Biotic Inventory:
Documenting Diversity at the Katharine Ordway Natural History Study Area
Northern Leopard Frog, Rana pipiens
Once one of the most widely distributed frogs in the US, the Northern
Leopard Frog was a common sight on high-school dissecting trays. They
were also quite popular in the food industry as well, hunted and killed
for their legs.
Taxonomy
Kingdom Animalia
Phylum Chordata
Class Amphibia
Order Anura
Family Ranidae
Genus Rana
Species pipiens
Common names: Northern Leopard Frog, Meadow Frog1, Grass Frog2
Identification
They are named for the three rows of irregularly shaped dark spots on
their back and legs.1 Each spot is encircled by a lighter cream ring.
They are greenish-brown in color with a pale underbelly and light
colored ridges (dorsolateral folds) on either side of their backs.
These ridges or folds run from the eye to the rump. They have pinkish
patches on their feet with extensive webbing on their hind toes.2Their
eyes are golden. Measuring 3-5 inches (7.6-12.7 centimeters) from
nose to rump they are considered medium-size. Females are slightly
larger than males.
Tadpoles are dark-brown/grey with tan tales and light blotches on the
underside. Their backs have flecks of light gold with higher
concentrations on the sides. Tails are usually less than 1.5 times
total body length. The dorsal tail fin is anterior to the tail when
viewed laterally. The anus is on the right side in front of the fin.
The eyes fall within the outline of the head when viewed from above.
The lower mandible is thicker than the upper. Tadpoles usually measure
between 5.5 to 10.0 centimeters.2
Diagnostic Characteristics
The distinctive spotting pattern of the Northern Leopard Frog is one
easy way to distinguish it from other ranids (true frogs). Most
noticeably, it has three rows of spots while others have only two and
its distinctive darker spot rimmed by a lighter halo easily separates
it from other species like the Columbia Spotted Frog or the American
Bullfrog (which has no spotting). The Northern Leopard Frog also
has a lateral stripe on the side of its snout that is yellowish in
color and absent in the American Bullfrog. Lack of reddish coloration
on the belly and legs distinguishes it from the Columbian Spotted Frog.
Ecology
They tend to live in marshy areas, near ponds, in streams, rivers,
valley bottom ponds, beaver stocks, potholes, lakes and warm water
springs. Their habitats are mostly freshwater but some have been found
in places with moderate salinity. During warmer months they will
venture into well-covered grasslands.3 They are well adapted to the
cold and can be found in slow moving streams up to 3000 meters above
sea level.
They are carnivores, tending to eat anything that they can fit into
their mouth. They lie in wait for prey, pouncing with their powerful
back legs when the prey ventures across their path. They are known as
opportunistic feeders, like many other amphibians.3 Their prey
includes: beetles, ants, flies, worms, smaller frogs, including their
own species, birds, and garter snakes. They are preyed upon by
raccoons, snakes, other frogs and humans. They are fast and rely on
speed to escape from predators. This makes them very difficult to catch
as they can avoid predators by moving quickly performing a series of
zigzag hops. Tadpoles are preyed upon by a variety of creatures
including: the Pied-billed Grebe, Tiger Salamander, garter snakes, and
American Bullfrog tadpoles.
Northern Leopard Frogs tend to be the most active after sunset (in warm
temperatures). They go into hibernation during winter which can
last from October to March or April depending on where the frog is,
i.e. in Wyoming ice does not usually melt until April while in
Colorado, ice melts by March. During winter they shelter under rocks in
streams provided there is enough oxygen in the water.4 They have also
been found inactive under rubble and debris in ponds/streams in water
as deep as 85 centimeters.2
Life History
Their average life expectancy is 2-4 years. Males emit a snore-like
call from spring to summer to attract a mate. The snore-like call lasts
for 2-3 minutes and is followed by 2-3 stuttering croaks.2 The earliest
known date for these frogs to come out of hibernation is March 1.3
During the mating season, males have a swollen thumb which aids in
copulation. These frogs breed from March-June, when they emerge from
hibernation. Exact timing depends on latitude. Males gather on warm
days (14-230C) and float on the surface of the water attempting to
attract females by their calls. They may gather together in groups as
big as 25 individuals in a 20 square meter area. Females lay eggs soon
after calling begins.
They breed in ponds or other stationary water
sources, like a stagnant stream. They can lay up to about 6,500 eggs in
the water at one time. The eggs can lie at the bottom of ponds, become
attached to vegetation or clump together in flattened masses of 300-800
eggs measuring less than 6.0 millimeters across. The eggs are black on
top and white on bottom. Eggs may reach from a few hundred per hectare
to more than a thousand, depending on favorable conditions. Tadpoles
usually hatch within ten days of laying and several hatchings can occur
at one site.2 Tadpoles develop in the breeding pond within 4-5 months
(58-105 days).4 Fully metamorphosed juveniles appear as early as June
in lower elevations and as late as September in higher elevations. When
they morph into frogs they resemble smaller versions of the adult only
measuring 0.75-1.2 inches (2-3 centimeters). They will reach sexual
maturity at about three years old.
During their life history they can be found in different habitats
depending on what stage of their life history they are undergoing. For
example, ponds used for breeding tend to be shallower and warmer,
different than those used for wintering which maybe deeper. Juveniles
and adults may venture out into marshy fields with dense, but short
vegetation. Taller vegetation seems to not be as preferable.2
Distribution
They have a wide range covering most of the Northern United States
except the Pacific coast. They range from the Northern-most parts of
Maine, down into New Hampshire and New York, across the mid-West to
Minnesota and further west to Montana. They can even range as far south
as Arizona and parts of New Mexico. They are found in Canada as well
along the southern Labrador coast and further inland from Quebec to the
southern parts of Alberta. They were once one of the most commonly
distributed frogs in the US, but suffered massive declines in the
1970’s in the US and Canada as well. Scientists are still trying to
pinpoint the causes of this massive decline but have yet to determine
the source. It is generally thought to be a combination of increasing
water acidity, deforestation and pollution. They are now classified as
‘Threatened’.
Voucher Information
This specimen was collected at the Katherine Ordway Natural History
Study Area in Inver Grove Heights, Minnesota. The specimen was found in
a marshy area located in a small depression in the prairie grasslands.
The area was wet with low lying vegetation.
References
1) National Geographic http://animals.nationalgeographic.com/animals/amphibians/northern-leopard-frog.html
2) Montana Field Guides http://fieldguide.mt.gov/detail_AAABH01170.aspx
3) Marshall Herpetology http://www.marshall.edu/herp/Toads_Frogs/Leopard_Frog.htm
4) Ray’s Web for the Alberta Wildness Association http://raysweb.net/specialplaces/pages/frog.html
Compiled by Kristen Ross.
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.