Have you ever wondered how grapefruit (and larger)-size masses of up to 2,000 eggs come out of a 2- to 3-inch-long Wood Frog that weighs about a third of an ounce? It turns out that a Wood Frog’s eggs are highly compressed into a mass the size of a golf ball when laid! In under an hour they have absorbed enough water to have expanded to the size we usually associate with Wood Frog egg masses. (Photo: three freshly-laid (darker) egg masses amongst older egg masses)
Usually it’s your ears that tell you that Wood Frogs (Lithobates sylvaticus) have emerged from hibernation. They are one of the first amphibians to announce themselves, often appearing before snow and ice are completely gone. Male frogs congregate in shallow ponds and vernal pools, where their ardent courtship ensues.
Unlike most frogs, which have a single vocal sac below their mouth, Wood Frogs have two paired sacs, called paired lateral vocal sacs, located on either side of their body just behind their head (see photo). With their mouth and nostrils are closed, male frogs pump air back and forth from their lungs to their inflated vocal sacs which vibrates their larynx and produces a duck-like quacking sound. The vocal sacs act as resonating chambers, amplifying the frogs’ calls so as to attract females from far and wide.
Vocal sacs serve a dual purpose for some frogs. As soon as the eggs of Darwin’s Frog (Rhinoderma darwinii), found in Chile and Argentina, hatch, the male scoops the tadpoles into his mouth and they spend the next six weeks metamorphosing inside his vocal sac. The male does not eat until the tadpoles have matured into adults and exited his mouth.
Male Green Frogs are extremely territorial. During June, July and August they establish a series of territories, often claiming each one for less than one week before moving on.
The desirability of a given territory is of utmost importance – the better the quality (amount of vegetation) of your territory the more likely it is that a female will choose you to fertilize her eggs. The more time a male spends in a high quality territory the greater the number of mates he will acquire.
Considerable aggression is displayed against other males when an individual is claiming and maintaining a territory. When a competing male arrives, a wrestling match often ensues (see photo).
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Wood Frogs have emerged from their hibernacula and migrated to their ancestral woodland breeding pools, or vernal pools, to sing (males), mate and lay eggs (females). A chorus of duck-like quacking reveals where these hidden temporary bodies of water are located. The male Wood Frog in this photograph is on top of a female, grasping her behind her front legs in a hold referred to as “amplexus.” They will remain in this position until she lays her eggs and he then fertilizes them externally. Note that even though he has attracted a female and is in the process of mating with her, the male is continuing to sing (one of his two vocal pouches is inflated on the near side of his body).
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Every year in late July or August a young Spring Peeper crosses my path and I cannot resist photographing it. At this time of year they are roughly the size of a Japanese Beetle, and during early mornings and late afternoons can be found in shrubbery where they intermittently rest in the shade and feed on insects even smaller than themselves. If you live near a pond where it sounded like sleigh bells were ringing last spring, keep an eye out for these irresistible frogs that are no bigger than your tiny finger nail.
At this time of year one would expect to find Leopard Frogs lying on the bottom of a pond, partially but not completely covered with leaves or mud as they hibernate their way through winter. Because of the depth of a pond, and the fact that in winter the water temperature is around 39°F., ice isn’t an issue at the bottom of a pond, and the frogs and turtles that overwinter there don’t usually freeze.
However, sometimes ponds freeze over before amphibians or reptiles that overwinter in them arrive at their hibernacula. Apparently this is what happened to these Leopard Frogs, and they took refuge in the only open body of water they could find – a large but shallow puddle about 10’ wide by 20’ long in a dirt road. Shortly after they arrived temperatures dropped and the frogs were trapped under (and eventually will be encased in) the ice. Unlike Wood Frogs, Spring Peepers and Gray Treefrogs, Leopard Frogs are not freeze tolerant, so their demise is inevitable. (Thanks to Kelly Maginnis for photo, and Jim Andrews for his herpetological expertise.)
The migration of amphibians from the woodlands where they overwinter to their breeding pools in March is a familiar spring phenomenon to most. However, some of these amphibians engage in a fall migration as well. Amphibians (and reptiles) need to find a good overwintering spot, where they won’t freeze. In some cases, that might be a few feet upstream or into a seepage area, and in others it is a few hundred yards uphill. The extent of the migration and the species participating vary in different parts of the state.
In western Vermont, Jim Andrews, Director of the Vermont Reptile and Amphibian Atlas(http://vtherpatlas.org), has observed Blue-spotted and Four-toed Salamanders moving down into wetlands in the spring, staying close to the wetland for moisture and feeding during the summer, and then moving uphill back to wintering areas in the fall. On the other hand, he has found that most Spotted and Jefferson Salamanders, as well as Wood Frogs, move down in the spring to breed, and then head back uphill in stages soon after they are done breeding.
