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QUESTIONS
& ANSWERS '96
- Do frogs lay different
amounts of eggs?
(Submitted by the O.H. Andeson School, Mahtomedi, MN)
Moriarty: Yes. Frogs and toads can lay from two to over 20,000
eggs at a time. In Minnesota, the number of eggs varies from 200 (Western
Chorus Frog) to 20,000 (Bullfrog) eggs per year. Frogs lay their eggs
in small clumps, long strings, floating masses, or large surface films.
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- What kind of fish eat frogs?
(Submitted by the O.H. Anderson School, Mahtomedi, MN)
Moriarty: Most predatory fish, from a stickleback to a muskie,
will eat frogs. Smaller fish will normally prey on eggs, while larger
fish prey on tadpoles and adults. Northern Leopard Frogs are commonly
sold as bait for bass and northern pike fishing. Most frogs pick lakes
and wetlands that are absent of fish for breeding ponds. This way,
frog eggs and tadpoles will not be eaten. When humans add fish to
a pond or wetland it can lead to the elimination of amphibians from
that area.
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- Why do frogs mainly come
out in the rain?
(Submitted by the Clayton Public Schools)
Moriarty: Frogs come out in the rain because they do not have
to worry about drying out. An amphibian's skin is permeable to water
and if the humidity is too low they will lose water and start to dry
out. This is very stressful to the frogs. Frogs have adapted to water
loss by developing less permeable skin, as in toads, or seek micro-habitats
with higher humidity.
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- Why are so many frogs killed
on the roads?
(Submitted by the Clayton Public Schools)
Moriarty: Humans have built too many roads! Roads fragment
the habitats that frogs use for breeding, feeding, and hibernating.
When frogs migrate from one area to another they normally have to
cross a road. A frog's defense when a car drives by is to crouch down.
When that happens they become "sitting ducks." In Europe
people are building frog corssings or tunnels to allow frogs and toads
to safely get across roads.
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- How small can frogs be?
(Submitted by Elizabeth Winters' class, Rochester, MN)
Moriarty: As adults, the smallest frogs are the narrow-mouth
toads of Africa at 1 cm. (less than 1/2 inch). In Minnesota the smallest
frog is the Spring Peeper at 2-3 cm. (about 1 inch). A newly transformed
Spring Peeper can sit on a dime and not have any part of its body
sticking over the edge.
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- Why are Minnesota frogs
so different from brightly-colored rainforest frogs?
Harris: That is a very good question. Actually, scientists
are not sure why they are so different, and there are many different
ideas for why we don't have really colorful frogs here in Minnesota.
You may already know that the bright red and blue frogs in the rainforest
are poisonous, and their colors are used to warn other animals to
leave them alone. Some scientists think that most of the animals in
Minnesota that like to eat frogs are actually color-blind. That is,
they can't tell if a frog is yellow or gray, so why should a frog
bother being so colorful? Our frogs are usually the color of the ground
(okay, maybe that isn't so exciting) so that frog-eaters can't see
them very well. Some biologists think that the tropics have had more
time to allow more types of animals to develop because they were not
covered by huge glaciers ten thousand years ago. Minnesota WAS covered
by glaciers during this time, and so there were no frogs here then.
Also, many people don't know this, but there are
actually A LOT of really drab frogs in the tropics as well. Obviously
something is going on, though, because we see the same pattern with
birds. Many of the birds in the rainforest are
much more colorful than our birds. We may never completely answer
this question, so maybe you should consider studying this topic in
college.
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- What is causing the deformities
in Minnesota's frog populations?
Helgen: No one is sure yet. But MPCA staff and other researchers
are testing the mud, water, and frog tissues for abnormal levels of
metals such as arsenic, mercury, selenium and cadmium. They're also
looking for chemicals like PCBs, herbicides and pesticides, and also
at parasites as potential causes. There are many theories. But so
far, there's no evidence pointing to any one possible cause over another.
Researchers speculate the deformities may be the result of genetic
mutations passed on from the parent frogs to their offspring. There
is no evidence at this point to suggest that human health is being
endangered by whatever is causing the deformities.
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- Do the pollutants go up
the food chain?
Helgen: Many of the pollutants we are considering as possible
causes and analysing do "bioaccumulate" and move up the
food chain. This is especially true of some of the heavy metals like
mercury. Many of the organic compounds we are considering such as
many of the herbicides can bioaccumulate especially in the fatty tissues
of frogs and other animals.
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- What causes frogs to malform?
Helgen: This is the question everyone wants to know the answer
to. Right now we don't know. We have some ideas we are considering
and we're doing tests to find out how likely they are to be the cause.
These inlcude:
1. Increases in the ultraviolet radiation due to thinning of the ozone,
2. Heavy metals which are know to cause abnormal development, particularily;
Selenium, Arsenic, cadmium, and Mercury,
3. Organic compounds like PCBs (Poly-chlorinated biphenyls) and Biocides
(Herbicides and pesticides),
4. Parasitic cysts physically disturbing the developing limb bud in
tadpoles and,
5. Synthetic chemicals that act like hormones to disrupt the endrocrine
system.
We probably won't know the answer until after several years of tests.
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- How many deformed frogs
were in the population first discovered by students at the Minnesota
New Country School?
Reinitz: On the day the frogs were first discovered, the students
from the Minnesota New Country School looked at 22 frogs, half of
which were deformed. Within a month they had collected roughly 400
deformed frogs from the Ney Pond. Most of those frogs were turned
over to researchers for study.
(For the full story, jump to the Minnesota New Country School website:
http://www.mncs.k12.mn.us/frog/frog.html.)
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- Where do amphibians come
from?
David Hoppe: Amphibians are thought to have evolved from a
group of extinct fishes known as the rhipidistians. Those fishes had
fleshy, muscular fins to move along the bottom and push themselves
through thick vegetation in shallow waters--fins that are logical
forerunners to amphibian legs. They also had a tube connecting from
the gut to the swim bladder, so they could swallow air and have some
get to the swim bladder as a way to receive some oxygen. Thus, the
swim bladder served as a primitive lung, useful to the fishes in warm,
shallow waters that had little oxygen, and became logical forerunners
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- Can tree frogs swim?
(Submitted by Brianna in Rochester)
Harris: Yes, tree frogs are able to swim, and swimming is actually
a very important part of their lives. Many tree frogs lay their eggs
on the underside of leaves, and when the tadpoles hatch, they fall
into a body of water that--hopefully!!--is directly beneath the leaf.
As you can probably imagine, these tadpoles must be able to swim immediately.
When they develop into adult frogs, they will climb into the trees
and spend the rest of their lives away from the water, until it is
time to lay eggs of their own.
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- Can an African frog change
to a different sex or a different color? Why or why not?
(Submitted by Kelly and Shannon in Rochester)
Harris: From your question, it seems that you have learned
that certain frogs change sex and many frogs change color on a regular
basis. Unfortunately, there are so many types of frogs in the world,
and I can't say for sure if there are specific frogs in Africa that
change sex. My guess is that there are frogs like this in tropical
Africa, because sex change in frogs is not uncommon around the equator.
These changes seem to have something to do with temperature changes
around the tadpole eggs but scientists aren't sure why this would
be useful to frogs. I AM sure that African frogs do change color,
because this behavior is typical of frogs everywhere, including Minnesota.
Sometimes they change in response to time of day, other times they
change because of their level of activity. I have seen a frog in South
America change from white to yellow to orange in just a few minutes,
and I think the changes were due to the frog waking up.
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