Orchids mimic alarm pheromones of bees to attract wasps

August 26, 2009 • 7:00 am

At the end of The Origin, Darwin constructed a justly famous metaphor of nature as a “tangled bank.” In the case of mimicry, the metaphor might be Walter Scott’s “tangled web,”* since the network of interspecific interaction and deception can become quite intricate. Some of the most famous examples of mimicry are those of orchids that mimic bees and wasps.

Natural selection has molded the flowers of these orchids (many in the genus Ophyrys) into mimics of the insects that pollinate them. Horny male insects, thinking that the petals are a female, land on them and engage in fruitless attempts to copulate (“pseudocopulation”). During the barren act, the insects’ heads or bodies contact the orchids’ pollen sacs, which break off and attach to the insect. The frustrated insect flies off, but soon tries to copulate with another orchid, which puts the hitchhiking pollen in contact with the new orchid’s stigma. In such a way the bees/wasps serve as “flying penises,” helping the orchids have sex. Here are some specimens:

Ophrys insictiferaFig. 1. Ophyrys insectifera (fly orchid), which deceives male digger wasps.

Mirror orchid

Fig. 2. Mirror orchid (Ophrys speculum), which attracts scolid wasps.

By adopting the “female mimic” strategy, the orchid sacrifices half its potential pollinators (the female bees/wasps), but there’s no obvious way to attract a coy female insect.

Here’s a David Attenborough video showing pseudocopulation of a mirror orchid by a wasp. As Attenborough notes, the orchids also have olfactory mimicry of the wasp: they produce a chemical similar to the mating pheromone of the female wasp, further increasing the flower’s allure. (This form of mimicry was recognized only recently since it is far less obvious than the visual similarity).

In a new paper in Current Biology, Jennifer Brodman and her coauthors show that a Chinese orchid, Dendrobium sinese, has an even more intricate strategy for attracting wasp pollinators. Rather than mimicking the wasp’s mating pheromone, the flower produces a chemical that mimics the alarm pheromone of two species of honeybees that are likely to be the wasp’s prey. (I posted on such bee-eating wasps a few days ago.) The paper is short and easy to read; it should be accessible to the non-scientist. And the abstract says it all:

Approximately one-third of the world’s estimated 30,000 orchid species are deceptive and do not reward their pollinators with nectar or pollen. Most of these deceptive orchids imitate the scent of rewarding flowers or potential mates. In this study, we investigated the floral scent involved in pollinator attraction to the rewardless orchid Dendrobium sinense, a species endemic to the Chinese island Hainan that is pollinated by the hornet Vespa bicolor. Via chemical analyses and electrophysiological methods, we demonstrate that the flowers of D. sinense produce (Z)-11-eicosen-1-ol and that the pollinator can smell this compound. This is a major compound in the alarm pheromones of both Asian (Apis cerana) and European (Apis mellifera) honey bees and and is also exploited by the European beewolf (Philanthus triangulum) to locate its prey. This is the first time that (Z)-11-eicosen-1-ol has been identified as a floral volatile. In behavioral experiments, we demonstrate that the floral scent of D. sinense and synthetic (Z)-11-eicosen-1-ol are both attractive to hornets. Because hornets frequently capture honey bees to feed to their larvae, we suggest that the flowers of D. sinense mimic the alarm pheromone of honey bees in order to attract prey-hunting hornets for pollination.

This is the kind of new result that keeps us evolutionists juiced up: you can never predict what kind of bizarre adaptation will crop up in the next issue of a journal.

And if you look at the D. sinense flower, it doesn’t look like a bee or a wasp:


Figure 3. Dendrobium sinense Flower and Vespa bicolor Forager. D. sinense flower (A) and V. bicolor forager with pollinia stuck onto the thorax (B). (Figure from Brodman et al.)

Apparently the fragrance alone is enough to attract the wasp. And, as predicted from the scent mimicry, the wasp doesn’t try to copulate with the flower; instead, it “pounces” on it, exactly as a wasp pounces on an alarm-pheromone-emitting bee. But that pouncing is enough to stick the pollen sac onto the wasp. As the old saying goes, “Natural selection is cleverer than you are.”

h/t: Matthew Cobb


Brodman, J. R. Twele, W. Francke, L. Yi-Bo, S. Xi-quiang, and M. Ayasse. 2009. Orchid mimics bee alarm pheromone in order to attract hornets for pollination. Current Biol. 19:1368-1472.

