Biotic Inventory:
Documenting Diversity at the Katharine Ordway Natural History Study Area
Rana pipiens, The Northern Leopard Frog
Taxonomy
Kingdom Animalia
Phylum Chordata
Class Lissamphibia
Order Anura
Family Ranidae
Genus Rana
Species pipiens
Common names: the Northern Leopard Frog, the grass frog, the meadow frog
Diagnostic Characteristics
Identifying a Northern Leopard frog can prove to be
difficult for many inexperienced frog enthusiasts mainly because these
frogs are very similar in size, color and morphology to many other
frogs. The leopard frogs grow to 3 ½ inches long and produce a low,
snoring or rumbling croak mixed with grunts and squeaks, different from
many loud croaks of familiar frogs because the leopard frogs do not
possess an air sac 5. Generally, their advertising call is a hoarse
croaking followed by two clucks, and their aggressive calls consist of
short chuckles. Leopard frogs make four different kinds of calls. Their
vocalizations can represent advertisement, aggression, courtship
rituals, or defense1. Calls vary among individuals, however, and are
hard to interpret. Along with which, these frogs have three
morphological types, the common type, the Kandiyohi and the Burnsi type
1.
The most common type which is found in adulthood with black spots
scattered randomly about the back and on the sides with a plain white
underside. There are two light (though sometimes a shade of gold)
dorsolateral ridges down the back with spots on the forelimbs and
“tiger stripes” or bars on the hind legs. The groin and thighs have a
greenish and sometimes yellowish tint to them. The brown specimens,
pictured above in pictures 1 and 2, differ from the closely related
pickerel frogs by having round spots scattered about the back and a
greenish wash on the thighs. The Kandiyohi form is relatively rare in
Minnesota and has not been catalogued in Ordway, but has been
catalogued elsewhere in Minnesota 2. These are brown with distinct
dashes of white and brown or black. The spots on the back and legs are
still discernable, as well as the dorsolateral fold. Lastly, the Burnsi
forms have been documented in Ordway, although they are relatively rare
and a picture was not able to be taken. It is brown with no dorsal
spots, yet has spots or bars along the limbs, and may have black
stippling on the back and sides 2. In most of the Burnsi types found in
Minnesota, they are known to remain light and have a yellowish tint to
them.
The hydrophobic skin covering the frogs is moist and smooth, making
them elusive for predators and quick underwater. Their skin contains
mucus that they secrete in order to keep the skin from drying out and
also these secretions are sometimes found to be poisonous1. Although
not harmful to humans, these secretions are powerful enough to deter
their predators in the marshes and wetlands.
Ecology
As seen in picture 3 above, these leopard frogs are
found in permanent ponds, swamps, marshes and slow moving streams
throughout forest, open and even urban areas. They are normally found
in ponds with abundant aquatic vegetation, upon which they tend to
hide. These frogs are well adapted for living in a cold environment and
can be found well above 3,000 meters. During the harsh Minnesotan
winters, these frogs move to the bottoms of lakes and ponds, where it
is naturally warmer 3.
The tadpole’s and froglet’s diet differs from those of the adult
because they feed primarily on algae and microscopic organisms1. The
adult frogs, however, are fierce predators, feeding on the excess of
insects and mosquitoes present in the Minnesotan and tundra climate.
Not only do they feed on the insects such as crickets, flies and
mosquitoes, but they also feed on slugs, snails, worms and smaller
vertebrates such as the chorus frogs that they live beside and often
will hear with their calls3.
Save from being fierce predators in their ecosystem, they are also
fiercely preyed upon. Herons, snakes and mammals (including humans),
and even other frogs prey on the leopard frog.
Life History
Copulation occurs in the water mainly between the
months of March into late June. The females release eggs while swimming
and the male attaches himself to the female with specialized thumbs,
allowing him to fertilize the eggs. The females are known to lay more
than 6,000 eggs at a time which may vary from submerged egg masses in
northern populations to a surface film of eggs in southern
populations2. The black eggs are attached to aquatic vegetation and
hatch within 13-20 days and transform in 70-120 days2.
