Why does the skin on your fingers and toes wrinkle after immersion in water? A panselectionist answer.

June 22, 2022 • 9:45 am

All of us have noticed that after a period of immersion in water, the skin on both our fingers and toes wrinkles up, but not the skin anywhere else on our body. Here are two photos of the crenulated digits:

From The Conversation


This raises two questions:

a.) What is the mechanism for the wrinkling?

b.) Is there any usefulness or “adaptive significance” of the wrinkling? That is, did natural selection favor it because the wrinkles are useful. 

The two articles below, the first a new popular summary from the BBC and the second a year-old scientific paper discussing the “adaptive significance” of the wrinkling, suggest answers to both questions.

It turns out that we know the mechanism of wrinkling pretty well, but, despite the assurance of both articles, we still have no idea whether it’s an “adaptive” response to water or merely some epiphenomenon that makes no difference to our well being or reproductive output.  That both articles immediately look for an adaptive “reason” why natural selection promoted finger and toe wrinkling is an example of what Steve Gould called “naive pan-selectionism”: assuming that every feature has natural selection behind the evolution of that feature, and favoring the production of that feature—in this case, wrinkling.

Panselectionists often accept pretty scanty evidence as being supportive of their theory, and I think you can see that here.

Click on both screenshots to read the article; the pdf of the scientific article (in PLOS One; reference at bottom) can be downloaded here.



I’ll use facts from both articles, but quotes will be attributed to one or the other.

First, how long does it take to wrinkle up? It depends on the temperature, with 3.5 minutes in warm water to begin wrinkling (40º C or 104° F) and 10 minutes in tepid water (20º C or 68° F). But even in cool water we will wrinkle.

How does it happen? Scientists first thought that it was simple osmosis: the skin cells absorbed ambient water and that made the cells swell up, causing wrinkles. But then they noticed that if the median nerve in the arm is severed, there is no wrinkling. That rules out the osmosis theory as a complete explanation. Osmosis may contribute a bit to the wrinkling, but nerves and blood vessels are also involved. Author Davis of the PLOS ONE paper says this:

Explanations for the wrinkling of the skin include a passive response of the skin to immersion, or an active process that creates the wrinkles for a functional purpose. There is overwhelming evidence that finger-wrinkling is an active process. The small blood vessels of the fingertip constrict, which creates valleys in the skin surface, triggered by water entering sweat pores . Note that a passive explanation would usually assume that water absorbs into the skin, pushing up ridges. This vasoconstriction appears to occur most readily at a temperature of around 40° Celsius, or the temperature of a warm bath [2]. People with autonomic neurological conditions including Parkinson’s, cystic fibrosis, congestive heart failure or diabetic neuropathy may show abnormal or asymmetric wrinkling in the affected parts of the body.

Note that in the first sentence he conflates an “active process” with “an adaptation that has a functional purpose.” This isn’t necessarily true. We get wrinkles, gray hair, and liver spots with age, which are “active processes,” but that doesn’t mean those features are the direct products of natural selection. (What is the adaptive function of liver spots?). The BBC adds a bit more about the mechanism:

Wilder-Smith and his colleagues proposed that when our hands are immersed in water, the sweat ducts in our fingers open up to allow water in, which leads to an imbalance in the salts in our skin. This change in the salt balance triggers the firing of nerve fibres in the fingers, leading to the blood vessels around the sweat ducts to constrict. This in turn causes a loss of volume in the fleshy area of the fingertip, which pulls the overlying skin downwards so that it distorts into wrinkles. The pattern of the wrinkles depends on the way the outermost layer of skin – the epidermis – is anchored to the layers beneath it.

The involvement of nerves explains why some conditions that affect nerves (see first indented para above) affect skin wrinkling.

Let’s assume, then, that we have a pretty good idea of how this happens in fingers, though nobody says much about toes or the rest of the body. (Toes are also sorely neglected in the “adaptive” explanation.

Both sets of authors then set about explaining why natural selection would favor such wrinkling (again, they discuss only fingers, not toes). The experiment describe in the second link above, which gives results in line with previous studies, suggests that the wrinkled skin allows you to grab wet objects with more force than if your skin is unwrinkled and wet. And if your fingers are wrinkled, you’re likely to be in an environment where there are wet objects.  The purported mechanism for this is the same one for treads and valleys in tires: the “channels” in our finger wrinkles suposedly help squeeze out the water when we’re gripping a wet object, allowing better contact with the object. (But what about the toes?)

Davis, then, did a study estimating the strength it took to grip a small and initially DRY plastic disk under three conditions:

a. dry unwrinkled fingers

b. wet wrinkled fingers (note: they apparently didn’t use dry wrinkled fingers, but it’s not clear from the paper. In fact, if they used dry unwrinkled fingers, it would make the adaptive explanation less credible.)

c. wet unwrinkled fingers

Not did they use wet objects, which is crucial for their adaptive hypothesis, though of course gripping a plastic disk with wet wrinkled fingers will make the object wet. Note also that the object is small and light (the BBC says it weighed as much as a couple of coins).

I won’t go into the detail to measure force, but they had an apparatus that measured both grip strength and the ability of the subject to lift up the object and hold it sufficiently tightly so it could be manipulated to follow a computer track. Here’s a photo from the paper:

(From paper): Fig 1. Picture of the equipment in use. The participant is gripping a load cell between finger and thumb. The participant’s task is to pull up on the second load cell to match a force trace that appears on the laptop monitor. The current load force is shown as a red circle, and the history of the participant’s force is shown as a trail of green dots.

