Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
This one is pretty easy—so easy, in fact, that I won’t give the answer. It’s a photo of a single mallard hen resting under a tree in yesterday’s heat. I put it up just to demonstrate how cryptic these females are in a woodsy situation. The colorful drakes, of course, would stick out like a sore thumb: the price they pay (via predation risk) for being attractive to the hens. Clearly the risk of predation is outweighed by the “need” to attract a hen.
Click on the photo to enlarge it.
We have a tripartite piece today, with everyone’s notes and IDs indented:
First, from biologist/naturalist/photographer Piotr Naskrecki‘s Facebook page, a wonderful grasshopper from Mozambique. It was such a good example of crypsis that I asked him if I could post it here, and he kindly gave me permission. His caption:
Although one of the largest insects in Gorongosa, the Gladiator grasshopper (Acanthoxia gladiator) is also one of the most difficult species to find. Not surprisingly so. Its coloration and the body form are designed to look just like a dry stem of grass. Both its head and the abdomen carry long, blade-like extensions (hence the name) that make the illusion complete. The grasshopper fauna of Gorongosa is incredibly rich, with over 160 species that we have recorded so far.
Spot the grasshopper!
A lovely bird from Duncan McCaskill of Canberra, Australia:
Here are a few photos of just one bird, but a very special bird. Back in mid-January, at the height of our severe summer when a lot of the country was on fire, a spectacular and very special bird turned up in a small patch of woodland in Canberra: a Regent Honeyeater (Anthochaera phrygia). The Regent Honeyeater is a critically endangered species, with a population estimated to be only around 350 individuals, scattered over a range exceeding 600,000km2. They were once fairly common throughout south-eastern Australia, and were seen in the suburbs of Melbourne and Sydney, but the population has declined with land clearing of their woodland habitat, and crashed from the 1960s to the point where is it on the verge of extinction.
When the presence of this bird in January was reported in birding circles, birders from far and wide went to see it. It remained in the area for a week or so and seem unbothered by the attention of birders. There were around half a dozen people watching it when I visited.
It was named Regent Honeyeater in the early twentieth century due to its black and gold colouring which is similar to that of the Regent Bowerbird (Sericulus chrysocephalus), which in turn got its name a century earlier due to its resemblance to the colourful attire of the Prince Regent of Britain (later George IV). Prior to the twentieth century, it was known has the Warty-faced Honeyeater, an unattractive but accurate name.
With an insect (I think it’s a winged ant):
The woodland was bone-dry, like the whole region. Many of the trees looked like they were severely stressed, if not dead. But one large, old Yellow Box (Eucalytpus melliodora) was in flower, and this was the reason the bird was there. Here it is feeding on a flower in the canopy of the Yellow Box:
From James Blilie:
I’m not the big wildlife photographer in the family—that’s my son, Jamie. But I did get these this winter. Birds on one of our feeders in a snow fall.
A Black-capped Chickadee (Poecile atricapillus):
Two shots of a female Northern cardinal (Cardinalis cardinalis):
Reader Tony Eales from Australia sends some camouflaged beasties and a brightly colored spider. His notes are indented.
Some camouflage.
A Brisbane Two-tailed Spider, Tamopsis brisbanensis, on a mangrove tree trunk. These are very hard to spot. They have a leg span of around 3 or 4 cm. I believe the one in the photograph has some securely wrapped prey in front of it on the bark but even this is well camouflaged. I found this video of a slightly different species of Two-tailed spider catching prey: https://youtu.be/6uo1rPHxlRI
I’ve sent a Flat-headed Leafhopper (Ledromorpha planirostris) nymph as a spot the before, but I saw another one just recently. I’ve attached a long shot, a close up and the reveal.
At the opposite end of the spectrum from camouflage, I found another species of Peacock Jumping Spider. This one is Maratus nigromaculatus.
I’ve also included a picture of one of our really common and colourful bugs. It is a male Cotton Harlequin Bug (Tectocoris diophthalmus). Females are orange with iridescent green blotches, and the nymphs are iridescent dark blue with red patches. They love hibiscus, especially the tree-sized ones we have around here called Cotton Trees (Hibiscus tiliaceus).