*Oh! What a tangled web we weave
When first we practice to deceive!

Walter Scott, Mamion

The Asian giant hornet

August 22, 2009 • 10:37 am

In WEIT, I begin the chapter on natural selection with a particularly gruesome example of an adaptation: the predatory Asian giant hornet (Vespa mandarina), the world’s largest wasp and a viscious killing machine.

They are horribly fricking huge, with a two-inch body (tipped with a quarter-inch stinger that injects a potent venom) and a three-inch wingspan. Their stings kill several dozen Asians yearly. Here’s what they look like:


Fig. 1. Vespa mandarina on a brave Homo sapiens.

I won’t recount the whole story here, except to say that a small band of these wasps, incited by a single scout wasp, who finds a nest and marks it for doom by depositing a drop of pheromone that attracts its confreres, can decimate a colony of 30,000 Asian honeybees in a few hours, decapitating the hapless bees with their slashing jaws. They then raid the bee colony of honey and grubs, which they bring back to their own nests to deposit in the maws of their own voracious larval wasps. Here’s a video of a wasp raid:

But the local honeybees have evolved a marvelous counter-adaptation: they mob the first scout wasp that tries to mark the beehive with a pheromone, covering the wasp with a thick ball of bees that vibrate their abdomens, raising the temperature inside the ball so high that the hornet is cooked to death (the bees can survive that temperature). Here’s a video of the cooking process:

As one might expect, the introduced European honeybee, which hasn’t coevolved with the Asian wasp, has no counteradaptation.

A relative of the hornet, the not-quite-so-dangerous species Vespa veluntina (also a predator of honeybees) , has invaded Europe in the last few years, as reported this week in the Telegraph:

The bee-eating hornets, instantly recognisable by their yellow feet, are rapidly spreading round France and entomologists fear that they will eventually cross the Channel and arrive in Britain.

Hundreds of the insects attacked a mother on a stroll with her five-month-old baby in the Lot-et-Garonne department, southwestern France, at the weekend before turning on a neighbour who ran over to help. The baby was unharmed.

They then pursued two passers by and two Dutch tourists on bikes. The victims were treated in hospital for multiple stings, which are said to be as painful as a hot nail piercing the skin

. . .The Vespa velutina, which grow up to an inch in length, is thought to have arrived in France from the Far East in a consignment of Chinese pottery in late 2004.

So far the honeybees in Europe, like the European honeybees in Asia, have no defenses against the wasp. It will be interesting to see if, over time, they evolve a cooking behavior (or, in the case of some honeybees in Cyprus, a variant in which a mob of bees surrounds the wasp and suffocates it by preventing it from expanding and contracting its abdomen).

How the tapir got his spots III

August 13, 2009 • 12:49 pm

by Greg Mayer

The two great classes of phenomena that Darwin set out to explain were those of adaptation– the fit between an organism’s features (structure, behavior, etc.) and its conditions of existence; and unity of type — the similarities of basic structure among organisms in diverse conditions of existence (e.g., the one bone-two bones-many bones pattern of tetrapod forelimbs, whether they be burrowers, swimmers, climbers, runners, etc.). The unified explanation that Darwin provided for these phenomena was descent with modification: the similarities were due to inheritance from a common ancestor (i.e. descent), while adaptation arose from the process of modification (i.e. natural selection).

The methods of studying adaptation are thus crucial for biology.  How can we tell what (if anything) the spots of the baby tapir are adaptive for?

There are three basic ways of studying adaptation, in the sense of determining what a trait is an adaptation to. The first is engineering: does the feature conform to what we would expect if it is performing some adaptive function?  Study of hydrodynamics enables us to understand the shapes of the bodies, flippers, and fins in fish, dolphins, icthyosaurs, etc. as adaptations to movement within a fluid environment.  The dorsal fin of an ichthyosaur, for example, stabilizes the reptile in its forward movement through water, preventing unwanted roll (for recent discussions of ichthyosaur aquatic adaptations, see here, here, and here). For another example of the engineering approach, see Richard Dawkins’ delightful account of bat sonar in chap. 2 of The Blind Watchmaker.

Second, there is the method of correlation (also called the comparative method): does the feature evolve repeatedly in particular environmental circumstances? Thus even if we were wholly ignorant of hydrodynamics, the repeated evolution of dorsal fins in aquatic fish, reptiles, and mammals provides evidence that dorsal fins are adaptations to an aquatic existence.


