Little Known Frog Facts continued . . .
"There are certain species of marsupial tree frogs who live up in the
treetops of the Andes in South America. There are no ponds, so the female
carries her eggs in a pouch on her back. Live froglets come out; there's
no tadpole stage at all," says Maxson, who is a professor of ecology
and evolutionary biology as well as a UTK associate vice chancellor.
The young of the turtle frog of western Australia also bypass the tad-pole
stage, although the hatching arrangements are a little different.
"The adults burrow in the sand about three feet down," Maxson
says. "Their nests are at the bottom of the holes, and they lay and
fertilize eggs that develop directly into little froglets."
Another Australian frog, discovered in the late 1970s and now extinct,
earned the distinction of being the only vertebrate in the world where the
mother actually carries her babies in her stomach.
"The eggs are fertilized and the female swallows them," Maxson
says. The cells lining the stomach undergo a change, and the female stops
feeding.
"She carries about a dozen eggs for four to six weeks until the little
froglets are ready to emerge. Then she literally throws them up. As soon
as the froglets are born, the tissue in the stomach becomes normal, secreting
acid, and the female feeds."
These frogs are among the many unusual species that Maxson has encountered
through her research. She uses her expertise in biochemistry and genetics
to determine the evolutionary history of frogs. Other researchers often
request her help in learning more about particular species' family tree.
Frogs, whether or not they belong to an unusual species, are very useful
to humankind, Maxson explains.
"They are major consumers of mosquitoes and other noxious insects.
Some species produce toxins that have potential medicinal value. In addition,
frogs have been used in biology classrooms for many years, because they
produce externally fertilized eggs that can be brought in and studied. A
lot of what we understand about human development comes from some very early
studies on the development of frog eggs."
Many frog species are now extinct, Maxson says. 
"They are disappearing because of habitat degradation. That's particularly
true in South America and some places in Europe, Asia, and Australia, where
humans are expanding, paving things over for homes, or removing forests
for agricultural and lumbering purposes."
However, the frogs closer to home, while perhaps less in danger of eradication,
can serve as warning signals of imminent environmental problems, Maxson
says.
"Most species start out as aquatic tadpoles and metamorphose into land-living
animals.
As tadpoles in an aquatic system, they are very sensitive to pollutants.
So a number of my colleagues use tadpoles as monitors of water pollution.
A tadpole's skin is so permeable that if you do something to the water quality,
the first thing you do is endanger and/or kill the frogs," she says.
Froggie Went a Courtin'
Who's singing down at the pond? In the frog world, males are the vocal sex,
calling to attract females, says Dr. Linda Maxson.
What's more, the male frog is a single-minded swain, Maxson says.
"Most species have an external fertilization mode known as amplexus.
The male grabs the female from behind, waits until she extrudes her eggs,
and then fertilizes them." If a male accidentally grabs another male,
he will make a "release call" to indicate that he's the wrong
sex.
Woe be to the mute would-be mate in the vicinity of an amorous amphibian.
"If you float an orange or an apple by a little frog that's in breeding
condition and he grabs on-well, that [orange or apple] doesn't make a sound,"
Maxson says. "He'll stay on hours, maybe days, until something happens
that makes him realize his mistake."