And lastly a katydid genus Polichne. There’s a lot of undescribed and hard to distinguish species in this genera.
Reader Tony Eales from Oz sent some camouflaged arthropods. These aren’t all that hard to see, as they were specially photographed to show the beast, but the photos do underscore the wonders of natural selection. His notes are indented:
Camouflage. A series of the cryptic to the near invisible.
First is a geometrid moth caterpillar. I was very lucky to see it on the bark of this native caper tree (Capparis mitchellii):
Next is the nymph of the world’s largest leaf-hopper Ledromorpha planirostris. These are very common on the bark of blue-gum trees but because of how they are flat and cryptically coloured they are often missed. Almost worthy of a spot-the! [JAC: this is a hard one!]
Next is a type of Crab Spider (Thomisidae) Stephanopis sp. These spiders have such good patterning and hairs and knobs to break up their outline they sometimes make your eyes swim looking at them on bark and trying to make out where the spider ends and the bark begins.
I was lucky to see this caterpillar for the White Banded Plane (Phaedyma sheperdi) out on a green leaf or I would have over-looked it. As it was I thought it was a weird chrysalis but it was just the way the caterpillar holds itself.
This one is amazing:
Last is one of my favourites, the Wrap Around Spider (Dolophones conifera). The way they perfectly wrap around small twigs and just look like a small bump is extraordinary. The only way I’ve ever found them is as they flee from their orb-web as I approach because once they’re on a twig they’re pretty much invisible.
Here’s a photo from The Daily Mail:
And another photo from Real Monstrosities:
From a tw**t by J. Rowley (h/t: Matthew Cobb), we have a very cryptic frog; the caption is “From #Moss to #Frog in a single move. It’s no wonder this species is called the Vietnam Moss Frog (Theloderma corticale)!” Actually, it’s called the “mossy frog”, is semiaquatic, and lives in the primary evergreen forests of Southeast Asia.
To enhance the crypsis, they curl up in a ball, like the one on the left, to hide their froggyness:
A photo from Wikipedia (go to this page to see a lot more):
And from WildFacts:
And here’s a short video:
Well, the answer is here, so I just wanted you to see this. The caterpillar is, of course, a larva, and the lappet moth is Phyllodesma americana.
Well concealed caterpillar. Lappet moth, using lappets to blend in pic.twitter.com/ApszpzKMou
— Richard Lewington (@rlewington2) September 21, 2016
The adult, when resting on bark, is also cryptic. Note how the head is tucked down and hidden:
Here’s another picture of the caterpillar; don’t ask me whether they can change colors (either within one period or depending on their habitat) or come in different colors”
h/t: Matthew
We have another visual stumper today, and this is a hard one. It comes from reader Mark Sturtevant. First his notes and the picture, and I’ll give the reveal later:
A funny thing happened when I was preparing this picture. I found a large underwing moth [Catocala sp.] on a dead tree trunk, and immediately set about taking pictures. One picture was taken at a distance so that the readers of WEIT might enjoy trying to find it. That moth is actually not too hard to find, but when I was preparing the picture to be sent to you I found a second underwing moth in the picture! I was at this tree for nearly an hour (there was a huge syrphid fly that also needed its picture taken), and I had no idea that the second moth was there. I am still pretty giggly about it.
Anyway, the readers will know what to do. But that 2nd one…. Let’s say your readers might go through a pot of tea before they find it. Good luck!
I’ll put up the reveal at about 1 pm Chicago time, just to give you plenty of time to spot the two moths.
Although these moths have brightly marked hindwings, they’re always covered by the highly cryptic forewings when the moths are hiding (they probably evolved to startle predators). You can see some photos of underwing moths here.
Oh, and try not to give away the locations of the moths in the comments. But if you found both, feel free to proclaim your perspicacity!
And click (twice if you want to eliminate the overlapping text) to enlarge.
Here’s today’s “where’s the __” reveal. Above the fold I’ll show the original photos, and then click “Read more” at the very bottom to see the reveals.