The results: people with wet wrinkled fingers and those with dry fingers had similar grip forces, but those with wet, unwrinkled fingers needed significantly more force to grip the disk. Here’s one graph (just look at the top three lines) showing no significant difference between wrinkled-finger force (red) and dry-fingered force (purple), but significantly more force needed using wet, unwrinkled fingers. (The paper give statistics). This shows no real benefit of wet, wrinkled fingers over dry fingers when gripping the disk, but if your fingers are wet and unwrinkled, it’s harder to grip (the plastic get slippery).

(From paper): Fig 2. Comparison of performance across conditions. Mean grip force (thinner traces) and load force (thicker traces) when participants tracked a load weight target (black line). Participants with wrinkled fingers produced a grip force that did not differ from that used by people with dry fingers in the static hold phase, however people with wet but non-wrinkly fingers used a significantly higher amount of grip. The shaded area indicates the pointwise ±1 standard error of each mean trace. Lines below the trace indicate the attack phase (A) of the trial, the static phase (S) and the decay phase (D).

Here’s another graph that shows pretty much the same thing, but showing the grip force needed to sustain the load of the plastic disk under the same three conditions but with varying “load force” (weight, which could be manipulated). Green is wet, unwrinkled fingers, red is wet, wrinkled fingers, and blue is dry unwrinkled (normal) fingers:

(From paper): Fig 4. Relationship between grip and load force in Dry, Wet and Wrinkly conditions. This illustrates the grand mean of the grip and load forces for the whole duration of the trail, minus the first 1000 ms. The target force is shown as a dashed line. The three grip force traces lie above this line, indicating the safety margin. The ‘easiest’ condition, Dry (blue trace) follows the target force most closely. The ‘hardest’, Wet (green trace), shows a higher safety margin, and looser coordination. Participants with Wrinkly fingers (red trace) lie between the two.

Wet unwrinkled fingers require more force to hold the disk than do dry ones. Wet, wrinkled fingers aren’t superior to either, but intermediate between them. (No statistics are given, but another graph implies that none of the differences between the lines in the plot right above are significant.)

The overall conclusion is not strong. Clearly, wet unwrinkled fingers make it harder to grip a smooth plastic object than either dry fingers or wet wrinkled fingers (DUH), but wet wrinkled fingers don’t make it easier to grasp an object than dry unwrinkled fingers. In other words, any advantage of wrinkling is only when it’s compared to wet unwrinkled fingers. Otherwise, dry fingers grasping a dry object are marginally (and nonsignificantly) better than wet, wrinkled fingers.

What can you conclude from this? I’d say, “not much”, but the author of both the BBC article and of the paper seem to think that wrinkling is an adaptation that evolved in our ancestors to enable them to grip objects under wet conditions:


This suggests that humans may have evolved fingertip and toe wrinkling at some point in our past to help us grip wet objects and surfaces.

“Since it seems to give better grip under water, I would assume that it has to do with either locomotion in very wet conditions or potentially with manipulating objects under water,” says Tom Smulders, an evolutionary neuroscientist at Newcastle University who led the 2013 study. It could have given our ancestors a key advantage when it came to walking over wet rocks or gripping branches, for example. Alternatively, it could have helped us when catching or foraging for food such as shellfish.

From the paper:

Grip and load force coordination is an important aspect of object handling. The ability to generate the correct amount of grip force for a given load means that the minimum necessary amount of energy is used by the muscles that control the fingers and hands, and means that objects are less likely to be dropped or to be crushed. Efficient grip force coordination is seen in many extant primates, and is likely to have evolved early in the primate lineage [13]. The grip force required to stabilise a wet object is greater than the force required for a dry object, since the coefficient of friction of the digit-object interface is reduced [8]. It would therefore benefit an animal to gain an advantage in handling wet objects, as this would increase success in hunting and foraging in watery environments. The skin of the fingertip is already adapted for regulation of moisture at the contact surface [14]. Fingertip wrinkles would seem to afford an enhanced advantage in object handling, and may plausibly aid travel and clambering in wet areas, especially if combined with wrinkled toes.

Ergo, it helped us “hunt and forage in watery environments.” But this raises a number of questions:

a.) If you’re hunting or foraging in a watery environment, but your hands have been immersed for fewer than 20 minutes so they’re unwrinkled, you’re better off gripping a dry object with dry hands instead of wet ones. You have an advantage with wrinkled fingers only if they’ve been underwater long enough to get wrinkled, and that advantage is only over unwrinkled wet fingers so long as you’re gripping an object that is itself wet, like a plastic disk that your fingers have wetted.  If you’re trying to grab a dry object when your hands are wet and wrinkled, you’re worse off than when using dry hands.

b.) They did not test the three conditions when gripping large dry objects like a tree branch or an animal, which may not behave like plastic disks! This is essential if you think that either grabbing dry objects was important for our ancestors even when our fingers were wrinkled from having been immersed in water.

b.) We did not evolve in a watery environment; the “aquatic ape” hypothesis has long been dispelled. As for our relatives, the BBC article says “only one other primate has so far been found to have water-induced wrinkling of the fingers—Japanese macaques.” (Naturally, they show a photo of a macaque sitting in water.) I’m not sure if other primates have eve been tested (no such tests are referenced), but if chimps, bonobos, and orangs show finger wrinkling, that would imply that it did NOT evolve to enhance grip strength in watery environments. These primates don’t live in those environments!

d) What about doing the study with dry wrinkled fingers? (You quickly dry them before grasping the object.) The adaptive hypothesis would predict that there would be no grasping advantage of dry wrinkled fingers over dry unwrinkled fingers. They didn’t do that experiment (as far as I can see).