Eastern Red-backed Salamanders (pictured) migrate as needed during the summer to maintain their body moisture, and in the fall, search for an animal burrow or crevice to hibernate in. Migration often occurs at night, while it’s raining, so as to prevent their skin from drying out. (Amphibians need to have moist skin because a large portion of the frog’s “breathing” occurs via oxygen diffusing through the skin. Oxygen will not diffuse through a dry membrane.) After the very dry summer we’ve had, a large number of amphibians were observed taking advantage of the recent rain and migrating.
Despite their name, Green Frogs are not always green. They can be brown or tan, as well as many shades of green. Usually Green Frogs in the Northeast are a combination of these colors, but occasionally one sees greenish-blue coloring on a Green Frog. An understanding of what causes a frog’s green color sheds light on why sometimes all or part of a Green Frog may be close to turquoise than green.
Basically there are three types of pigment cells (chromatophores) which stack up on top of each other in a frog’s skin. The bottom layer (melanophores) of pigment cells contain melanin, a pigment that appears dark brown or black. On top of these cells are iridopores, which reflect light off the surface of crystals inside the cells. When light hits these cells, they produce a silvery iridescent reflection in frogs, as well as other amphibians, fish and invertebrates. In most green frogs, sunlight penetrates through the skin to the little mirrors in the iridophores. The light that reflects back is blue. The blue light travels up to the top layer of cells called xanthophores, which often contain yellowish pigments. The light that filters through the top cells appears green to the human eye.
The pictured turquoise-headed Green Frog most likely lacks some xanthophores in the skin on its head, and thus we see reflected blue light there.
Congratulations to Naturally Curious readers for their familiarity with Eastern Gray Treefrog (Hyla versicolor) toe pads! This is the easiest time of year to find these amphibians, mainly because of their habits. During the fall, winter and spring treefrogs are usually silent, unlike the summer, when males can be heard calling their melodic trill from bushes and trees near bodies of water after the evening air temperature rises above 59°F. Because they are nocturnal, well camouflaged, and hibernate in the winter, you don’t often come across one except for the warmer months when males are calling. The colors of an Eastern Gray Treefrog (brown, green and pearl-gray) vary with the colors of its background and environmental factors such as season and humidity, but shades of gray are most common. Their green color is more prominent during the breeding season and in young frogs.
When hunting insects, or when disturbed, treefrogs can leap great distances and, thanks to advanced toe pads, when they land they can cling to practically any surface, including vertical branches and leaves that are wet. A very low angle between the toe pads and substrate as well as mucous glands located in channels between the hexagonal pattern on a treefrog’s toe pads have inspired the design for treads on car tires.
You can listen to an Eastern Gray Treefrog’s call by going to http://langelliott.com/calls-of-frogs-and-toads-of-the-northeast/ (Sound recording © Lang Elliott – langelliott.com)
The loose banjo string call of the Green Frog is a familiar sound near wetlands this time of year. Males (to the right in photo, with bright yellow throat) have been busy serenading females (to the left in photo, with white throat), in an attempt to breed with one. Female Green Frogs select their mate, a choice which is based partially on the suitability of the male’s territory (underwater plants are a plus). After inspecting several males’ territories at night, the female chooses one and slowly approaches him, turning to face away from him as their bodies come in contact. External fertilization takes place as he clasps her while she lays her eggs (known as “amplexus”).
Unlike Wood Frog eggs that are laid in clumps, or Spring Peepers’ individually-laid eggs, Green Frog eggs are laid in a loose cluster that often floats on the water’s surface (see photo) or is draped on underwater plants. Each cluster usually consists of 1,000 to 5,000 eggs that hatch in three to five days. Females sometimes return to breed a second time with a different mate, in which case the second egg clutch is usually smaller, consisting of about 1,000 to 1,500 eggs.
The larval, or tadpole, stage of a Green Frog lasts from 3 to 22 months, which explains why you might have already seen large Green Frog tadpoles this summer.
(Outstanding theories were submitted on yesterday’s mystery. Be sure to read comments!)
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Pickerel Frogs emerge early in the spring from their muddy, pond bottom hibernacula, and mate in April and May in the Northeast. As part of the mating ritual, males call to attract females, with the calls resonating inside their internal vocal sacs located between their tympanum (ear drum) and foreleg (unlike Spring Peepers and American Toads, whose vocal sacs are located directly under their mouths).
These low-pitched calls resemble short “snores.” Occasionally Pickerel Frogs call from under water, but even when they are above water, their calls do not carry very far, frequently making it difficult for human ears to hear them. Their call is similar to that of the Leopard Frog’s but lacks the short grunts of a full Leopard Frog call. You can compare these two calls (and several others) by going to http://langelliott.com/calls-of-frogs-and-toads-of-the-northeast/ (Sound recording © Lang Elliott – langelliott.com)
Every year in early spring on a rainy night spotted, blue-spotted, Jefferson and four-toed salamanders, wood frogs, and spring peepers leave their leaf litter and subterranean winter hibernacula and migrate to their ancestral breeding pools to mate. Sometimes there are roads between these two sites. Vehicles driving these roads inevitably kill thousands of salamanders, wood frogs and spring peepers every year. Concerned citizens have set up teams to try to scoop up these nocturnal travelers and escort them to the side of the road in which they were headed. While this action definitely helps, it is hard to find enough generous volunteers to man every crossing on every road all through a rainy night.