From reader Barn Owl: there’s a stick insect here:
And a hidden grass moth from reader Gabe McNett:
Reader Gayle Ferguson, you may recall, is a biologist at Massey University in Aukland, New Zealand (she once worked with Matthew in Manchester), and rescues batch after batch of orphaned kittens, for which she gets the title of Official Website Kitten Rescuer™. (One of the kittens she saved is Jerry Coyne the Cat.) She also does scuba diving, and took this photo of a camouflaged baby scorpionfish.
Gayle’s notes say “Photo taken on a scuba dive at the Poor Knights Islands off Tutukaka on the East Coast near Whangarei.”
Can you see it? It’s not terribly hard, but does show some nice camouflage:
Answer at 11 a.m. Chicago time.
Africa harbors three living species of zebras: the plains zebra (Equus quagga), with several subspecies, the mountain zebra (Equus zebra), and Grévy’s zebra (Equus grevyi). The taxonomy of the group is in fact disputed, as distinct species sometimes produce fertile hybrids when they live in the same place, but let’s not worry about that now. The most distinctive feature these species have in common is, of course, their stripes: they are the only fully-striped members of the genus Equus (zebras, horses, and asses), a group containing eleven species. Here’s what the three zebras look like (notice the difference in pattern):
Now why do they have stripes? If you’re asking the question as an evolutionary one, then one way to pose it is to ask, “What were the selective advantages to the ancestor of modern zebras of having the striped pattern?” (Stripes evolved only once, in the ancestor of zebras, so we don’t require a separate adaptive explanation for each species.)
But that presumes that the stripes were or are an adaptation. Perhaps the pattern wasn’t selected for itself, but is a byproduct of some other adaptive aspect of the zebra’s biology. Perhaps, for instance they’re simply a physiochemical result of a developmental constraint in the production of any body color in zebras. That’s possible, I suppose; but I suspect, given the distinct pattern, that it really did confer some reproductive advantage to zebras, or does so now. Let us assume that it did and does enhance the survival and reproduction of zebras, and see if we can find an explanation of how it does so.
I’ve previously discussed one theory for the evolution of stripes: resistance to biting flies. I’ll highlight a recent paper on that soon, but today we’ll examine another widespread hypothesis: stripes provide camouflage that hide zebras from predators, chiefly hyenas (which take young zebras) and lions.
First, though, let’s list all the hypotheses for striping: camouflage; resistance to biting flies, which won’t land on striped objects (more on that this week); “species recognition,” so that the stripes help zebras find other individuals or the herd; “aposematism”: conspicuous coloration that tells predators, “Stay away! I can bite and kick!”; and a way to cool off by reducing thermal load.
The “camouflage” hypothesis is actually several hypotheses: the stripes hide the zebras in grassland or woodland, making them harder to see; or the stripes break up the body outline so predators can’t discern them as “prey”; or that the stripes confuse predators when they’re attacking a herd, making it hard to single out one individual to nom.
A recent paper in PLoS ONE by Amanda Delin et al. (reference and link below) tests the first two parts of the camouflage hypothesis. They did this by determining the distance at which four species (lions, hyenas, other zebras, and humans) could discern the striped pattern under three light conditions: daylight, twilight, and darkness (moonless nights). (Most predators hunt zebras at twilight). They used measurements of these species’ eyes, photographs of mounted skins as well as of live zebras in the wild, and estimates of visual acuity taken from what we know about vision in domestic cats and passing photographs of zebras through filters mimicking cat’s vision.
The researchers wanted to know the distance at which the four target species (especially the hyena and lion predators) could discern the stripes. They used the “discern stripes” criterion for a good reason: predators attack from a distance, and if they can’t see the stripes at that distance, then those stripes can’t really function to hide the animal. (They could, however, still act to confuse a predator in the midst of an attack.)
The upshot: predators are lousy at discerning stripes from even moderate distances, well short of distances at which predators commit themselves to attack. Zebras and humans (especially the latter) are much better at seeing stripes at a distance than are hyenas and lions, and the ability to discern stripes gets exponentially worse as night falls.
Conclusion: at present, stripes don’t seem to camouflage zebras from predators.
