It’s great when you can get really close to the wildlife, i.e., right in their “faces”, to grab a “mug shot”. I didn’t get many this year, but there were a few opportunities, assisted either by my extremely long (600 mm) telephoto lens or my trusty macro, to get up close and personal with the local wildlife on our travels and in my backyard.
Hiding in plain sight by matching the color of your skin to the color of the background is useful for predators stalking their prey or prey trying to escape predation. It’s also useful in confusing photographers about where to aim their cameras. Case in point: I saw the bushes moving, wondered if it was a bird, thought it might be a chipmunk, but stared right at the little critter for a full minute before I figured out what it was.
Many species of fish, amphibians, and reptiles have the ability to change color and outward appearance by contracting or expanding the chromatophore (pigment-containing) cells in their skin. This chameleon-like ability can assist them in better regulating their body temperature, or as suggested, in achieving the kind of camouflage that helps them evade their predators or fool their prey. It is thought that the animals’ eyes perceive the color spectrum of their environment and send neural signals to skin chromatophores to effect the appropriate color changes.
In cases where there doesn’t seem to be a clear message about the color of the background, Gray Treefrogs are often mottled gray and green, or just remain gray, as illustrated below.
I heard a few faint, froggy croaks coming from the dying perennials in the garden. It seemed a little late in the season for the local amphibians to be active. But wind and coolish temperatures didn’t deter an appearance by this little American Toad — hiding in the litter under the aging peony vegetation.
Amphibian skin is really quite special. It can absorb water like a sponge when its owner is dehydrated, and it can secrete a variety of chemicals its owner might use for defense. Poison dart frogs aren’t the only amphibians to use toxic chemicals to deter predators; several Toad species (in the genus Bufo) secrete a combination of chemicals called “bufotoxin”, which induce a variety of debilitating reactions in animals that try to eat them. Although concentrations of bufotoxin vary from species to species, one website claims that “the skin of an average-sized toad can cause significant symptoms and even death in humans and other animals”.
Pet owners beware — don’t let your dogs lick any toads.
Last year I wrote about the possible basis for the many variations of eye color in birds (click here to read that post). Unlike most mammals which sport a limited variety of pale blue, gray, green, to dark brown shades, I’ve noticed that frogs and toads, like birds, also exhibit a rich variety of eye colors. And so, “eye” wonder why?
But these highly colorful eyes are tame, compared to those of a couple of South American frogs.The variety of eye color in frogs and toads is astounding, as captured in this collage by Jodi Rowley.
Surely all this ocular advertisement has purpose — beyond frightening would-be predators? Any speculation from you, dear readers?
In addition to the flash of color provided by the wide-open frog eye, you may have noticed that frogs have the ability to project their eyes outward from their head, or retract the eyes inward level with their skull.
Extrinsic eye muscles that elevate the eye above the level of the head actually give the frog a 360 degree view of its environment. Movement of the eyes downward presses on the roof of the frog’s mouth, helping to propel food down the back of the throat toward the stomach. Eyes — the multiple use organ!
Green is a popular color for frogs, and birds too, but that lovely green color doesn’t come from a green pigment as you might expect, but from the interaction of multiple layers of specialized color and light-reflecting cells in the upper layers of their skin.
Frogs have three layers of chromatophores (color-producing cells) in their skin. The deepest layer are melanophores that produce melanin pigment giving skin a brown to black color. The middle layer are iridiphores which contain no pigment but instead have mirror like plates capable of producing iridescence, or when viewed from a certain angle, reflect blue light. The most superficial layer of chromatophores are xanthophores that contain a yellow pigment. When the middle layer of iridophores interact with the top layer of yellow-pigment containing xanthophores, you get what elementary school students have learned: yellow paint + blue paint = green color.
Animals that change their skin color (like some frogs and reptiles) not only can change the shape and size of the chromatophore but the dispersion of the pigment within the cells. Thus, tightly contracted chromatophores with no dispersion of pigment might appear white, and the opposite pattern would look black. Moderate dispersion of melanin pigment in the deepest layer of chromatophores looks gray. Dispersion of pigment in the xanthophores coupled with dispersion of light-reflecting particles in the iridophores produces variation of yellow, blue, and green colors in the skin. Combination of all of the above produces the mottled pattern of coloration the animal uses to blend in with its background — a protective camouflage.
Color change seems to be temperature dependent: warmer frogs are lighter in color to reflect incident light; darker frogs are generally cooler and the darker skin helps them absorb heat. Skin color becomes lighter when these frogs are placed on a brighter background, and darker when placed on a dark background. This color matching is part of the effective camouflage protection. And lastly color change may reflect the mood of the animal — e.g., sexual display, territorial display, etc. The stimulus for all of this change begins in the brain, is transmitted by hormones, and carried out by the actions of the chromatophores — all in a matter of seconds to minutes.
A really detailed and fascinating discussion of how color change is achieved in animals (in this case a Chameleon) is shown below.
The Eastern Gray Treefrogs are most often found on vegetation, usually high up in the trees, rather than near water, as their name implies. Actually, I most often see them in the outdoor tank we use to water potted plants.
Their skin has a warty appearance, like that of toads, and they manage to stay hydrated without taking refuge in pools of water, even during hot days. Gray Treefrogs tend to hunt at night, often near an outdoor light fixture where they can capture insects coming to the light.
Skin color in these frogs is highly variable (hence their scientific name — Hyla versicolor). Three layers of chromatophores in the skin (color-producing cells) combine to produce the color pattern, which can change (very slowly) from near white to very dark brown, with variations of mottled green and gray in between. Treefrogs don’t match their environment perfectly the way a chameleon does, but their color changes do help camouflage them in their environment somewhat. More on how this is accomplished in the next post! Stay tuned.
