way.  Howard Ochman at Yale has done many studies on the acquisition of new bacterial functions by uptake of DNA from other species (and the source of the new DNA is often mysterious).

2.  In relatively short-term lab experiments there has simply not been enough time to observe the accumulation of complex FCTs, which take time to build or acquire from a rare horizontal transmission event.  Finding adaptation via point mutations or loss of function is much more likely.  Behe admits this much, but downplays it by saying this:

After all, one certainly would not expect new genes with complex new properties to arise on such short time-scales. Although it is true that new complex gain-of-FCT mutations are not expected to occur on short time-scales, the importance of experimental studies to our understanding of adaptation lies elsewhere. Leaving aside gain-of-FCT for the moment, the work reviewed here shows that organisms do indeed adapt quickly in the laboratory—by loss-of-FCT and modification-of-function mutations. If such adaptive mutations also arrive first in the wild, as they of course would be expected to, then those will also be the kinds of mutations that are first available to selection in nature. This is a significant addition to our understanding of adaptation.

and this:

A third objection could be that the time and population scales of even the most ambitious laboratory evolution experiments are dwarfed when compared to those of nature. It is certainly true that, over the long course of history, many critical gain-of-FCT events occurred. However, that does not lessen our understanding, based upon work by many laboratories over the course of decades, of how evolution works in the short term, or of how the incessant background of loss-of-FCT mutations may influence adaptation.

What he’s saying is this:  “Yes, gain of FCTs could, and likely is, more important in nature than seen in these short-term experiments.  But my conclusions are limited to these types of short-term lab studies.”  Well, good, but then let us not hear Behe’s ID colleagues tout these results as giving strong conclusions about microbial or eukyaryotic evolution in nature, particularly because the lab studies deliberately exclude sources of gain-of-FCT mutations that we know are important in nature.

3.  Finally, Behe does not mention—and I think he should have—the extensive and very strong evidence for adaptation via gain-of-FCT mutations in eukaryotes.  While that group may occasionally acquire genes or genetic elements by horizontal transfer, we know that they acquire new genes by the mechanism of gene duplication and divergence:  new genes arise by duplication of old ones, and then the functions of these once-identical genes diverge as they acquire new mutations.   Or, new genes can arise by unequal crossing-over between different genes, so that new genes arise by mixing bits of old ones.  Behe would count both of these as type 2 mutations (“gain of FCT”).  Think of all the genes that have arisen in eukaryotes in this way and gained novel function:  classic examples include genes of the immune system, Hox gene families, olfactory genes, and the globin genes.  And in many cases the origin of new genes via duplication or swapping of bits is untraceable because the genes originated so long ago and have diverged so greatly in sequence that their origin is obscure.

Vertebrates are thought to be the product of two whole-genome duplication events, giving rise to many genes with novel functions.  This has probably happened in yeast at least once, and many plants are the results of ancient “polyploidy” events in which entire genomes were duplicated at least once.  More than 40% of the genes in the human genome arose via gene duplications; this rises to more than 75% if we count those ancient rounds of whole-genome duplication. And over a third of the genes in the invertebrate  Drosophila genome arose via duplication, with most of these having new functions. There are many, many papers describing and discussing the importance of duplicated genes (and regulatory elements) as a source of evolutionary novelty; see, for example, Long et al. (2003), Wray et al. (2003), and Kaessmann et al. (2009).

While Behe’s study is useful in summarizing how adaptive evolution has operated over the short term in bacteria and viruses in the lab, it’s far less useful in summarizing how evolution has happened over the longer term in bacteria or viruses in nature—or in eukaryotes in nature.  In this sense it says nothing about whether new genes and gene functions have been important in the evolution of life.  Granted, Behe doesn’t make such a sweeping statement—his paper wouldn’t have been published if he had—but there’s no doubt that his intelligent-design acolytes will use the paper in this way.

Finally, this paper gives ID advocates no reason to crow that a peer-reviewed paper supporting intelligent design has finally appeared in the scientific literature.  The paper says absolutely nothing—zilch—that supports any contention of ID “theory.”

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Behe, M. 2010. Experimental evolution, loss-of-function mutations, and “the first rule of adaptive evolution. Quart. Rev. Biol. 85:419-445.Kaessmann, H., N. Vinckenbosch, and M. Y. Long. 2009. RNA-based gene duplication: mechanistic and evolutionary insights. Nature Reviews Genetics 10:19-31.

Kaessmann, H., N. Vinckenbosch, and M. Y. Long. 2009. RNA-based gene duplication: mechanistic and evolutionary insights. Nature Reviews Genetics 10:19-31.

