- 3.10 C investigate and compare how animals and plants undergo a series of orderly changes in their diverse life cycles such as tomato plants, frogs and lady bugs
- 4.10 C explore, illustrate and compare life cycles in living organisms such as butterflies, beetles, radishes or lima beans
- 5.10 C describe the differences between complete and incomplete metamorphosis of insects
Through the years our labs have investigated the life cycles of many of the animals suggested in these standards, such as butterflies, beetles, lady bugs, and frogs, but to tell you the truth there are all kinds of examples in the animal world that will help you teach the concept of metamorphosis. So before I start suggesting some ways to illustrate metamorphosis with your students, lets discuss a little bit about metamorphosis and why it takes place.
The word metamorphosis when broken down means, meta: changing, morph: shape, osis: the process of. This word is used to describe animals that go through a physical structure change during its lifetime. This could be a complete structure change, like larvae to adult creature, or a partial change like gaining body parts such as wings as an adult. Did you know that the majority of the animals here on earth go through some type of metamorphosis during their life cycle? This is because the majority of the animals on this earth are insects. Insects are not the only animals that can go through a metamorphosis, other animals such as fish, crustaceans, jellyfish and most famously amphibians go through metamorphosis as well. Biologists believe these animals have evolved to experience metamorphosis as an aid to survival. When these animals are in their different stages of metamorphosis, they do not compete for the same food source and fulfil a different niche in their environment. For example, caterpillars do not compete for nectar with butterflies because their food source is the leaf of plants, tadpoles live in water and feed on water plants and microscopic animals at the beginning of their life cycle, which doesn't compete with their adult insectivore counterparts and so on.
Live Materials and Watching a Metamorphosis Happen
As with any science lesson, reading about metamorphosis is not as exciting as actually watching it happen in the classroom. The easiest metamorphosis to witness is from amphibians and insects because, to put it simply, those are the easiest to teach and the easiest to find. This post will address amphibian metamorphosis and next post will address insects in the classroom.
Amphibians in the classroom:
An amphibian is an animal that needs to have access to both land and water because it is an animal that needs both to survive. Living in the El Paso area, this might become an issue because you will need to provide the proper moisture for an amphibian to keep its skin moist. We have hosted salamanders, frogs and toads and I will tell you right now, all have benefits and drawbacks. I prefer to raise tiger salamanders, woodhouse toads and tree frogs to illustrate amphibian observations but for metamorphosis I prefer large frogs. Larger species of frogs are easier to order in tadpole form from companies such as Wards and Carolina Biological, as well as your local exotic pet store, we use THE ARK.
Raising Grass Frog Tadpoles (Leopard Frogs)
At first when I started raising tadpoles, I ordered grass frog (leopard frog) tadpoles from science lab companies like Carolina Biological. In the spring, when the spawn were available, I would order 50-100 small tadpoles and set them up in classroom aquariums for our 5th grade classes to observe. The survival rate wasn't very good. Out of about 75-100 tadpoles, only about 1/4 survived to adulthood, which I found out later is about right for nature. One year most all of them turned into little frogs, so I transferred the batch of 60 little frogs to our 75 gallon river tank so that they could take advantage of a land/water environment. After the first day I found 40 frogs, the next 20. I soon realized that placing these tiny leopard frogs in a tank with adult leopard frogs was a disastrous mistake, and before I realized what was happening my little frog population was decimated by my hungry cannibalistic grass frogs and my 5th graders were heartbroken. I vowed to try to raise another batch of baby leopard frogs, however changes in our environment made it so I couldn't.
You see many science companies harvest frog eggs and tadpoles from lakes and rivers in the US. Because of environmental changes, both with climate (a later winter season) and pollution (chemical dumping in waterways) the next year the tadpoles didn't have the same viability, and were not available until late April, early May (which was after Science testing.) I needed to find a heartier species, one that was available before the science exam and would actually survive, which brings me to, African Clawed Frogs.
Those of us who have been around for awhile and survived TAKS testing, may remember a question about the "African Clawed Frog" which made them a very popular lab animal. Xenopus frogs are a very hardy species. They are mainly aquatic and are very prolific. I didn't like them very much because, well quite honestly I find them kind of ugly. They have a weird shape and because they are mainly aquatic, its hard to show how amphibians need to get out of the water and be on land. The deal breaker for the african clawed frog for me, was the fact that they are invasive! There are a lot of people who at the end of the school year will release their animals into waterways and that is a bad thing. African clawed frogs will eat everything in sight, including dead animals, frog eggs, bird eggs, other frogs and its own spawn. Because they are so competitive, they have become an invasive species of California, Louisiana and Florida as well as other states. I didn't want that responsibility, so I moved on to an easier tadpole to raise and enjoy.
