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Wood Frogs

Biology of the Wood Frog

 

 

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Wood Frogs in the Classroom!

(Click links to see images)

By Doug Collicutt

Raising wood frogs, from eggs to froglets, is a practical and rewarding classroom project. Eggs are readily available in the spring and with proper care, froglets are ready for release back into the wild by mid-June. With a minimum of materials and time, students can experience one of nature's most spectacular transformations.

Legal Stuff (The times we live in!)

Removing wood frog eggs, tadpoles or adults from the wild to rear or maintain in captivity for educational purposes, either private or institutional, is an acceptable practice according to the Wildlife Branch, Manitoba Natural Resources. The sale of, or any other form of profiting from the removal of wood frog eggs, tadpoles or adults requires a permit from the Wildlife Branch. (See Manitoba Natural Resources' official position on this subject in our Snakes in the Class Room article - in this issue.)

Conservation

Does removing eggs from the wild hurt local populations of wood frogs? Not likely. Frog eggs and tadpoles are food for many other animals. The chances of any one egg surviving to produce a single adult frog are slim. That's why frogs produce so many eggs, so at least a few will survive. In nature, local frog populations will vary greatly over time. Long term weather patterns that determine the availability of both spring breeding ponds and food will have the greatest effects on local populations. Raising tadpoles to maturity and releasing them back into the wild may help to raise the local population, or it may just provide more food for birds and snakes. As long as their habitat remains intact, wood frog populations, like all wildlife, will take care of themselves. But be respectful of nature and don't collect more eggs, tadpoles or adults than you can properly care for! One batch of eggs (4 cm across) may contain 500 or more eggs, enough tadpoles for an entire school!

Getting Eggs or Tadpoles

To be sure of getting some wood frog eggs or tadpoles requires some planning. Peak breeding season will vary with the weather conditions from year to year, but usually occurs in mid-to-late April in southern Manitoba, and later as you proceed north. When winter's snow is almost gone and daytime temperatures first reach the mid-teen's, wood frog breeding will get underway. Wooded areas with water-filled depressions or nearby ditches are good places to look, or rather, to listen! Finding breeding sites is best done by visiting prospective sites and listening for calling males. Early evening is the best time to try and locate frog choruses (you need to find a spot where there are lots of frogs calling, not just one or two), but early on in the breeding season, males can often be found calling during mid-day. Small streams, creeks, ditches, ponds or depressions in fields or forests are the places to look (or listen) for wood frogs. Be prepared to do a little driving around if you don't already know of a site where there are wood frogs, but rest assured, they are out there!

Play the mating call of a Wood Frog

Female frogs tend to deposit their eggs on vegetation near the water's surface. Look for areas where there is lots dead grass, cattails or other debris in the water. The dark egg masses are usually easy to spot. (Check out our other Wood Frog articles for images of eggs and tadpoles: Frog Log, Wood Frogs )

Being Sure of Getting Wood Frog Eggs

Want to be sure you're getting wood frog eggs? If you're collecting eggs from mid-April to early May, in melt-water ponds or ditches, chances are pretty good that they are wood frog eggs. The other two species that breed at the same time in the same habitat, boreal chorus frogs (Pseudacris triserata) and spring peepers (Hyla crucifer), produce smaller egg masses and they hide them quite well. Leopard frogs (Rana pipiens)usually breed a little later and are seldom found far from permanent water bodies. It would be great to get some eggs from these species, and raise them as you would the wood frogs, but they are more difficult to find. We prepared this project guide with wood frogs in mind largely because of the ease with which eggs may be obtained and the short maturation period that almost guarantees they will transform before the end of the school year in late June.

If you're a little late and trying to catch tadpoles from the wild it may be difficult to know what you are getting. The Amphibians and Reptiles of Manitoba (see book references below) has a taxonomic key for identifying the tadpoles of species found in Manitoba. But don't worry, whatever they are they will provide you with an enjoyable experience.

How to Collect

When you're looking for frog eggs be prepared with rubber boots or hip waders, some sort of container (a wide mouthed jar, pail or sturdy plastic bag) and a long handled net or scoop of some sort. It's best to get up close to the eggs and scoop them, gently, directly into the container. If using a long handled net to reach eggs, scoop them into the net, then draw the net close to you WITHOUT lifting the eggs out of the water. Then carefully transfer them to a container. Fill the container with water from the pond or ditch in which the eggs are found! Use a container that will hold at least 1 L of water. This ensures that their will be sufficient oxygen in the water for your return trip and that the temperature in the container will only change slowly. If you are anticipating a long trip to collect eggs, then bring along a cooler so the water will not get too warm.