Long, M., E. Betran, K. Thornton, and W. Wang. 2003. The origin of new genes: Glimpses from the young and old. Nature Reviews Genetics 4:865-875.

Wray, G. A., M. W. Hahn, E. Abouheif, J. P. Balhoff, M. Pizer, M. V. Rockman, and L. A. Romano. 2003. The evolution of transcriptional regulation in eukaryotes. Molecular Biology and Evolution 20:1377-1419.

A remarkable “flying” snake

December 5, 2010 • 12:30 pm

An article in this week’s New York Times Science Observer column highlights the paradise tree snake (Chrysopelea paradisi) of Asia, long known to escape predators by hurling itself from a tree and sailing through the canopy to alight on a new tree.  Studies suggest it can travel as far as 300 feet in this way.  A new study published in the oddly-titled and hard-to-find journal Bioinspiration and Biomimetics analyzes the mechanics of this “flight”:

. . . a study in which scientists threw the snakes from a 50-foot tower and recorded their descent on video suggests that the snakes are active fliers, manipulating their bodies to aerodynamic effect.

“It essentially looks like they are slithering in the air, like a whip moving left and right,” said Jake Socha, the study’s lead author and a biomechanist at Virginia Tech. “The body itself moves up and down as well.”

Dr. Socha and his colleagues found that the paradise tree snake tilts its body about 25 to 30 degrees relative to the airflow to stay as aerodynamic as possible. The farthest a snake was able to travel from the tower was about 79 feet.

This is far better seen than described; here’s a nice video from PBS:

And Wikipedia says a bit more:

Upon reaching the end of a tree’s branch, the snake continues moving until its tail dangles from the branch’s end. It then makes a J-shape bend,[7] leans forward to select the level of inclination it wishes to travel to control its flight path, as well as selecting a desired landing area. Once it decides on a destination, it propels itself by thrusting its body up and away from the tree, sucking in its stomach, flaring out its ribs to turn its body in a “pseudo concave wing”[8] all the while making a continual serpentine motion of lateral undulation[9] parallel to the ground[10] to stabilise its direction in midair in order to land safely.[11]

The combination of sucking in its stomach and making a motion of lateral undulation in the air makes it possible for the snake to glide in the air, where it also manages to save energy compared to travel on the ground and dodge terrestrial bounded predators.[7] The concave wing that a snake creates in sucking its stomach, flattens its body to up to twice its width from back of the head to the anal vent, which is close to the end of the snake’s tail, causes the cross section of the snake’s body to resemble the cross section of a frisbee or flying disc.[10] When a flying disc spins in the air, the designed cross sectional concavity causes increased air pressure under the centre of the disc, causing lift for the disc to fly.[12] A snake continuously moves in lateral undulation to create the same effect of increased air pressure underneath its arched body to glide.[10] Flying snakes are able to glide better than flying squirrels and other gliding animals, despite the lack of limbs, wings, or any other wing-like projections, gliding through the forest and jungle it inhabits with the distance being as great as 100 m.[10][13] Their destination is mostly predicted by ballistics; however, they can exercise some in-flight attitude control by “slithering” in the air.[1]

The beakiest bird

November 27, 2010 • 10:24 am

I photographed a specimen of the South American sword-billed hummingbird, Ensifera ensifera, in the bird collection at the Universidad de Los Andes, using a pen for scale. The bird is found throughout the northern Andes, and is the only species in the genus Ensifera.

Most important, it’s the only living bird whose beak (3.5 to 4 inches long) is longer than the rest of its body (ca. 2-3 inches)!

Here’s a skeleton of the bird, showing how disproportionately long the bill is.  Wikipedia reports (and there’s verification in a video below) “since the Sword-billed Hummingbird’s beak is very long, it grooms itself with its feet”.

You’ve certainly guessed that the long bill is an adaptation for feeding.  These birds feed largely on passionflowers (Passiflora), which have long corolla tubes that contain the nectar.  The birds approach these pendant flowers from below, deftly inserting their beak like so [note: as several alert commenters note below, the flower shown is not Passiflora but Brugmansia]:

The paper by Lindberg and Olesen (citation below) strongly suggests that these birds are also important pollinators of Passiflora, since they carry pollen on their beaks from flower to flower.  But the authors also warn that their specialization on one genus of flower, and the increasing habitat fragmentation in the Andes, may put these birds on the verge of extinction.

There are some lovely videos and photos of this bird at The Internet IBC bird collection, including a female supping from a feeder and another female using her feet to groom herself.  Arkive has another grooming video and a marvelous video of feeding from a Passiflora.