Third, we can study the effects on survival and reproduction of variations in the trait of interest.  This can be done either by altering the features of the character experimentally (as in this neat experiment on sexual selection in widowbirds) or by studying naturally occurring variants (as was done with peppered moths by  H.B.D. Kettlewell).

The evidence for the adaptiveness of spotting/striping in mammals is primarily of the first sort (Hugh B. Cott, in his classic Adaptive Coloration in Animals, has a lot about optical principles, and what makes things hard to see), the second sort (pacas, bongos, deer, tapirs all have spots and/or stripes [and note that pacas are rodents, and that tapirs, which are perissodactyls, are not at all closely related to the artiodactyl deer and bongo, so it would be hard to argue it’s a retained ancestral feature]), and very little of the third sort– no one’s painted baby tapirs’ spots over to see what happens to them (at least as far as I know). I’ll touch on all three sorts as they relate to tapirs in later posts.

(For other examples of camouflage, see Matthew Cobb’s earlier post on the subject.)

How the tapir got his spots II

August 5, 2009 • 11:03 am

by Greg Mayer

I promised baby tapirs, so here are baby tapirs! (From Zooborns.)

Baby Malay tapir
Baby Malay Tapir (Tapirus indicus; from Zooborns)

Adult Malay tapirs, as you’ll recall, are particolored:

Malay Tapir with baby
Malay Tapir with baby (from Zooborns)

The three other species of tapir, all from the Americas, also have spotted/striped young. Here’s a lowland tapir, found throughout much of cis-Andean tropical South America; the others are very similar in appearance.

Brazilian tapir with Baby
Lowland Tapir (Tapirus terrestris) with baby (from Zooborns)

We can thus see that all baby tapirs look much alike, and quite different from adults.  Adults are either self-colored (the American species) or particolored (the Malay tapir). (It’s interesting that both young and adults have white edges to their ears.)  The question is, is this coloration of the juveniles an adaptation? Or is it an ancestral feature of no current utility, which makes a brief appearance in the young, but is then lost (like the coat of hair that human babies have in utero)?

How the tapir got his spots

August 4, 2009 • 7:11 am

by Greg Mayer

A while back Jerry posted a video of lion cubs at the Tulsa Zoo, and noted that they have spots, remarking

Many species of cats show this pattern in the cubs, even if the pattern disappears with growth.  It almost certainly reflects (as discussed in WEIT), an atavistic trait: the persistence in a descendant of traits that were adaptive only in an ancestor.  I suspect that the ancestor of lions had spots as adults, and that’s why they show up, briefly, in lion cubs.

I posted a comment to the effect that Hugh Cott, the great student of adaptive coloration, agreed with Jerry, although I wasn’t so sure:

Hugh Cott, in his classic “Adaptive Coloration in Animals” (Methuen, 1940) agreed with Jerry on this: “Among mammals and birds, first liveries acquired by the young– whether this happens before or after birth– often differ widely from the full dress of their parents. But it must not be assumed that such differences are necessarily adaptive. Lion cubs have spotted coats, and their tails are ringed…[Cott gives some more cat examples]… Since the kittens of all these animals…are born in sheltered dens or holes, carefully hidden or guarded by the mother, the spotted pattern can hardly be explained as protective.”(p. 21). I’m not so sure, though. Lions are not sheltered in dens or burrows, but rather are kept in thickets and kopjes, and may be on their own for a day at a time (George Schaller, “The Serengeti Lion” [Chicago, 1972], so the spots might be protective coloration for keeping the young hidden before they become formidable individually. (Protective coloration in the young is well known in mammals– whitetailed and mule deer, and tapirs, being good examples: their young bear dots and vermiculations that blend with sun- or moon-dappled forest floors.)

Since then, Jerry and I have conducted an off-blog discussion on this, and he has particularly challenged me with regard to tapirs.  While tapirs (and lions!) present many interesting aspects of natural history, the general question is one one of fundamental conceptual importance for evolutionary biology: how do you tell if a feature of an organism is an adaptation? So I’m going to pursue this question over a few posts.  To set the scene, let’s introduce tapirs. The best web source of info on them is the IUCN‘s Tapir Specialist Group.

