Tadpole Maturation
The eggs of Wood Frogs, the earliest species of frog to breed in the Northeast, are just hatching and tiny Wood Frog tadpoles can be found swimming about at this time of year. This Green Heron is devouring a tadpole, but it is anything but tiny – certainly not a Wood Frog tadpole. How can this be?
The answer is that the tadpole that the Green Heron caught did not hatch this spring – it hatched last summer. Unlike Wood Frogs and Spring Peepers that mature in roughly two months, Green Frogs and American Bullfrogs can take two or even three years to metamorphose into adult frogs. By their second summer they are of substantial size. The Green Heron has caught a Green Frog or Bullfrog tadpole that has overwintered and would probably have matured this summer.
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Male Green Frogs Defending Territories
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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Green Frogs’ Coloring
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.
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All In A Day’s Post
Every day I head out with the hope of finding something interesting enough to write about and share on my blog, and which I can also manage to photograph. There are days when it happens within minutes, but typically it consumes the better part of half a day. I thought I would take this opportunity to try and convey the sensory experience of this endeavor by describing yesterday’s outing.
I arrive at the beaver pond late in the afternoon, hoping for a glimpse of the two beaver kits that have been seen here in recent days. Hidden behind ferns and shaded by young white pines, I set up my tripod and camera and settle down, hoping my arrival has not been observed. I am accompanied by Emma, my lab, who for 12 years has patiently sat by my side motionless as we waited for the expected and unexpected to present itself. The sun is close to disappearing behind the trees, but lingering light provides a warm glow to the pond.
Silence greets me, but not for long. There is no sign of beavers, but I am serenaded by a lone Hermit Thrush, bidding a sweet goodnight to the surrounding woods and all who reside therein. Soon after the Hermit Thrush’s flute-like song ceases, a chorus of plunking Green Frogs starts up. Still no beavers, but I hear a splash from a corner of the pond that is hidden from view, and out flies a Broad-winged Hawk, whose empty talons tell the tale of a failed attempt to catch a frog or other aquatic resident. I suddenly hear the high-pitched whining of young beavers coming from within the lodge that is roughly 150 feet directly across the pond from where I sit. This often occurs when a parent leaves the lodge, so I am on high alert. Cedar Waxwings appear, perching on snags and flying out over the pond to snatch insects from a recent hatch before returning to their perch.
The sun is all but gone as a lone adult beaver surfaces and heads to the far end of the pond. As silently as possible I walk along the side of the pond until I hear the familiar sound of rodent incisors gnawing rapidly on wood. There, at the shoreline, the beaver is cutting a branch off a limb of a sloping tree that is within its reach. Soon the chewing stops and the beaver grasps the cut branch in its mouth and swims the length of the pond to the lodge where its young eagerly await a freshly-cut meal. When it gets to within several feet of the lodge, the beaver silently disappears beneath the water and moments later is greeted with the exuberant, anticipatory whining of its offspring. With luck, I may have captured more than one post’s photograph, but even if I haven’t, my ears and eyes (and soul) have reaped enormous benefit from the effort.
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Young Green Frogs Absorbing Tails
Generally speaking, green frog tadpoles that hatch early in the season will transform into frogs by mid-to late summer. Tadpoles that hatch late are likely to overwinter in their ponds and metamorphose late the following spring or early in the summer. Thus, the Green Frogs you see now with both legs and a tail spent the winter as tadpoles and are maturing now.
A specific process called “apoptosis” occurs as the tail is absorbed by the frog. It involves programmed cell death, and occurs in various forms in multicellular organisms. Humans experience apoptosis throughout their lives. It is responsible for the separation of fingers and toes in a developing embryo, as the cells between the digits undergo apoptosis and it is responsible for the death of between 50 and billion cells each day in the average human adult. Once it has begun, apoptosis cannot stop, and thus is a highly regulated process.
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Green Frogs Mating & Laying Eggs
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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Green Frog Tadpoles Maturing
When you think about the structure and form of a tadpole, a plant-eating, streamlined creature designed for aquatic life, and that of a meat-eating terrestrial frog which is adapted for jumping on land, and know that they are one and the same organism, one can’t help but be impressed. The visible changes in this transformation are dramatic enough – legs emerging, head shape changing, tail being absorbed – but the internal changes a tadpole undergoes are just as dramatic. Research has shown that everything from a tadpole’s respiratory (gills replaced by lungs), urogenital and sensory systems to its digestive system (intestines becomes much shorter due to change in diet) is undergoing significant changes.
The length of time these changes take varies according to the species of amphibian. Most Green Frogs (Rana clamitans, pictured) undergo metamorphosis within the same breeding season or they overwinter as tadpoles and mature the following summer. (There are records of Green Frog metamorphosis taking up to 22 months). Biologists in Michigan found that eggs deposited before roughly June 25 were capable of developing in one season, whereas eggs deposited after roughly July 10 remained as tadpoles until the following year.
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Aquatic Frogs Hibernating in Ponds
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.
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