e.) What about the TOES? They get wrinkled too. The paper posit that wrinkled toes would aid “travel and clambering in wet areas”, but that is pure speculation—not even a hypothesis. It could be fairly easily tested, but wasn’t.

f.) If wrinkly skin is pretty much as good as dry skin for gripping almost anything, why don’t we have permanently wrinkled skin? Author Davis has an answer:

A previous study of object manipulation with wrinkled fingers found that wet objects were moved more quickly when the fingers were wrinkly compared to dry [15]. Importantly, there is no difference in tactile sensitivity in wrinkled fingers compared to dry [16], meaning that people are not trading off acuity for friction at the fingertip. It is therefore reasonable to wonder why healthy people do not have permanently wrinkled fingers. The answer presumably lies in the changes in the mechanical properties of the finger tissues, where there may be lower shear resistance when the finger is wrinkled [17]. Previous studies have also suggested differences in manipulation across the lifespan [1820]; the present results show age-related effects, although they are rather weak in this sample. Our journey through life leads us to develop strategies for handling familiar and unfamiliar objects, so it seems likely that strategic changes, along with sensory and motor changes, will affect how children and adults perform tasks with handheld objects [21].

Here we have ultimate pan-selectionism: if your hypothesis fails to explain another phenomenon, you simply make up a reason why that’s also adaptive. In this case, Davis posits “lower shear resistance” for wrinkled fingers, which for a reason he fails to specify must confer a disadvantage (presumably because you can’t hold onto an object as tightly).

I’m not at all convinced by this explanation or the supporting data, as they’re contradicted by evolutionary observations and by the absence of data on wrinkled toes. As the BBC says, some believe that wrinkling “could just be a coincidental physiological response with no adaptive function.” (Go have a look at that link!). I am one of those skeptics. What surprises me is that that statement is the sing caveat (and doesn’t reprise what’s at the link) in a whole article pushing the “adaptive wrinkling in wet environments” hypothesis.

Other venues have also picked up this result, and I guess they are either overly credulous or didn’t read the paper carefully enough. Or they didn’t ask probing questions.

h/t: Peter


Davis NJ (2021) Water-immersion finger-wrinkling improves grip efficiency in handling wet objects. PLOS ONE 16(7): e0253185. https://doi.org/10.1371/journal.pone.0253185

Stick insects can disperse like plant seeds: in bird poop

June 1, 2018 • 12:45 pm

One of the striking observations about life on oceanic islands—those islands, like Hawaii and the Galapagos, that arose, bereft of life, from volcanic activity below the sea—is the prevalence of native birds, insects, and plants, and the paucity of native reptiles, mammals, and amphibians. (Continental islands, like Great Britain, that were once connected to larger land masses, don’t show this pattern.)

Darwin was the first to make this observation and show that it supported his theory of evolution. Plants, insects, and birds can more easily get to islands, where they evolve in relative isolation into new species, while mammals, reptiles, and amphibians can’t easily cross large expanses of seawater to colonize distant islands. His view could be summed up as biogeographic patterns = dispersal + evolution.

One of the ways that plants get to islands (besides via their seeds floating in seawater) is through bird movement: birds eat fruit and seeds, fly to an island, and the seeds germinate from the bird’s post-migration poop. In fact, I think a lot more plants have arrived on islands this way than by seed flotation, but don’t quote me on that.

But more than plants can get to islands in bird poop. A new short paper in Ecology by S. Kenji et al. (reference below, free pdf here) shows that stick insects (phasmids) produce hard-shelled eggs that can remain viable and hatch after they pass through a bird’s digestive tract. Moreover, since the eggs don’t require fertilization (they’re from “parthenogenesis”), they don’t have to be fertilized right before being laid, as most insect eggs are. They can simply be nommed by the birds right after being laid, or ingested by gobbling a pregnant female.

The eggs of many stick insects are sculptured like seeds and, more important, have a hard layer of calcium oxalate on the outside that is dissolved only by acidic environments like bird stomachs (this layer appears unique to phasmids). You can see some of these tough eggs in part “B” of the figure below, taken from the paper.

The authors fed eggs of three species of phasmids, mixed with an artificial diet, to Japanese brown-eared bulbuls (Hypsipetes amaurotis), which they claim is one of the main predators of stick insects. They then collected fecal pellets of from the birds when they were pooped out within three hours, and measured hatchability of the eggs. Those hatchabilities were 5%, 8.3% and 8.9% (sample sizes between 40 and 60 eggs per species).

The figure below shows bulbuls eating a phasmid, the eggs, and a nymph of one phasmid species:

(From paper): FIG. 1. (A) The parental brown-eared bulbul Hypsipetes amaurotis feeding the stick insect Ramulus irregulariterdentatus to its chicks. (B) Intact Ramulus irregulariterdentatus eggs defecated by the brown-eared bulbul Hypsipetes amaurotis. Bar = 2 mm. (C) First instar nymph of R. regulariterdentatus hatched from the egg defecated by H. amaurotis.

One obvious conclusion is that, given that bulbuls can fly about 40-60 km/hour, they could disperse phasmid eggs over a hundred kilometers (eggs are produced at about the time the Japanese brown eared bulbuls migrate). The authors fed eggs excised from adult phasmids to the birds, which suggests further that the birds could ingest a bunch of eggs at once simply by eating a pregnant female (they didn’t do this test).

The next questions are these:

1.) Did the eggs evolve that hard coat to facilitate dispersal? There are advantage to dispersing your offspring widely, especially if local predation is high or environments uncertain, and many species of animals have evolved elaborate dispersal mechanisms. (Fruits with seeds inside are one of these!). This is possible, but the authors prefer the idea that the tough eggs evolved to reduce parasitism by wasps. But of course the coat could have evolved for several “reasons” (and by that I mean there could have been more than one reproductive advantage to toughening up your eggs).