Now we're talking! I was at a conference and a vendor was selling bullfrog tadpoles on the vendor floor. I couldn't resist, I had to take them home with me. You should have seen me try to take my little guys through airport security. Luckily I could buy a bottle of water at the other end of the security line because I had to dump out all but an inch of water from their little tank in order to get them through security and on the plane. So what made me fall in love with them? First of all, THEY ARE HUGE!!! Much bigger than the tadpoles of other species. I find this a good thing, especially for 3rd graders because a big tadpole means that you can see the gills up close and the buds of legs start to form. Also they take a while to metamorphosize. A bullfrog tadpole can take up to two years to make the change to an adult. Bullfrogs have been in the El Paso waterways for a while, (my father remembers them being in the canals and river when he was a kid,) but they do not originate in the western U.S.. There is suspicion that they came as a result of fish stocking. The reason I like to use them for observation in the lab is because they are very hardy creatures, easy to care for and feed (they like timothy hay pellets, which is rabbit food) when they are in their gill stage, and once they are baby frogs they eat small to medium sized crickets just like my green tree frogs. The kids absolutely love watching them change and its something the kids can definitely monitor for the entire second semester of school.
When they make it to frog stage you need to be careful. After years of experimenting I have found that my new bullfrogs like a muddy terrarium with a deep dish to swim in (you definitely need both land and water.) They are nocturnal so they like to hide under logs or any cave like feature you place within the terrarium. They are also very competative with other animals, so don't place them with green tree frogs even if they seem to be the same size. Bullfrogs grow quickly and will starve out any frogs you place in a tank with them. The rule for amphibians is that you need to put similar sized creatures together or they will either eat each other or starve the other.
Lesson plans with with tadpoles:
Generate Questions: when introducing an animal into the classroom the best thing to do is to generate a list of questions about the creature that the kids want to know. This is a great way to get started because it develops a purpose for reading when the students do their research and it also gets the kids to start observing structures and behaviors of the animal right away.
Create a table about the animal: Have the students organize the information about bullfrogs in a table for quick reference. Critique the table designs to see which ones are the most efficient for extracting information. Things to be charted might be: lifespan, environment, food, predators, etc.
Draw and label a chart: Have the students draw and label the animal as it goes through its stages. Make sure the students identify when the tadpole is going from the gill to lung stage. Things to observe is the shortening and absorption of the tale and widening of the mouth.
Topics for discussion: Things to discuss with the kids about frog metamorphosis can be competition for environment (do tadpoles and frogs compete for the same food?) What type of structures or adaptations do the different body stages have that help them survive in their environment? What is the difference between breathing air with gills and breathing air with lungs? This would be a great time to discuss oxygen as a solution in water. Gills can draw oxygen out of a water solution Many kids (and adults) think that the water molecule of H2O is where the oxygen is drawn out, but actually the gill cannot break that chemical bond. When water is moving (like with an air pump in an aquarium) water becomes oxygenated, it becomes a solution of gas and water that the gills can extract oxygen from. That is the reason why water plants and water pumps are both good for aquarium life because it allows the water to become oxygenated so that animals with gills can breathe oxygen to survive.
To learn more about setting up a bullfrog tadpole habitat, check out the Live Materials 101 page and download the Bullfrog Tadpole Habitat recipe card
Safety with Amphibians in the classroom:
Our standards ask that children be given opportunities to do observational studies using aquariums and terrariums. This does not mean that the kids should be handling the amphibians. Amphibians can carry salmonella which can be a very dangerous bacteria for children. If you or your students insist on touching the frogs, this danger can be resolved by washing hands with an antibacterial soap. Most of the dangers with handling amphibians actually are to the specimen. Their bone and body structure is fragile and leaps from table to floor can damage the frog. Another thing to consider is that oils and dirt from human hands can damage their skin. As a rule we try to only observe animals rather than touch them. Lastly, at the end of the school year do not release amphibians into our waterways. Try to find a home over the summer or take the animal to a local petshop or live material center.
Studying tadpoles in the WILD
After some good rains in El Paso, ponded areas will experience the very quick metamorphosis of woodhouse and spadefoot toads. These amphibians are native to the El Paso area and your students probably already have experiences capturing either the tadpoles or the adult of these creatures. Please note that toads are different than frogs in the respect that their skin harbours toxins. Please teach your students to wash their hands after handling any animal. Our native toads need water to survive, so they have very short metamorphic cycles (as short as a few days) and will gorge on insects and then bury themselves underground until the next rain. This is a great topic to research as well as observe in our own backyard.
Next topic for metamorphosis: Insects and complete and incomplete metamorphosis!
-Techscichick!