In Monkton, Vermont, Jim Andrews, Steve Parren and Chris Slesar, along with the Lewis Creek Association and the Monkton Conservation Commission, spear-headed an effort this past year to do something about the mortality of hundreds of migrating frogs and salamanders. Grants, plus a large number of organizations and citizens, provided the manpower and finances to construct two concrete culverts under a road in a location where the road separates the breeding pools of amphibians from their upland wintering grounds.
In the past, hundreds of migrating amphibians were killed in this location by automobiles during every spring and fall amphibian migration, and it is doubtful whether the population would have been able to sustain itself over time. (In the past month, 673 salamanders and 329 frogs have safely passed through the tunnel. In addition, a few early migrators (16) have already started coming back up hill.) Hopefully, these will be the first of many such culverts in the Northeast. For further information on the Monkton underpasses, go to: http://www.burlingtonfreepress.com/story/news/2016/03/28/hundreds-saved-new-vermont-salamander-crossing/82336084/. (photo: spotted salamander)
American crows obtain most of their food on the ground as they walk along in search of seeds, insects, frogs, snakes, bird nests and small mammals. Their hunting techniques are varied and most involve the use of their bill. In search of invertebrates, crows will probe the soil with their bill, flick aside leaves, dig in the soil and even lift cow paddies. They fish for tadpoles and dig nearly an inch deep with their bill for clams. In winter, their foraging continues and as these tracks indicate, when the snow is only a few inches deep they will walk around and around in a given area, probing tufts of grass for hibernating insects, mice, voles, or any other form of life these opportunists find.
Most Green Frogs have disappeared over the last few weeks, but this male (eardrum, or tympanum, is larger than his eye) was basking in the last bit of sunshine he will see or feel for the next five or six months. Soon he will take the plunge and bury himself in leaf litter at the bottom of the pond or lay, partially exposed, on the mud beneath the leaves. (Green frogs typically hibernate in water, but occasionally overwinter on unfrozen stream beds or seeps, as well as underground.) Aquatic turtles can shut down their metabolism to a greater extent than frogs, so they are able to survive hibernation buried in mud, where there is little oxygen, but frogs overwintering in a pond must have their skin at least partially in contact with oxygen-rich water. Green frog tadpoles will typically, but not always, overwinter prior to metamorphosing the following spring.
The peeps of male Spring Peepers can be heard fairly consistently this time of year. Unlike in the spring, these calls are coming not from bodies of water, but from the woods nearby. And they are single peeps coming from individual peepers, not the chorus of “sleigh bells” one hears in the spring. This phenomenon occurs so regularly in the fall that herpetologists have given it a name – “fall echo.” They speculate that the calling of peepers is spurred by light and temperature conditions, when fall climate conditions are similar to those of spring.
Next to the Green Frog, the Pickerel Frog is the most abundant frog in New England. It is often confused with the Northern Leopard Frog, which it closely resembles. The spots on a Pickerel Frog’s back are squarish and aligned in rows, whereas the Leopard Frog’s spots are rounded, and randomly scattered over its back. In addition, the male Pickerel Frog has bright orange on the inner surface of its hind legs, which the Leopard Frog lacks.
Recently male Pickerel Frogs have started calling to attract mates. Each species of frog, just like each species of bird, has its own distinctive call. Spring Peepers “peep, “ Wood Frogs “quack” and Pickerel Frogs “snore.” Their snore isn’t long – it only lasts a second or two — but it is unmistakable. Pickerel Frogs call from under water, as well as on top of mounds of vegetation, so if you hear one, and then search for it, you may not find it. To hear a Pickerel Frog, go to http://langelliott.com/mary-holland/pickerel_frog_VA.mp3. (Sound recording © Lang Elliott – langelliott.com & miracleofnature.org)
Starting with today’s post, my blog will occasionally be enhanced with the sound recordings of Lang Elliott. For those of you who may not be familiar with his work, Lang Elliott has made world-renowned recordings (that are commercially available) of the vocalizations of birds, mammals, insects, frogs and toads. If you’ve ever wondered what out-of-sight creature was singing, screaming, trilling or buzzing, his CDs and books will give you the answer. To learn more about the work of this author, speaker, cinematographer, sound recordist, and nature poet, visit http://www.langelliott.com.
Most aquatic frogs such as this Green Frog have been deep in hibernation for several months. A common misconception is that frogs spend the winter the way aquatic turtles do, dug into the mud at the bottom of a pond or stream. In fact, hibernating frogs would suffocate if they dug into the mud for an extended period of time. A hibernating turtle’s metabolism slows down so drastically that it can get by on the mud’s meager oxygen supply. Hibernating aquatic frogs, however, must be near oxygen-rich water and spend a good portion of the winter just lying on top of the mud or only partially buried. They may even slowly swim around from time to time.