2.) Have phasmids actually dispersed this way? We don’t know, as the biogeographic studies haven’t been done. As the authors note, this should show up as evidence for wider dispersal of parthenogenetic phasmids than of their sexually-reproducing relatives:

If avian dispersal is important to stick insects, the phylogeographical patterns should reflect occasional long‐distance dispersal events (e.g., Miura et al. 2012). In addition, the patterns of spatial genetic structure will differ among stick insects with parthenogenetic reproductive capability (and hence potential avian dispersal) and non‐parthenogenetic stick insects. The phylogeographical patterns in these stick insects thus deserves further studies.

Further, stick insects themselves should in general show dispersal different from non-stick insects (I don’t mean Teflon ones!), since some of the former have the ability to get their eggs dispersed hundreds of kilometers. But all this awaits further study, as there was no reason to investigate those patterns before this new paper appeared.

h/t: Dom


Kenji, S., F. Shoichi, T. Asuka, I. Katsura, and Y. Takeshi. 2018 Potential role of bird predation in the dispersal of otherwise flightless stick insects. Ecology. doi: 10.1002/ecy.2230

Why exaptation is an unnecessary term in the science of form

December 28, 2015 • 2:30 pm

by Greg Mayer

The most important finding of vertebrate comparative morphology and paleontology is that most of evolution is the gradual, adaptive, modification of pre-existing structures (or, better, pre-existing developmental programs, which result in the structures).  The point about pre-existing structures is very important– the history of evolution is to a great extent the history of making due with what you have– “tinkering”, as François Jacob called it. The current phenotype is where you start, and there is a range of mutationally accessible phenotypes that is determined by the developmental system, the environment, and the genotype. This mutationally accessible region will only rarely include completely new structures.

As mammals, WEIT readers may have wondered why it is so difficult for them to get food to come out of their noses while eating (it usually requires vigorous coughing, laughing, or sneezing), while their pet aquatic turtles can do so effortlessly, small food particles floating gracefully out of their nostrils while they eat. The reason is that mammals have a secondary palate– reach in and tap the roof of your mouth– that’s it, right there. It is a shelf of bone that separates the air passage, which goes from the nostrils to the glottis, from the food passage, which goes from the mouth to the esophagus. In (most) turtles, there is no secondary palate, so the air and food passages are one passage, thus allowing food to exit the nostrils. Where did the secondary palate come from? It is not a new bone(s), but a series of medial processes, off the very same bones found in turtles, that meet in the midline to form a complete shelf. Turtles, having a low basal metabolism, do not need to breathe incessantly, while mammals, with their high metabolism, must generally be breathing and eating all the time– the separated passageways allow mammals to breathe while chewing. The gradual modification of these bones to form a secondary palate (and, somewhat less clearly, the crucially associated soft palate) can be traced in the fossil record. The drawback to the way mammals eat is that when finally you have to swallow the food, the lungs are ventral to (i.e. below) the digestive tract, even though your nostrils are dorsal to (i.e. above) your mouth, and the air and food streams must cross. So, if you sneeze/cough/laugh real hard just as your food is crossing over the air passage, it can be blown into the air passage, and come out your nose.

Mammalian swallowing and breathing is thus based on the same bones, digestive tract, and lungs, arranged in the same basic way, as are found in turtles, and in our common reptilian ancestor. In mammals, the pre-existing structures have been “tinkered” with, in a series of documented steps, to arrive at the current state, which allows for a lot of eating and breathing, which in turn allows for a high metabolic rate (and hence being warm-blooded), although since the lungs remain below the gut, you can choke on your food. (Had engineers designed vertebrates, they never would have crossed the air and food passages.)

What brings all this to mind is Jerry’s mention yesterday of Steve Gould’s concept of “exaptations”, noting that the 7th day of evolution video stated that penguin wings are an”exaptation”, because “not every trait is an adaptation, and they don’t all have a point.” This is surely one of the most unproductive, and, indeed, wrong, ways to look at penguin wings. Penguin wings are in fact adaptively modified pre-existing structures; the earliest known penguins in the fossil record, were flightless swimmers, but not as modified for this as later penguins. The notion that a change in function (flying to swimming, in this case) necessitates a new terminology was argued, quite unsuccessfully, by Gould and Elizabeth Vrba in 1982.

Composite skeleton of Waimanu tuatahi from Slack et al. (2006), via March of the Penguins.
Composite skeleton of Waimanu tuatahi of the Paleocene, one of the earliest penguins, from Slack et al. (2006), via March of the Fossil Penguins.

“Exaptation” is an unnecessary term in the science of form. It confuses more than it clarifies. It ignores the historical component of adaptive evolution (which is curious for Gould, since he frequently, and often correctly, argued for the importance of historical considerations; his animus against natural selection must have overcome his historical instincts in this case).  The great paleontologist George Gaylord Simpson attempted to rehabilitate the term “preadaptation” to cover the not uncommon situation where a pre-existing structure undergoes a change of function; once there is a new function, then “postadaptation” will refine the structure to fit the new function. The morphologist Carl Gans suggested that what he called “excessive construction”– the ability of a feature to perform at least tolerably in circumstances other than the usual ones– could often form the basis for preadaptation. (Gans substituted “protoadaptation” for “preadaptation”, finding the latter term too freighted with unfortunate associations with mutationism, of which Simpson sought to cleanse it, to be used.)