I won’t go into all the details, but below are the data tables showing distances at which the four species can resolve stripes in open habitats—under three light conditions. The table below gives the maximum distance in meters at which stripes can be recognized in different body regions (three species of zebras, two body regions for each). The graph went across the page, so the six rows can be identified from the first shot below:
Humans are pretty good at discerning stripes in daylight, but at dusk (and especially at night), you can’t see the stripes more than 170 meters away on any zebra.
ZebraVision:
Zebras aren’t as good as humans at discerning stripes, but can still see them pretty well at daylight (at least as far enough away to see conspecifics within 75-200 meters), so maybe the stripes can help animals find their herdmates.
What the charts below show is that predators are lousy at seeing the stripes, particularly under low-light (hunting) conditions. At dusk, lions can’t see the stripes when zebras are more than 50 m away, and on moonless nights they have to be right on the zebras before they can see the stripes. Since lions commit to attack at distances much greater than 50 meters, it seems as if the stripes don’t protect zebras from being seen by lions. At distances of 50 m or more, zebras look much like unstriped prey: waterbuck and topi.
Hyenas are even more myopic for stripes: beyond about 30 meters, a zebra looks to a hyena just like any uniformly-colored prey.
Here are some photos showing what zebras would look like to humans and lions at only 16.4 meters away. The caption is this:
Fig 3. A small group of plains zebra taken at a real-world equivalent of 16.4 m as they may appear to a human (a,c,e) and lion (b,d,f) under photopic (bright; daylight), mesopic (dim; dusk) and scotopic (dark; moonless night) conditions. Stripe visibility falls off from human vision to lion vision and as ambient light decreases.
This is from the close distance of 16.4 meters. But even at that distance the stripes are not very visible at twilight (“mesopic”) conditions, while under moonless (“scotopic”) conditions the zebras are just gray blobs, looking much like antelope. As I said, lions attack from distances much greater than this, so zebras at twilight would look like any other prey item. In other words, the stripes don’t appear to camouflage the zebras at distances relevant to protecting them from predators.
It’s still possible, though, that the stripes could confuse a predator once it’s in the midst of a fleeing zebra herd. But I don’t find that particularly plausible, as an attacking lion tends to single out only one individual for attack, while hyenas act as a group when taking down an individual.
The authors did find that stripes did render zebras less conspicuous in woodland, as the vertical stripes tend to hide them amidst the vertical saplings. But this still obtains only at close distances, and, as the authors note, “Thus, stripes cannot help zebras blend in with the background except when a zebra is close to a predator, distances at which predators could likely smell or hear zebras moving or breathing as they are particularly noisy herbivores.” (There’s a hint of special pleading here!)
What about the other hypotheses? Aposematism may still be viable, as once a lion is close to a zebra it might shy away it because of the stripes signalling “don’t mess with me.” That could be tested by dyeing zebras in the wild, but I don’t find that theory very plausible.
The “social recognition” hypothesis for stripes is still viable. The authors try to dismiss it, though, by saying this:
We therefore cannot reject the hypothesis that stripes may assist recognition of conspecifics or individuals, although stripes promoting species recognition seem improbable given the limited extent of allopatry in the three species of zebra. Field observations do not support the idea of stripes enhancing allogrooming, social bonding, individual recognition or being an indicator of phenotypic quality or health. Nor is striping related to crude categories of social organization, namely harem defense polygyny or to resource defense across equids where social requirements might differ. Finally, domestic horses are capable of sophisticated individual recognition using visual cues in the absence of stripes and so it seems somewhat implausible that their close relative, the zebra, needs stripes to do this.
Well, “species recognition” is not a particularly viable hypothesis anyway, and I’ll accept the authors’ notes (there’s a reference given) that stripes don’t enhance individual recognition or other forms of bonding. But stripes still could help zebras find their herds more easily, and there are of course many advantages to finding your herd and rejoining it if you wander off. And the fact that horses can find their herds even though they’re not striped is irrelevant: the question is whether zebras’ stripes give them an enhanced ability to stay with their herd. We simply don’t know the answer to that.