Baird's Tapir at Franklin Park Zoo, Boston (from Wikipedia)
Baird's Tapir at Franklin Park Zoo, Boston (from Wikipedia)

Tapirs, along with horses and rhinoceroses, are odd-toed ungulates, members of the mammalian order Perissodactyla, which is the less species-rich of the two great extant orders of hoofed mammals. (Most hoofed mammals, such as deer, antelope, cattle, sheep, pigs, etc., are even-toed, members of the  Artiodactyla.) There are four species, all of which have short trunks.  Three are in the Neotropics (Tapirus bairdii, T. pinchaque, and T. terrestris), found from southern Mexico to northern Argentina. As adults, they are all more or less uniformly colored, brown to gray to black. The Malay tapir (T. indicus) of southeast Asia, however, is strikingly particolored.

Malay Tapir at Regents Park Zoo, London (from Wikipedia)

I’ll show some baby tapirs in the next post.

Why toucans have big bills

July 24, 2009 • 7:33 am

If you’re like me, you’ll have asked yourself many times, “Jerry, why do toucans have such ridiculously big bills?” (See Figs. 1 and 2.)  The first answer that might strike you is that the bill — like bodies, plumage ornaments, and other traits in birds — was driven to extreme size by sexual selection.  But that won’t wash because male and female toucans have identical-sized bills, and if the male’s bill is brightly colored, so is the female’s. (There are several dozen species of toucans in five genera, all Central or South American.)

The next most obvious hypothesis is diet: maybe toucans eat a type of food that requires large bills to handle.  But that doesn’t seem likely, either.  Toucans are frugivores (fruit eaters; Fig. 3), and there’s no obvious reason why they need such a big bill to handle fruit.  Indeed, there are many frugivorous birds, like parrots, and none of the others have such hypertrophied beaks.

A paper in today’s Science gives a clue: the bill is a radiator. You might suspect this because the bill is full of small blood vessels and is uninsulated. But can the birds control the flow of blood to the bill as needed?

The authors used thermal-imaging video cameras to record the temperature of the birds’ bills and bodies in rooms adjusted to different temperatures ranging from 10 degrees C to 34 degrees C (the species was the toco toucan, Ramphastos toco, which has the largest bill of all toucans).  What the authors found, and what you can see in the movies below, is that the toucan can adjust blood flow to the bill depending on ambient temperature.  When the room heats up, the surface of the bill heats up rapidly, allowing body heat to be dumped.  The reverse happens at cooler temperatures.

When birds are flying — a time when they produce 10 to 12 times more metabolic heat than when they are resting — the bills can heat up by as much as 6 degrees Centigrade.  And when the birds get ready to sleep, a time when their body temperature is reduced (this saves metabolic energy), the surface of the bill transiently heats up, allowing them to dump heat (see movie #1 below).  There are also, as you can see in movie #2, transient changes in bill temperature during sleep, presumably to regulate body temperature (like many birds, the toucan tucks its bill under its feathers while asleep, presumably also to buffer heat loss).

Movie 1.  Body heat moves to the bill right before the bird goes to sleep (note bill glowing bright orange, while body stays darker; temperature scale to right). “Heat dumping to the bill during entry into sleep. Thermal imaging video demonstrating transient movement of body heat to the bill during initiation of sleep in a toco toucan. Time-lapsed data obtained at 10-s intervals. Total frames = 724, total length = 2 hours.”

Movie 2. “Sleep-state transitions witnessed as changes in bill temperature. Thermal imaging video of transient changes in bill temperature that occur during sleep while the bill is tucked between the wings. Time-lapsed data obtained at 10-s intervals. Total frames = 724, total length = 2.7 hours..”

Now none of this answers the question of why the beaks are often brightly colored.  That probably has the same answer to the question of why some other non-dimorphic birds, like parrots, are also brightly colored.  There are lots of theories (ease of recognizing members of your own species is one), but, in short, we don’t know why.  And we also don’t know why toucans, but no other species, have beaks this large.  Why do toucans need to thermoregulate more than other species? A final question — one that probably can’t be answered — is this: did natural selection increase bill size because that increase directly helped with thermoregulation, or is the thermoregulatory function an exaptation, a beneficial byproduct of a feature selected for some other reason?


Fig. 1.  The ridiculously large bill of the toucan. This is a keel-billed toucan, Ramphastos sulfuratus.

normal_Toco ToucanFig. 2. The toco toucan, subject of this study.  Is that a banana in your mouth or are you glad to see me?


Fig. 3.  Toucan Sam


G. J. Tattersall, D. V. Andrade, A. S. Abe. 2009. Heat exchange from the toucan bill reveals a controllable vascular thermal radiator. Science 325:468-470.