I find preadaptation, shorn of its mutationist connotations, a perfectly serviceable concept. Thus, wings on aquatic birds are a preadaptation for swimming in the water. Behavioral flexibility allows the wings to be used in a new way, which then induces a new selective environment, and postadaptations will then further suit the structure to these new conditions of existence. But even more productive, I think, is the notion of “sequential adaptation”, based on Richard Swann Lull’s notions of primary and secondary adaptations (and related to W.K. Gregory’s related concepts of caenotelic vs. paleotelic and habitus vs. heritage), which provides a much better way of looking at changes of function. (I’ve long attributed the phrase “sequential adaptation” itself to Lull, but after a rereading I can’t find that he used it; it was my term for summarizing his views. Lull, by the way, was Simpson’s doctoral adviser.)

So how do we look at the wings of penguins under this view? Certain dinosaurs’ front legs (they already had front legs, of course– they were preexisting structures) became adapted for flight– that is, there were modifications in the structures that conferred higher fitness on their possessors by virtue of the presence of the modifications.  Are the wings of birds an “exaptation”? No. Wings, qua wings, are an adaptation for flying. The wings of certain birds became adapted for swimming– the “flippers” of penguins. The bones have become solid, more robust, dorsoventrally compressed and knife edged, all of which improve their function for diving (not all of these were fully present in the earliest known fossil penguins). Each of these modifications is an adaptation. In Lull’s terms, the wing is a primary adaptation (for flight), the flipper a secondary adaptation (for swimming). But the wing itself was a secondary adaptation of the primary adaptation of the locomotory forelimb of dinosaurs– and so on back in time. They are a series of sequential adaptations. The error of “exaptation” is to think of traits or features of organisms as unanalysed wholes without a history: penguin flippers are are not merely “flippers”, but a whole suite of features, including many bones, muscles, and behaviors for their use. And the flippers have a preceding history as wings, front legs, and so on ad not-quite-infinitum. If “exaptation” means that later adaptations are based on the pre-existing structures, then the term is vacuous– all adaptations are then exaptations.

A case noted by Simpson as preadaptation involves the predatory behavior of keas (Nestor notabilis), the large alpine parrot of New Zealand. Keas eat sheep (or at least parts of them, since the sheep may survive), by cutting through the skin with their sharp hooked beaks.

Keas are naturally omnivorous, so what we have is an expansion of the dietary range, enabled by the pre-existence of a wicked beak. The beak is an adaptation to wide ranging foraging on and in the ground, and on and in the vegetation (excavating logs and such).  If there arises an innate (as opposed to learned) aspect of sheep feeding, or the bill itself experiences selection for features that enhance the ability to feed on the sheep, there would would then be secondary, or sequential, adaptations. At this point, though, feeding on sheep may well be behavioral flexibility with the available tools (Gans’ “excessive construction”, providing the basis for “protoadaptation”).

We not only don’t need the term “exaptation”, it actually hinders understanding, by suggesting that non-adaptive processes are at work (which was Gould and Vrba’s explicit intention), when in fact we have a series of sequential adaptations.

[And I must give here a strong recommendation to the website March of the Fossil Penguins by Daniel Ksepka, which I discovered while writing this post.]

Gans, C.  1979.  Momentarily excessive construction as the basis for protoadaptation.  Evolution 33:227-233.

Gould, S.J. and E.S. Vrba. 1982. Exaptation- a missing term in the science of form. Paleobiology 8:4-15. pdf

Jacob, F. 1982. The Possible and the Actual. Pantheon Books, New York.

Kirsch J.A.W. and G.C. Mayer. 1998. The platypus is not a rodent: DNA hybridization, amniote phylogeny and the palimpsest theory. Philosophical Transactions of the Royal Society B 353:1221-1237.  pdf

Lull, R.S. 1917. Organic Evolution. Macmillan, New York.  Internet Archive

Simpson, G.G. 1953. The Major Features of Evolution. Columbia University Press, New York.

Slack, K.E., C.M. Jones, T. Ando, G.L. Harrison, R.E. Fordyce, U. Arnason, and D. Penny. 2006. Early penguin fossils, plus mitochondrial genomes, calibrate avian evolution. Molecular Biology and Evolution 23:1144-1155.

Readers’ wildlife videos

December 9, 2015 • 7:45 am

Because I’m having trouble braining this morning (I haven’t yet made my latte with three shots of espresso), I’ll eschew the laborious process of making a readers’ wildlife post, and instead present you with two treat videos by reader Tara Tanaka from Florida. The first shows a pileated woodpecker (Dryocopus pileatus) making a cavity; Tara has a detailed explanation at the Vimeo site. Pileated woodpeckers are amazing; I describe some of their adaptations on pp. 114-115 of Why Evolution is True.

And Tara sent notes on this one, called called “Thanksgiving Morning Bunnies”:

Part of this one is featured in the Windland Awards video that is now on display in the Smithsonian Museum of Natural History for the next year. Look for the cottontails in the background.

Her notes on Vimeo add that this was filmed Thanksgiving morning at Bosque del Apache National Wildlife Refuge in New Mexico:

This was shot with a GH4 and Nikon 105mm lens + a GH4 mounted on a Swarovski STX85 spotting scope. The bunnies appear in the background and you can see them out of focus at a distance until I get the scope on them and capture them with 1000mm.

I’m thankful for many things today, but right now I can’t stop smiling after witnessing two bunnies playing a game whose goal, after watching them in slow motion over and over, must be to touch noses while in they are both in mid-air.