Hummingbirds are truly the jewels of the avian world, and display some of the most remarkable adaptations seen in animals.  I am always amazed at seeing how these birds hover, absolutely rock still, while they feed.  No helicopter is as agile.  And how some of these nectar-guzzling species make a 600-mile nonstop journey across the Gulf of Mexico—20 hours of straight flight—is beyond belief.

Here are a few words, and some dramatic videos, about how the PBS film “Hummingbirds” was made (I haven’t seen it).

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Lindberg, A. B. and J. M. Olesen. 2001.  The fragility of extreme specialization: Passiflora mixta and its pollinating hummingbird Ensifera ensifera. Journal of Tropical Ecology 17: 323-329.

The surreal treehoppers

November 26, 2010 • 8:00 am

Last week’s Nature highlighted the sculptures of Alfred Keller (1902-1955), and the example, a model of the Brazilian treehopper Bocydium globulare, struck me as one of the weirdest animals I’ve ever seen:

Martin Kemp describes Keller’s work:

Keller was trained as a kunstschmied, an ‘art blacksmith’. From 1930 until his early death he was employed by the Berlin Museum für Naturkunde (Museum of Natural History), painstakingly labouring over his recreations of insects and their larvae. Each took a year to complete. Keller worked first in plasticine, from which he cast a model in plaster. This plaster reference model he then recast in papier maché. Some details he added, cast in wax, with wings and bristles in celluloid and galalith (an early plastic material used in jewellery). Finally he coloured the surfaces, sometimes with additional gilding. The levels of patience and manual control Keller exercised were incredible. His fly, for example, boasts 2,653 bristles.

. . . Keller was a sculptor of monumental one-off portraits. Each model is a masterpiece, with no effort spared. It is difficult to see how such a skilled artisan could survive in today’s museums, with their emphasis on cost analysis. Keller’s exacting models may be things of the past, yet they are far from obsolete. Like the great habitat dioramas, they exercise a magnetic attraction.

The first thing a biologist does on seeing a model like this is think, “This can’t be real,” and resorts to some Googling. Sure enough, it’s a real insect.  Here are two photos by Patrick Landmann (check out his other terrific nature photos):

The second thing one asks is, “What the bloody hell is all that ornamentation on the thorax?” (Note that the “balls” on the antenna-like structure aren’t eyes, but simply spheres of chitin.)  A first guess is that it’s a sexually-selected trait, but those are often limited to males, and these creatures (and the ones below) show the ornaments in both sexes.  Kemp hypothesizes—and this seems quite reasonable—that “the hollow globes, like the remarkable excrescences exhibited by other treehoppers, probably deter predators.”  It would be hard to grab, much less chow down on, a beast with all those spines and excrescences.

Note, though, that the ornament sports many bristles.  If these are sensory bristles, and not just deterrents to predation or irritating spines, then the ornament may have an unknown tactile function.

Membracids, related to cicadas, are in the class Insecta (insects, of course), the order Hemiptera (“true bugs”) and the family Membracidae.  Like aphids, which are also “true bugs,” adult and immature treehoppers feed on plant sap.

For a wonderful panoply of membracid photos, download this pdf file. Here are some of the images, showing that, as Kipling said, “The wildest dreams of Kew are the facts of Khatmandu.” If Dali invented insects, they’d look like these (all photos by Patrick Landmann):

The color and shape of this last one makes me suspect that it’s mimicking a wasp:

h/t: Matthew Cobb

Wasps: artists or robots?

November 15, 2010 • 5:13 am

by Matthew Cobb

[Continuing my lazy practice of re-posting material from elsewhere in the blogosphere and bringing it to the attention of WEIT readers, here’s one I posted last week over at Pestival (the insect arts festival – yes, honest!  Go look!)]

In case you weren’t listening to BBC Radio 4 at 06:15 am the other Sunday morning, I thought I would present to you the case of one of nature’s artists, the potter wasp. This small solitary wasp was the subject of the excellent Radio 4 programme The Living World. Anyone, anywhere in the world, can listen to it again here:

The female wasp makes a little clay pot about 1cm across, with a small hole in the end. She lays an egg in the pot, and then crams it full of living, semi-paralysed caterpillars which her offspring can eat. You can see quite how small the pot is in this photo from the BBC website, by Andrew Dawes. The “pot” is the tiny white thing underneath the middle finger!

[EDIT: The following (clearer) picture was taken by WEIT reader TrineBM (see comment 2 below)]:

According to Wikipedia (so it must be true, no?) the great entomologist Karl von Frisch, who was the first to study the honey bee’s waggle dance, claimed that the shape of the potter wasps’s pot inspired native American pottery designs. Quite how one could know that was true (or not) is hard to say – and what about other pots from round the world that look pretty much the same?

