The legs come fully into action only after metamorphosis is completed. While the hind limbs grow, the tadpole also grows and the tail remains the primary means of locomotion. They originate as little buds from the rear of the torso, grow into paddle-like structures and then elongate into muscular, articulated limbs at the base of the still-existing tail. The first external sign that a tadpole will not always remain a tadpole appears in the gradual development of hind limbs. And yet the two are inextricably connected the one cannot exist without the other. The fully aquatic, herbivorous tadpole bears no resemblance to the tailless, four-legged, carnivorous croaking and leaping frog. If you only observed, side-by-side, a tadpole and an adult frog, you would have no idea that the two animals have any connection with each other. But, more typically, they live as tadpoles for two to three seasons before metamorphosing. Bullfrog tadpoles ( Rana catesbeina) grow large - often around four inches (10 cm) long - and, depending on circumstances, can sometimes metamorphose into frogs in the fall (four to five months after hatching). The time is shorter when there are higher water temperatures and ample food, and longer when there is colder water and little food. A wood frog tadpole ( Rana sylvatica), for example, usually metamorphoses into a froglet within two or three months after hatching in the northeastern United States. How long a tadpole lives before it metamorphoses into a frog is dependent on the species and on outer conditions. Like fish, tadpoles have a lateral line organ, which runs along each side of its body and tail, through which they sense movements in water. They have no limbs, and swim through water via undulating movements of a long boneless tail fin. They remain submerged in water and breathe through their skin and gills. And yet, tadpoles are fish-like in many of their characteristics. With a thick squat body that abruptly tapers to a long finned tail, a tadpole definitely does not resemble a fish. Most of you have probably seen tadpoles in ponds and vernal pools. But my aim here is to provide a general picture of metamorphosis. They are fascinating and warrant consideration when you really want to understand the peculiarities of given species or genera and the variations within the amphibians. Because of this, for probably every characteristic I describe there are exceptions. There is an astounding variety of ways in which different species of frogs develop - some have no tadpole phase, some have tadpoles that are carnivorous rather than herbivorous, some frogs remain aquatic for their entire life cycle, and so on. I will focus on pond-dwelling tadpoles and their metamorphosis into land-dwelling frogs, as exemplified by many species that live in temperate climates. One caveat: I will be limiting my descriptions to those tadpoles that develop into frogs (not the tadpoles that develop into salamanders). New and exciting questions arise about how we conceive of development - including that trans-species developmental process we call evolution. We need to be keenly aware of what we mean, and what we don’t mean, when we say, “A frog develops out of a tadpole” - or a tadpole out of an embryo, or an embryo out of a fertilized egg, or an adult human being out of a child.Īs we will see, when we give careful attention to what is actually happening when a new phase of life develops out of a previous stage, there are large implications for our overall understanding of developmental processes. The frog comes into existence as the tadpole disappears out of existence. But just as surely, nowhere do we find the frog in the tadpole. But does it? Surely, without the tadpole the frog does not develop. Where does a frog come from? The answer seems obvious. It is also a chapter in Craig Holdrege’s book, Seeing the Animal Whole - And Why It Matters, published by Lindisfarne Books, 2021. The content of this article is incorporated into a longer monograph, Do Frogs Come From Tadpoles? Rethinking Origins in Development and Evolution. Do Frogs Come from Tadpoles? Understanding Development as Creative Activity Craig Holdregeįrom In Context #33 (Spring, 2015) | View article as PDF
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