I was sitting on the ground and had set up one video camera with a 105mm lens to video Gamble’s Quail in more of a wide-angle shot than my digiscoping gear would allow, and was digiscoping photos with my scope. I looked up from the viewfinder to see two Desert Cottontails playing in the background. I was going to take photos but decided it really needed to be captured on video, so I switched to video and started recording them through the scope. They were not in the focused area of the wide angle camera, but at least the whole scene was captured.

Are they fighting? Trying to mate? Or just playing? Readers with lagomorph expertise should weigh in below.


The cockeyed strawberry squid

August 14, 2015 • 1:45 pm

The strawberry squid (Histioteuthis heteropsis), also known as the cock-eyed squid, is famous (as you can also tell if you parse the Latin binomial) for the huge disparity in the size and form of its two eyes. That’s evident from this gif:


Now, before you watch the explanatory video below, or read Wire‘s recent piece on its eyes, form an evolutionary hypothesis about the disparity—one that involves natural selection. How could you test it?

Think! (It’s not obvious, and of course the explanations you’ll hear are speculative). The truth is, what the video and article say sounds good, but we have no idea if the form of natural selection they suggest really acted to produce this bizarre morphology.

Okay, watch the video, and then read the piece above.

And remember, folks, squids, like all cephalopods, are molluscs—in the phylum Mollusca.


A new and bizarre shape-shifting frog

March 29, 2015 • 9:50 am

Instead of going to church today, we can have our special Alain de Botton-Approved Religion Substitute by worshiping at the church of Our Lady of Natural History. There is in fact a wonderful new discovery about frogs, one described in a new paper in the Zoological Journal of the Linnaean Society by Juan Guayasamin et al. (reference and free link below; there’s also a precis in LiveScience).

I can state the results concisely: the authors found a new species of frog in Ecuador that can dramatically change its body shape from spiky to smooth in a matter of only a few minutes.  They then found another species, somewhat but not extremely closely related to the first, that can do the same thing. This kind of change in morphology, induced by the environment, is called phenotypic plasticity. And its observation in the frogs suggests two conclusions:

1. A lot more frogs can do this than have been described, but you need special conditions to see it, so it’s been largely undescribed. Other abilities of amphibians to change shape or color within a short time may also have been missed.

2. Since new frog species are often identified by their appearance after having been collected and pickled in alcohol, there may be described species that are identical to species with other names, but were misidentified because frogs collected at different stages of shape-changing could be mistaken for two different species. This is especially problematic because a large proportion of new species in both invertebrates and vertebrates (18% and 19%, respectively) are described from only a single specimen.

The paper gives other information as well, including genetic data, a phylogenetic analysis of the genus showing how the two shape-shifting species are related,  other genetic information about differences between populations, and a description of the frog’s call and morphology, important for describing it as a new species. But those issues are of more professional interest and need not detain us.

In amphibians, most variation among individuals of a species is in color, but those differences are permanent (like hair and skin color in humans) and don’t change over time. Those traits that do change over time in amphibian species, like crests in newts or tubercules in frogs, change during the breeding season, usually in males as a way to attract mates, and then revert back after the season. They thus change seasonally rather than over just a few minutes, like the frog described in this paper. The rapidity of change in the species is thus novel.

The new species, Pristimantis mutabilis (note the species name!), was first spotted in 2006 in the cloud forests of the Ecuadorian Andes, but its ability to change shape wasn’t detected until three years later. Under normal conditions the frog is spiky, with tubercules and points, but it changes when they’re picked up. As the authors describe in the paper:

All individuals of Prismantis mutabilis presented a markedly tubercular skin texture when found on vegetation or hidden in moss during the night. Large tubercles were evident on the dorsum, upper and lower lips, upper eyelid, arms and legs. After frogs were captured, they all showed a sudden and drastic change in skin texture; all tubercles became reduced in size, and the dorsal skin became smooth or nearly smooth (i.e., few tubercles are visible, mainly on the upper eyelid and heel). When frogs were returned to mossy, wet en- vironments, they recovered a tuberculate skin texture. We speculate that explanatory variables involved in frog skin texture change are stress, humidity, and back-ground. Our observations do not support light availability as a source of texture variation as we observed skin texture change at day and night. The time rate of skin texture variation might depend on the variables mentioned above; we only have one quantitative measure, which is summarized in Figure 2.

Here’s Figure 2: As you can see, the spiky frogs become relatively smooth within five minutes after capture. It’s not yet clear what physiological/biochemical systems are involved in this dramatic change:


Here are two more pictures of individuals changing:

In this photo, from Figure 3, a sub-adult male is first photographed in its natural habitat (A) and then in the laboratory (B). You can see the change very clearly:

Screen Shot 2015-03-29 at 8.28.26 AM

And here’s one more with the caption from the paper. These are small frogs, ranging in snout-vent length between 17 and 23 mm (0.7-0.9 inches):

Screen Shot 2015-03-28 at 8.35.04 AM

The authors also identified another species that does the same thing: a congener (frog in the same genus) named Prismantes sobetes. Since the two shape-shifting species are not closely related—a phylogeny shows many other species are more closely related to either than the two are to each other—either the ability to change body texture has evolved twice, or it’s present in some of the intervening species since it evolved in a common ancestor, or it is the remnant of a feature in their common ancestor that has been lost in all other species in the group. Since we don’t know about the abilities of those other species to shape-shift, more work is needed to distinguish among these explanations.