Off and on rainy days with warm(er) temperatures have really set the frogs in motion here. I sat on an upended bucket by one of the ponds in the backyard and trained my binoculars on the pond’s edge, hoping to finally locate those tiny little creatures that were making that hugely loud racket.
Eventually, by trying to ignore the loudest chirpers, I could distinguish three different types of calls, all at different pitches, frequencies, and volume. Finally, by staring at the water where I thought the calls were coming from (this takes sound localization ability, which apparently I am lousy at), I found the frogs making them.
Wood frogs are found almost everywhere in central and eastern North America; they are one of the first frogs out in the spring, as soon as the ice melts off the ponds. They are unique in that they tolerate being frozen solid over the winter, as they “hibernate” in a state of suspended metabolism under the leaf litter. Click here to view a short video of wood frogs calling (in Minnesota). And here is an amazing video of Wood Frogs defrosting from their frozen winter state.
By far, the loudest, shrillest, and almost deafening calls were coming from the Boreal Chorus Frogs (Pseudacris maculata), which were almost impossible to find, mostly because I was looking in the wrong places. They were sitting at least 3 feet from the shore, out in the submerged vegetation, oriented vertically, with their head, vocal sacs and abdomen out of the water. Perhaps the volume of noise from multiple frogs confuses predators as well. Are herons and egrets immune to this racket?
So, I moved to a better viewing spot, sat, and waited until they got used to my presence, and then got closer-up views of a calling Chorus Frog. These frogs (once you see them) are easily distinguished by the three longitudinal stripes down their backs.
If you haven’t had the pleasure of standing next to a group of calling Chorus Frogs, you can view a short video here. These tiny frogs can emit sound at nearly 90 decibels, which is about as loud as the human ear can stand without damage, about the same as standing next to a truck without a muffler. And, they call continuously for hours and days on end. Now how is that for amazing!
For good photos and descriptions of MN frogs and toads, you can click here.
I happened to notice blue speckles on the face of the frog in the last post and thought it might be an aberration of the image capture.
But it’s not — it’s due to the chromatophores in the frog’s skin. It’s the answer to why frogs are green, or blue, or red and orange, or how they change color from one shade or pattern to another.
The pattern of color in this subadult Gray Tree Frog (also inhabiting the rain barrel along with the adult in yesterday’s post) shows the range of colors in this species — from almost white to dark green.
If you zoom in, you see patches of skin that are white, various shades of gray, and a range of shades of green almost to black. On the right side of the photo are patches of skin reflecting blue-green instead of green, which suggests the use of structure to produce color (as discussed earlier in birds).
It turns out that frogs have three layers of chromatophores (color-producing cells) in their skin. The deepest layer are melanophores that produce melanin pigment giving skin a brown to black color. The middle layer are iridiphores which contain no pigment but instead have mirror like plates capable of producing iridescence, or when viewed from a certain angle, reflect blue light. The most superficial layer of chromatophores are xanthophores that contain a yellow pigment.
Frogs that lack xanthophores are various shades of blue, like this blue poison dart frog found in Brazil. Black spots are created by the melanophores underlying the iridophores.
(photo from Wikipedia)
So the answer to why frogs are green comes from the interaction of the chromatophores in the top two layers — iridiphores that produce blue color and xanthophores that produce yellow color. As any school kid knows, mixing yellow plus blue yields…
(photo by co-blogger Alison)
And, more amazing is the fact that Gray Tree Frogs can change color to suit their background (or their mood) within seconds, not as fast as a chameleon, but fast enough. They do this by dispersing (darker color) or contracting (lighter color) the pigment granules within the chromatophores or even changing the entire shape of the cell.
Where did that expression “like shooting fish in a barrel” come from? Meaning it’s so easy, you can’t miss? Well, you can’t really miss when you take photos of tree frogs in a rain barrel either, except when you have absolutely no room to get behind the camera viewer and have to hold it over your head and point randomly downward hoping the subject is in the photo, like this.
Gray Tree Frogs hang around landscaped dwellings in the summer after they breed in the nearest pond. They especially like outdoor lights…and rain barrels, where they find lots of tasty insects to eat. They climb vertical surfaces with ease using those super-sticky toe pads.
(Photo from Hanna and Barnes, 1991, Journal of Experimental Biology 155: 109)
However, there is no glue or adhesive secreted, just the simple physics of the surface tension developed by a large area of moist skin pressed against a flat surface. It’s the same principle used for suction cups that stick to vertical glass surfaces. For greater sticking power, frogs press their belly and thigh skin against the surface as well, and can stay put at angles of as much as 150 degrees from horizontal (i.e., tending towards upside down). Read more about how this was determined here.
(Photo by co-blogger Alison)
I was filling the rain barrel we use to automatically water the potted plants and found a visitor sampling the fresh water. When the frog saw me, it dropped to the bottom of the 50 gallon tank, but eventually made his way up to the top again and allowed me to take its photo before departing.
The yellow coloration inside the hind limbs and the slightly warty skin, along with the rounded disk-like suckers at the tips of the toes make this a Gray Tree Frog (Hyla versicolor). It can change color (slowly) from gray to green, which is why they are not Green Tree Frogs. They are found all over eastern North America in deciduous forests, and are usually active at night up in the trees, although they do descend from the trees to breed. Perhaps this was a female, checking out my rain barrel as a potential egg laying site.
This is somewhat of an unusual frog because it has double the number of chromosomes of its close cousin, Cope’s Gray Tree Frog (a southern species). Usually doubling of genetic material is a bad thing, but apparently not for this species.