A Permanent Container

Plan ahead and have an aquarium ready before you collect the eggs! If possible bring extra water from the breeding pond to fill your aquarium. This way you can simply fill it, then gently empty the eggs into it. If you wish to prepare an aquarium or other container ahead of time using tap-water, make sure the water has been "aged" for a couple of days, to allow chlorine and other chemicals to evaporate. If you are using a brand new container or aquarium be sure to add some "pond water" to it several days before adding eggs or tadpoles. This ensures that bacteria and other micro organisms will be present to process the wastes from the tadpoles, so these do not accumulate and become toxic. (This is why fish often die in brand new aquariums!)

Any size container or aquarium from 1L and up can be used, but you should limit the number of tadpoles kept in small containers. A 1L container should contain no more than 6-8 tadpoles. The larger the container, the more tadpoles it can hold. Resist the temptation to keep all the tadpoles from a batch of eggs, even in a large container. Even a small batch of eggs may contain 400-600 eggs! You can release excess tadpoles back to the breeding pond or find other people who wish to raise some. (To release tadpoles reverse the process used when transferring eggs from the wild.) To split up a batch of eggs use sharp scissors. Yes, sharp scissors! The individual eggs are surrounded by a sphere of hard jelly and attached together into a mass with softer, more viscous jelly. The scissors neatly separate the individual eggs and cut the viscous jelly between them.

Place the container or aquarium in a bright location to encourage the growth of algae, which the tadpoles will eat. Algae also helps to purify the water and increase oxygen levels. If the location receives full sunshine, monitor the water temperature. It should not be allowed to rise to more than 27C. Move or shade the container if it gets too hot. If the container is outdoors, make sure it doesn't freeze, tadpoles aren't freeze-tolerant!

Transferring Eggs to the Aquarium

Float the container of eggs in the aquarium water so the temperature of the eggs will change slowly to that of their new surroundings. This could take a couple of hours for a large container. As the temperature of the holding container approaches the aquarium, another good practice is to mix some of the aquarium's water into the egg container. At regular intervals, about once every couple of hours, add water to the egg container until it has been diluted by half with water from the large container. If possible let the containers sit overnight before transferring the eggs, but set up a bubbler to ensure oxygen levels are maintained in the egg container. Transferring eggs to new water should be done over no less than 4-6 hours to prevent temperature or other physiological shock.

Aquarium Maintenance

It is important to keep the water clean and well oxygenated. The smaller the container you are using the more diligent you must be to keep the water clean and oxygenated. Too much food and lack of circulation will cause the water to go foul. More importantly, bacteria feeding on excess food quickly use up the available oxygen and the tadpoles will suffocate. This can happen overnight! Keep the water circulating and well oxygenated by having an aquarium "bubbler" in the water. (A proper aquarium filter is useful, but not a necessity for raising tadpoles.) Adjust the rate of air bubbles according to the size of the container. Too much agitation in a small container interferes with the tadpoles normal behaviour. They like to loaf around! If the water goes cloudy or begins to smell foul, exchange part of the water (up to 30% at a time, once every 6 hours or so) until it is clear again. Keep some "aged" water on hand at all times for this purpose. Larger aquariums with modest numbers of tadpoles will be less work to maintain than smaller, or overcrowded ones!

Feeding Tadpoles

Tadpoles of wood frogs, like most frogs and toads, are primarily herbivorous and can eat just about any kind of plant material, but they will eat dead animal flesh (even other dead tadpoles!) when they are larger. Boiled lettuce is a good food item. Boiling helps to break down the plant tissues. (Freezing the lettuce does the same thing.) Commercial fish foods (the kinds that are thin flakes) are excellent. Use the cheap, "goldfish foods", not the expensive "tropical fish foods". Be careful not to overfeed! Tadpoles don't begin to actively feed until about 3-5 days after hatching. They live off their egg yolk to this point. When the tadpoles begin swimming around it is time to begin feeding them. Try to keep some boiled lettuce available in the container, but don't add more until the previous batch is eaten. Remove excess lettuce if it appears to be going foul. Add small amounts of fish food as required. Small tadpoles don't eat much, but they will become quite ravenous as they grow larger. Watch for uneaten food on the bottom of the container. Only add food when they have finished what you've given previously.