This leaves one big question: Why on earth do the frogs do this? Let’s assume as a working hypothesis that the shape change is an evolved one, and that individuals that could change shape had a selective advantage in the ancestral lineage. (It’s also possible that this is simply a nonadaptive physiological response to stress.) The authors suggest, probably correctly, that the tubercles and spiky appearance help camouflage the frog in the cloud forest, where it often sits among moss, vegetation, and epiphytes (plants growing on other plants); and they also raise one possibility for how they change their shape:

We suggest that skin plasticity is associated with environmental camouflage rather than sexual selection or dimorphism. Pristimantis mutabilis and P. sobetes are geographically distributed in montane cloud forest habitats that are abundant in epiphytes, vegetation, and moss. In these habitats, skin texture that has the appearance of moss or detritus likely conceals the individual from visual predators, such as birds and arachnids. While the physiological mechanisms of how texture changes in such a short time are unknown, we speculate that it could involve allocation of more or less water to existing small structures (e.g. warts and tubercles) on the skin.

But what’s missing here is an explanation for the change itself, which I can’t find in the paper. That is, why do they change from the presumably camouflaged shape to a smooth shape? And here I, who have no knowledge about amphibians, come up short. Perhaps being smooth helps you escape from predators if you’re caught, or helps the frogs jump better.  Experiments (some of them involving predation!) could help settle this.  I suspect some readers who know more about frogs than I (I’m looking at you, Lou Jost) can suggest evolutionary reasons why shape-shifting may be adaptive.  Please give your suggestions in the comments.


Guayasmin, J. M. et al. 2015. Phenotypic plasticity raises questions for taxonomically important traits: a remarkable new Andean rainfrog (Pristimantis) with the ability to change skin texture.  Zool. J. Linnaean Soc. 173:913-928.

h/t: Barry

Readers’ wildlife photos

September 1, 2014 • 3:44 am

We’ll start with three raptor photos from Stephen Barnard in Idaho, and then proceed to the cats.

First we have a Swainson’s hawk (Buteo swainsoni):


Then a red-tailed hawk (Buteo jamaicensis):


And a Northern Harrier (Circus cyaneus):


Reader John sent some cheetah photos and notes (indented):

Following your recent call for photographs I decided to dig out some of a feline variety. The attached were taken in 2004 in South Africa’s Kruger National Park. The Kruger is SA’s largest ‘park’ at 20,000km2 – about the size of New Jersey. Its size means that it doesn’t have the feel of a park and if you wander of the beaten track, you can spend many hours exploring the bush or siting by waterholes without seeing many other visitors. The trip was not long after I made the switch to a Canon 300D, an early digital SLR; up to that point having been reluctant to discard traditional film.  The sharp eyed will notice I hadn’t quite got to grips with the auto focus.

This Cheetah crossed a dirt track in front of our vehicle and then spent some time in a small tree sharpening its claws whilst also keeping a watchful eye on us.

2004-12-19_Kruger National Park_Cheetah_Coyne-0002

It then wandered into the scrub, which was quite lush because it was December and the rainy season. Shortly afterwards it reappeared from the bush and walked slowly alongside the track as we inched our vehicle along and took photographs.

2004-12-19_Kruger National Park_Cheetah_Coyne-0006

It was clearly mindful of our presence but otherwise carried on as usual including regularly marking its territory.

2004-12-19_Kruger National Park_Cheetah_Coyne-0005

Later during the same holiday my daughter was lucky enough to enter an enclosure and spend some time petting a Cheetah!

(Professor Ceiling Cat doesn’t ever get to do that. . . )

2004-12-23_South Africa-0068_Wilderness_Alex Cheetah

 I’ve been lucky enough to see wild cheetah a number of times but sadly, on a trip to the Okavango in 1988, I have also seen the darker side with Cheetah skins hanging in a tannery in Maun. I’ll spare you the depressing image.


The grasping reflex of babies: a vestigial trait?

April 8, 2014 • 11:56 am

This is the type of post I originally intended to publish on this website, and the only type of post, for the website was created, at the behest of my editor at Viking/Penguin, to support my book WEIT. My idea then was to post a bit of cool evidence for evolution every few weeks or so. Then things got out of hand. . . But today we are back to the original mission.

One of the pieces of evidence I use for evolution, in both my book and my undergraduate classes, is the presence of vestigial traits. And there are some nice behavioral ones. I wiggle my ears for my students, which they love, but I do it to demonstrate our vestigial ear muscles, useless in modern humans but adaptive in our relatives, which can move their ears widely to localize sounds. (Check out your cat when it hears something.)

Humans have another vestigial behavior: the “grasping reflex” (also called the “palmar reflex”). Young infants can hold onto objects with both their hands and their feet—and hold tightly and tenaciously. They lose this behavior—which is instinctive, prompted by inserting a finger or a stick in their hands or feet—a few months after birth.

While we’re not 100% sure what it represents, I’d bet that it’s a genetic holdover from our ancestry as hairier primates. (Remember: we’re the only “naked apes.”) In primate species, the young are carried about by hanging onto their mother’s fur with both hands and feet, and they keep this behavior throughout infancy. Their ability to hold on is important for their survival.

Humans aren’t hairy, and aren’t carried about by clinging to their mother’s fur. But we still, at least for a short period, show genetically-based behaviors that testify to our descent from furrier creatures.

Here are some photos of the daughter of a friend. This one shows the grasping reflex at 7 days of age. Note that  she’s holding on so hard that her fingers are white!

7 days

I took this one about three days later, showing the grasping reflex of the pedal extremities:
Grasping reflex

For years I tried to persuade my friends who had infants to let them hang from broomsticks (I have a drawing of this behavior in an evolution textbook from the 1920s), so I could photograph it or make a video. But for some reason they always refused, even though I claimed that one can do this safely: just put the infant over your lap or a bunch of pillows. No dice.