But is the wasp really an artist? Does it know what the pot should look like? John Walters, who’s been studying the potter wasps on this Devon heath for the last four years, says that he thinks each wasp has a different style – some pots are symmetrical, others have distinct twists. That doesn’t mean to say they know what they’re doing. Indeed, it seems certain they do not.

One of my favourite studies of animal behaviour was carried out on a potter wasp in 1978 by Andrew P. Smith, then of the University of Sydney. On the other hand, I seem to be one of very few people who rate this work – it has only been cited 13 times in the last 32 years. I feel your pain Dr Smith!

Andrew’s wasp – Paralastor – makes a rather more elaborate nest than the Devon potter wasp, a kind of odd umbrella shape, made up of a mud column and a bell-shaped entrance, leading to an underground chamber where the larva can munch its way through its living lunch, as seen here:

This picture shows the female wasp in action:

So how does she know what to build? Construction takes place in stages:

But does she have an image of what the final nest should look like? Or does she simply know that she’s carried out a series of behaviours and simply do them in sequence? The answer to both these questions is “No”.

Through a series of experiments involving changing the ground level, or altering the angle of the column, or making holes at various points, Smith was able to show that the wasp in fact proceeds by a series of steps, each of which is induced by a particular stimulus. If she sees a hole, she makes a column – even if this ends up with a bizarre double-umbrella nest:

The conclusion of the paper – apart from a lot of very tired and confused wasps – is this rather neat flow diagram, showing how the wasp decides what to do next:

So the wasp is not an artist, it’s more like a simple robot, carrying out a task when the appropriate conditions are provided. Less romantic, but still amazing!

Andrew P. Smith (1978) An investigation of the mechanisms underlying nest construction in the mud wasp Paralastor sp. (Hymenoptera: Eumenidae). Animal Behaviour 26:232-240.

The evolution of cat coat patterns

October 27, 2010 • 8:35 am

Why are some species of kittehs plain, while others have spots, stripes, or more elaborate patterns? A provisional answer comes from a new paper by William Allen et al., “Why the leopard got his spots: relating pattern development to ecology in felids”, in the Proceedings of the Royal Society.  The paper’s title, of course, comes from Rudyard Kipling’s Just So Stories.  And the short answer is this: the coats of wild cats help camouflage them, and what pattern evolves depends on where the species lives.

The simple answer comes from a rather elaborate analysis.  The authors set up the paper with what I think is a good specimen of clear scientific writing.  It’s not Joyce, of course, but these guys know how to write. I love the alliteration of “flanks of felids” and the breeziness of “pounce or quick rush.”

The patterns displayed on the flanks of felids are intriguing in their variety. Previous studies of the adaptive function of cat coat patterns have indicated that they are likely to be for camouflage rather than communication or physiological reasons [1,2]. The primary hunting strategy of felids is to stalk prey until they are close enough to capture them with a pounce or quick rush [3,4]. As hunts are more successful when an attack is initiated from shorter distances [5,6], cats benefit from remaining undetected for as long as possible and camouflage helps achieve this. Many smaller cats are also likely to be camouflaged for protection from predation [7].

The authors first note that others before them have suggested—and supported with some data—the idea that spotted or stripey cats live in forested habitats, and plain cats in open habitats.  But they quantify this “complexity” by doing a developmental analysis of coat patterns on pictures taken from the internet.  I won’t go into the details, but they match the photographs with patterns generated from a mathematical model in which pattern results from the interaction of two diffusible chemicals along gradients of the body.  Given a model that matches an existing pattern (they used 35 species of felids), they could then encompass “pattern” in the mathematical constants involved in generating it.  They could then correlate these constants with various aspect of cat ecology: where they live, preferred times of activity, how big they are, what they eat, and how social they are.

Here’s an example of a cat that came out “plain” in their analysis: the caracal (Caracal caracal), from Africa and the Middle East:

Nine of the 35 species were considered “plain.” Here’s a cat considered “patterned and complex”: the gorgeous clouded leopard (Neofelis nebulosa), from southeast Asia:

Sixteen species were considered patterned, with four of these, including the clouded leopard, as “always complex.”  The other ten were considered “variable”,” since there was polymorphism: individuals within a species can look quite different.

The results?