The life cycle of the leopard frog is fairly simple and is very similar
to that of many other frogs. When the eggs first hatch they are
tadpoles which lack any kind of limbs or lungs. The tadpole then
undergoes metamorphosis, growing hind legs initially, then the front
legs and the head simultaneously. The gills are later replaced with
lungs and the froglet begins to slowly adapt to life on land. On land,
the frog then loses the tail and continues to grow in size until fully
developed. The leopard frog grows until it reaches sexual maturity in 2
to 3 years, mating and beginning the cycle once again 1, 2.
Distribution
The Northern Leopard frog is endemic to the northern
United States and into much of Canada. These frogs have even been
reported to live in climates such as the mountains of New Mexico and
north into Alaska 4. The leopard frog’s range varies so intensely
because of its migratory habitats and abilities to go lengths of time
without water. The range of the northern leopard frog in southern New
England overlaps somewhat with that of the southern leopard frog, a
sibling species that can be deceptively similar in appearance. Their
migratory habits are, however, limited between aquatic breeding and
wintering habitats and nearby summer feeding areas. Summer movements of
adult frogs include mostly short distance (e.g. a few meters to 45
meters) nocturnal travel. More extensive summer movements have also
been documented, upon which they are known to have traveled miles in
one night 4.
Threats and Conservation Issues
Recently, malformed frogs have become a serious
issue. Amphibian malformations have been recorded in 44 states and in
nearly 60 species. These malformations are found throughout the United
States and Canada, but first became a real problem in Minnesota. An
abnormally high number of mutations occurs in the leopard frogs, and
between 1995 and 2000 this stemmed a vast study to determine the
causes. Missing or extra limbs, digits and eyes, partial limbs and
malformed jaws, among other problems, threatened the livelihood of many
frogs, threatening the population to dangerous levels 5. Nearly all the
malformed frogs found are juveniles, indicating that they rarely
survive to adulthood. Scientists do not know the exact causes of these
malformations, but they have started to present several theories.
Malformations at several sites in Minnesota have been linked to the
presence of a parasite Ribeiroia ondatrae, a flatworm that burrows into
tadpoles5. Frogs infected with this parasite at these sites develop
severe deformities. Another cause linked to northern leopard frog
malformations is pesticides. However, tests done at the research sites
in Minnesota did not find high quantities of pesticides in the water.
Researchers also believe that man-made chemicals other than pesticides
may have an adverse affect on frogs as well. However, some studies show
conflicting results. Some chemicals that are thought to be linked to
malformations, such as methoprene, have been found not to affect the
malformation of R. pipiens at levels commonly found in the environment5.
Another hypothesis is that the increasing amount of ultraviolet light
hitting the earth’s surface may cause some malformations5. Most
researchers believe a combination of all of these factors, human and
naturally caused, that result in the deformities found in so much of
Minnesota.
Voucher Information
This specimen pictured on the first page was found
in a permanent, fish-less pond with ample vegetation at the Katherine
Ordway Natural History Study Area in Inver Grove Heights, Minnesota.
Along with these frogs, there were an abundance of other frogs, such as
small chorus frogs, living beside them. Also, these frogs had just
begun to breed, for these were caught in mid May during an unusually
warm year.
References
1. Minnesota Department of Natural Resources:
http://www.dnr.state.mn.us/reptiles_amphibians/frogs_toads/truefrogs/northernleopard.html
2. Reptiles and Amphibians of Minnesota:
http://www.herpnet.net/Minnesota-Herpetology/frogs_toads/NorthernLeopard_frog.html
3. USDA Forest Service: http://www.fs.fed.us/r5/snfpa/final-seis/vol1/chapter-3/3-2/3-2-2/32210.html
4. National Geographic: http://animals.nationalgeographic.com/animals/amphibians/northern-leopard-frog/
5. US Fish and Wildlife Service: http://www.fws.gov/nevada/nv_species/nleopard_frog.html
Compiled by Kai Cowger.
Biodiversity & Evolution (BIOL 270) Professr Sarah Boyer. Spring 2010
Specimen collected at Macalester College’s Katharine Ordway Natural History Study Area on April 15, 2010.