But I was recently shown this video from the 1930s showing two infants “competing” to see who can hang the longest. Here are the YouTube notes:

Fragment of “Johnny and Jimmy” (twins), a silent film by Myrtle McGraw, recorded in 1932. from McGraw, M.B. (1975). Growth: A study of Johnny and Jimmy. New York: Arno Press. [1935]

One baby makes it for only 4 seconds (what a wimp!), but the other is still hanging after 37 seconds! I love the blotting out of the genitals.

Here’s a more recent video in which the infants are suspended more humanely. The genitalic blur has also been made spiffier: it’s now a fig leaf.

This isn’t the only primitive reflex displayed by human infants. Wikipedia has a whole list of them (the foot-closing is called the “plantar reflex”), and you might amuse yourself by speculating about which of them might have been adaptive in the infants of our ancestors, and why.

Why are there no more large flying birds?

February 6, 2014 • 6:33 am

by Matthew Cobb

As is well known, Professor Ceiling Cat can’t be doing with Tw*tter. Here’s yet another example of why he’s wrong, and should learn that that micro-bl*gging site is not just for knowing what celebrities had for breakfast or for launching cyber lynch mobs.

I was listening to Radio 4’s ‘Tweet of the Day’ this morning at 05:58. It featured the bizarre call of the Great Bustard (it sounds roughly like someone blowing their nose and farting at the same time). The Great Bustard is a large bird that was hunted to extinction in the UK, but has recently been reintroduced and is now successfully breeding. Chris Packham, who did the commentary, claimed that at 16 kg the Great Bustard is one of the heaviest extant flying birds.

This struck me – 16 kg isn’t much. Is this an absolute limit to flying? What about those pterosaurs – some of them were HUGE. How come they got so big and flying birds don’t? What’s the upper limit on the weight of a flying animal?

So I got out my iPad and tweeted @TetZoo aka Darren Naish, who knows about all things tetrapod. (I got the weight wrong. It was early in the morning. This caused some confusion, as you’ll see.)


Both Darren and Dave Hone, another pterosaur expert chipped intw3




The ‘different take-off’ caught my eye. I know there’s been a suggestion that pterosaurs lived on cliffs, so could simply soar without having to take off from the ground (the modern swift, hardly a chunky bird, can’t take off from the ground). But some pterosaurs would dive and eat fish – how did they take off from the sea?:


Dave replied:


Darren had to set the record straight regarding bustard weight, when David Watson rightly questioned my figure:tw10tw13


Then Mike Habib joined in and pointed out:


Then he asked the Big Questiontw8

Tommy Leung chipped in:

[JAC comment: Why is Habib so sure that “birds and bats can’t get giant pterosaur size”?]

So, as in most interesting questions, the answer to ‘Why are there no large flying birds now’ appears to be ‘We don’t know’.

Any ideas?

[JAC comment 2: I doubt this demonstrates that I’m wrong about Tw**ter. All that scientific brainpower results in the verdict that “we don’t know”?? They might as well have tw**ted what they had for breakfast!]

Links: Dave and Mike’s piece on how pterosaurs took off, the PLoS One paper from Mark Witton and Mike Habib, looking at whether giant pterosaurs could fly, cited by Darren.

An unusual antipredator defense

December 16, 2013 • 8:38 am

Yesterday, reader Roo sent me the Torygraph‘s photo of the day, which is an assassin bug. The caption is below (I’m not sure why they use the past tense):

These ruthless Assassin bugs hid from potential predators using a camouflage cloak – made from the bodies of ants they had killed. The deadly insects paralysed the ants by injecting them with a toxic enzyme before sucking them dry. They then piled the dried-out corpses on their sticky backs to act as a defence against other predators, such as jumping spiders. Picture: Guek Hock Ping/Photoshot/Solent News

Picture 1

Note that assassin bugs (unlike “ladybugs,” which are beetles in the order Coleoptera) really are bugs : they’re in the order Hemiptera, or “true bugs.” (If I want readers to learn anything from this site, it’s to use the word “bug” properly!) They’re also in the order Reduviidae, some of whose New World species—probably not the one above—carry the protozoans that cause Chagas disease, an often asymptomatic but sometimes fatal illness. For many years people thought that Darwin had been infected with Chagas on his Beagle voyage, accounting for his frequent and lifelong bouts of illness, including malaise and vomiting. We’ll never know for sure, for doctors have suggested many other causes, ranging from simple nervousness to the latest Darwin-illness fad, cyclical vomiting syndrome.

Assassin bugs are so called because they stick their snout (“rostrum,” if you want to be technical) into the prey, injecting a saliva that liquifies the prey’s insides. They then suck it dry.

It’s interesting to speculate how this evolved. This adaptation (and who can deny that it is one?) involves both a morphological trait (a sticky back) and a behavioral trait (the tendency to put the husks of your prey onto your back). Without that sticky back, you have no initial advantage, so I suspect that the evolution of this mimicry began simply because the bug had a back that could adhere to dead insects, perhaps because of cuticular lipids that served other functions, like desiccation resistance or attracting mates. Perhaps a prey accidentally adhered to one of the bugs with a particularly sticky back, and that individual gained an advantage, as it was simply harder to attack and eat. This would give an advantage to genes producing not only stickier backs, but also  promoting any tendency to place sucked-out prey on your back.  I am curious whether the ant carcasses are inherently sticky too—as they appear to adhere to each other—or whether the bug actually puts something on them to help them stick together.

But this is all speculation. What is on firmer ground is the idea (still probably not demonstrated through experiment) that this is a remarkable adaptation to deter predation. I wouldn’t call it “mimicry” (unless predators avoid piles of dead ants), for this ant-covered bug isn’t really deceiving the predator by “pretending” to be something else. It’s simply making it harder for predators to grasp and eat them.