  • Pattern itself, whether complex or not, was significantly associated with habitat, with more patterned cats in more “closed” habitats (forest, jungle, etc.).  Plain cats are found in open habitats like grasslands, deserts, and mountains.
  • More irregular patterns, like the cloud leopard, are significantly associated with tropical forests and other “closed” environments.
  • “The more time cats spent in trees, the more likely they were to be patterned.”
  • Pattern polymorphism, as in the melanism of “black panthers,” was significantly associated with living in temperate forests that vary seasonally and also with habitat generalism. This supports the idea that “disruptive selection,” that is, selection for different patterns in different places, maintains the intra-specific variation in coat color.
  • There were a few “outliers,” or exceptions—cats that had patterns not fitting into the habitat correlations given above.  One is the very rare bay cat (Catopuma badia; I’ve posted on it before), which is plain though it lives in tropical rainforest:

And another outlier is the black footed cat of Africa (Felis nigripes), which is patterned though it lives in open habitat (savannah, grassland, and semi-desert):

The authors note that the tiger is the only wild cat with vertical stripes, and the common notion that this camouflages them in grassland is unfounded: tigers don’t live in grasslands.

The conclusion, then, is that the patterns of cat coats reflect, in large degree, selection for camouflage in their natural habitats. This camouflage almost certainly evolved to hide them from prey, and, in smaller cats, predators as well.

I love the inclusion of a Kipling quote in their conclusion (reference “45” is to the Just So Stories):

These findings support the hypothesis that felid flank patterns function as background matching camouflage. Evolution has generally paired plain cats with relatively uniformly coloured, textured and illuminated environments, and patterned cats with environments ‘full of trees and bushes and stripy, speckly, patchy-blatchy shadows’ [45].

Now the sample size—35 species—is not large, and some of the associations were barely significant from a statistical standpoint.  This could reflect the low power of tests in small samples. Nevertheless, the study offers a good working hypothesis for the evolution of pattern not just in cats, but other species that “need” to be cryptic.  What remains to understand are those outliers like the bay cat, and also the existence of developmental change of pattern, in which some species are patterned when young and lose the patterns when they get older.  Lions, which are spotted as cubs, are a good example of this:

This change might not be adaptive per se, but simply be an atavism: a holdover from an ancestral spotted pattern that still persists in the young.

_________

Allen, W. L., I. C. Cuthill, N. E. Scott-Samuel and R. Baddeley. 2010.  Why the leopard got his spots: relating pattern development to ecology in felids.  Proc. Roy. Soc. B online: doi: 10.1098/rspb.2010.1734

Shark jaws

October 23, 2010 • 5:50 am

Here are two more photos from my immensely edifying visit to Jim Krupa’s lab at The University of Kentucky.  They show the extreme diversity of morphology that evolution can produce in a single group.

The first shows the jaw of what I remember as a tiger shark, Galeocerdo cuvier.  National Geographic notes that “They have sharp, highly serrated teeth and powerful jaws that allow them to crack the shells of sea turtles and clams. The stomach contents of captured tiger sharks have included stingrays, sea snakes, seals, birds, squids, and even license plates and old tires.”

The rows of teeth are lined up, waiting in the wings, to be replaced after one on duty is lost. The teeth aren’t embedded in the jaw, but merely in the gum tissue.  Wikipedia has a good article on them.

Sharks are in the class Chondrichthyes:  they have cartilage rather than bone.  The subclass Elasmobranchii includes sharks, skates and rays.  And the tiger shark is in the largest order of elasmobranchs, the Carcharhiniformes.

And here’s one of the weirdest elasmobranchs—the jaw of the Port Jackson shark (Heterodontus portusjacksoni), endemic to Australian waters.  It’s in the order Heterodontiformes (“bull sharks”), distinguished, among other things, by having the mouth completely in front of the eyes. The “Heterodontus” part of the genus name means “different teeth,” and that’s indeed what you see, spectacularly, in the jaw.  Having differentiated teeth in the jaw is very rare in sharks:

The small teeth in front are for grabbing and piercing, the ones at the rear for grinding up stuff, especially molluscs.  The Florida Museum of Natural History site notes:

This species feeds primarily on echinoderms, crustaceans, molluscs, and some small fish. Sea urchins and large gastropod molluscs are noted in almost every study on the diets of Port Jackson sharks. Stomach contents are typically ground up too small for full identification, thus leading researchers to believe Port Jackson sharks grind their food thoroughly before swallowing. This is also supported with juvenile diets, since it has been noted that juveniles eat more soft-bodied animals, and contain less molar-like teeth.

Here’s what the jaws look like in situ:

For $750 you can actually buy a Port Jackson shark for your aquarium, but I’m not sure why anyone would do that, as they grow over five feet long.

You can see the variety of sharks’ teeth here, and if you’re into buying recent or fossil teeth, here’s a place to start,