Transformation

The transformation from tadpole to froglet begins slowly with the appearance of tiny hind legs at about 20-30 days from hatching (at room temperatures). When the tadpoles are full grown and ready to transform (usually between 40-60 days from hatching) they will stop eating. There will be some variation in growth rates among a batch of tadpoles so you will need to adjust the amount of food given to allow the slower ones to keep growing. Be careful not to overfeed at this point. Once they've stopped eating, transformation becomes much more rapid. Plump tadpoles with dangling legs will become tiny frogs with shrunken tails in just 5-7 days.

Once the final transformation of tadpole to froglet has begun, make sure there is some available "land" for the froglets to climb out of the water. Rocks sticking out of the water or floating plastic aquarium plants will suffice. As they transform and switch from gills to lungs they need to be able to emerge from the water and rest. If no land is available the froglets will exhaust themselves trying to get out of the water and can drown!

Releasing Transformed Frogs

Once the froglets have completely lost their tails they can be released back into the wild. When you are ready to release them, or any remaining tadpoles, return them to the site where they were collected. Do not release unhealthy froglets, it is better to euthanize them than to risk introducing disease into the wild population. If animals appear diseased the containers and equipment used in their rearing should be sterilized by boiling prior to reuse. Resist the temptation to "introduce" or "stock" your frogs into areas where they do not currently occur. If the frogs aren't already there, there is probably a good reason; the site may be lacking in some habitat requirement. The exception to this guideline is when new habitat has been created that frogs haven't had the chance to colonize or cannot because of some natural barrier. For example, a new storm water retention pond or school yard habitat creation project could be a good place to "introduce" wood frogs.

Watch Wood Frog eggs become froglets in this NatureNorth Critter Video!

Experiments with Tadpole Rearing

The simple rearing of wood frogs can be a rewarding project for all grade levels. However, for advanced grades there are opportunities to incorporate some experimental elements. Students could study the effects of various environmental factors on the development of the frogs. In addition, regular data gathering could help with important biological monitoring of frog populations and add to the general knowledge of frog species.

Simple observations gathered over time, and from year to year, can provide significant insight into the biology of this and other wildlife species. Keep track of the dates at which:

• you hear calling frogs
• you collect eggs
• the eggs hatch
• the tadpoles begin actively swimming and feeding
• legs begin to appear
• the transforming froglets begin to gulp air
• transformation is complete (tails completely resorbed)

You could also measure the length of tadpoles over time as a measure of their growth rate.

Effects of Environmental Variables on Tadpole Development

Here are some variables to consider if you would like to add experimental aspects to raising wood frogs in the classroom.

Temperature

How does water temperature affect the rate of growth and maturation of tadpoles? Is there an optimum temperature? Does this factor interact with any of the other factors? Related discussion could include: the effects of weather on timing of reproduction in this and other species or the implications of global warming for the distribution of wood frogs in North America.

Water Quality

Does pH, Oxygen concentration or other water quality factors affect growth and maturation? What are the environmental tolerances of the tadpoles for various factors? Take advantage of opportunities to apply water quality testing equipment in a controlled environment with simple variables? (Obviously, care must be taken to impose quality changes slowly so as not to harm the tadpoles.)

Size of container

An issue of water quality or population density? Wood frogs typically breed in ever-shrinking pools. Would a smaller container, which might result in changes in water chemistry owing to the greater concentration of tadpoles, cause them to transform earlier or at a smaller size?

Type of food

Does the type of food offered affect the rate and nature of development? Low protein plant materials should produce tadpoles with larger guts for prolonged digestion while foods higher in animal proteins should result in smaller guts.

Availability of food

Can the tadpoles adjust their rate of growth and maturation in response to different or changing levels of food availability?

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More Resources:

Our record of raising Wood Frog tadpoles: Frog Log.

Other NatureNorth articles on frogs: Wood Frogs and Frog and Toad Sounds.

For kids: The Amphibians of Manitoba

Some good books:

• The Reptiles and Amphibians of Manitoba by Bill Preston.
  
  
• The Audubon Society Field Guide to North American Reptiles and Amphibians by J. L. Behler and F. W. King.
  
  
• A Field Guide to Reptiles and Amphibians of Eastern Central North America by R. Conant and J. T. Collins. A Peterson Field Guide.