Northern Leopard Frog
Last Update/Review: June 17, 2002
Status | Description | Habits | Reproduction | Food | Limiting Factors | Management and Outlook
Status
The northern leopard frog was once the most widespread frog species in North America. Its range extended from California and New Mexico in the southwestern United States to the Alberta/Northwest Territory border in the north, and eastwards to the Atlantic coast of eastern Canada and the northeastern United States. In addition, the species was successfully introduced onto Vancouver Island and Newfoundland. In Alberta, dense populations of leopard frogs occurred along lakes and rivers in prairie and parkland areas. A few scattered small populations were present in forested areas.
During the mid-1960s the number of northern leopard frogs in eastern North America began to decline. The frogs now are gone from some regions, and are scarce in many others. In Alberta, most populations remained healthy until approximately 1979 when the frogs mysteriously disappeared from many sites in central and south-central Alberta—sites where they had occurred only one year earlier. A few populations remained in southeastern Alberta, but many of these were so small and localized they were unknown until 1990.
In 1990, leopard frogs were known to occur at 32 sites in Alberta; however, only half of these populations had evidence of breeding. Fewer than 10 adults were seen at most sites and only one site had over 20 adults. Most sites are located in southeastern Alberta in the Mixed Grassland Natural Region and the Cypress Hills. Most populations occur in uncultivated, natural habitats.
The declines seen in leopard frogs in Alberta fit the general pattern of worldwide disappearance of many amphibian and reptile species. Unfortunately, scientists have been unable to determine the exact cause (or causes) of the declines. The disappearances may involve a combination of unusual local conditions (such as drought, frost, disease, acid rain, and loss of habitat), global changes of increasing temperature (global warming), changes in ultraviolet radiation levels, and generalized contamination of air, soil, and water.
The local changes make it more difficult to detect the possible impact of large-scale changes. On the other hand, perhaps the generalized loss of habitat experienced by wildlife throughout the world may simply be more obvious and dramatic in amphibians and reptiles.
For more information, check out the Status of the northern leopard frog in Alberta report.
Description
Northern
leopard frogs are the familiar frog of biology books and laboratories.
Usually, they are green or greenish brown with creamy white undersides.
The back, sides, and legs are dotted with well-defined round dark spots
with a light border. Two distinct light-coloured glandular ridges run
down the back, from the eyes to the tail. There may also be a light
stripe on the upper jaw. Some newly transformed frogs are distinctly
brown with black spots and a few individuals, particularly in southern
regions, may be quite pale, almost yellow in colour. Occasionally unspotted
leopard frogs occur.
Like all true frogs, leopard frogs have smooth moist skin and glandular ridges (dorsolateral folds) that separate the back from the sides of the body (the ridges in leopard frogs form the white lines seen on the back of the frog). The hind legs are long with large feet and well developed webs between the toes.
The leopard frog is our largest common frog and, on average, it is 50-130 mm long from the tip of the nose to the base of the tail. Young adults (newly transformed) may be as small as 25 mm. Adult females can be much larger than the males. The tadpoles of northern leopard frogs are "giants of their kind" and may be up to 80 mm long and quite fat in the body.
In Alberta, leopard frogs may be confused with the wood frog. However, wood frogs are smaller, browner, have a dark mask around the eyes, and may have a light stripe down the middle of the back. The glandular ridges are low and usually NOT offset by light colours. Wood frogs are frequently seen in moist habitats in the parklands and boreal forest of Alberta where they occur in large numbers.
Habits
Northern leopard frogs are usually found in moist habitats along the edges of streams, springs, ponds and lakes. They like clear clean water in open or lightly wooded areas and rarely occur in dense forest. Their relatively large size helps them conserve water and this feature, along with their long legs and big feet, allows them to travel to fairly dry habitat, perhaps 0.5 km from water. In wet weather, or after a heavy dew, these frogs may be found even farther from water.
In
Alberta, northern leopard frogs are active from April to October. During
the day, they rest in wet areas and may even make shallow pockets in
the soil, which help them to absorb moisture and avoid predators. Basking
in the sun also raises their body temperature and enables them to digest
their food more rapidly. After sunset, the frogs become active and
either stay in water or go out onto land to hunt for food. If they
themselves are chased on land, they leap and hop in an erratic, zigzag
manner as they quickly return to water.
Anyone who has tried to catch up with one of these frogs knows how good they are at not being caught! They are most active in warm, wet weather or at dusk and dawn provided it is not too cool.
During the winter leopard frogs rest on the bottom of lakes or hide under stones in springs, streams, or rivers.
As long as there is enough oxygen in the water, these sites provide a stable environment in which the frogs can overwinter. However, these sites are not completely safe and some frogs may be eaten by fish or aquatic mammals such as mink and otter. In the spring (April or May), the frogs usually migrate a short distance to find warm shallow water where breeding occurs and eventually eggs are released. In Alberta, overwintering, spawning, and feeding areas are close together and often are in different parts of the same body of water.
Reproduction
In many ways leopard frogs, like all amphibians, lead a double life. They also replay the history of evolution as they change from a water-dwelling fishlike animal to one that lives on land and breathes air. All frogs begin life as eggs in water. These hatch into tadpoles that are superbly adapted for life in water. They have gills, a fishlike tail, and are streamlined with no appendages (legs and arms) sticking out from the body. These stages of the life cycle can exist only in water. During transformation to the adult form, a series of remarkable changes occur as the animal completely changes inside and out. The gills become lungs, legs appear at the four corners of the body, the tail disappears, the eyes migrate to the front of the head, and the mouth widens into a broad "froggy" grin. At this time, frogs also change from a plant-eating diet and digestive system to one that uses only insects and other "meat" sources.
In
Alberta, the cycle begins as male northern leopard frogs begin calling
in ponds, backwaters, beaver dams, marshes, and slow moving streams
in late April and early May. They are trying to attract females for
breeding purposes. The males may call during the day; however, calling
gets more persistent after dark as the intensive search for females
begins. In areas with high frog populations (unfortunately, not in
Alberta), the calling becomes so loud and intense that you can hardly
hear yourself think.
The voice of a leopard frog has been described as a low guttural snore lasting about 3 seconds followed by several clucking or grunting noises. Others describe it as sounding like a balloon full of air being rubbed the wrong way! The sound is interesting, to say the least.
In Alberta, most breeding and spawning occurs in May and is completed by early June. Each female may deposit up to 3000 eggs in a large flattened spherical mass of gray/black jelly. The eggs must remain in water to survive and so the egg masses are attached to underwater plants, logs, or rocks. Some masses are stuck to the bottom of the pond, marsh, or stream. The eggs hatch in 10 to 20 days and the tadpoles, so familiar to almost any child that has looked in a pond, swim away from the egg mass. The tadpoles develop slowly (compared to our other frogs) and transform into adults in late July or early August. Newly transformed frogs are often found in shallow water near the shore or in moist habitat along the shore.
Food
Tadpoles are herbivores and as such, they eat plants and algae. They graze on the delicate edges and tips of plants and on algae floating on the water or growing on rocks, logs, and larger plants found under water. A few dead tadpoles and other small dead invertebrates may be included in the diet.
Adult northern leopard frogs are opportunistic predators (meaning they will eat almost anything they can catch) and their diet includes a wide variety of animals encountered in their habitat. Although insects and other invertebrates are eaten most often, small vertebrates such as mice, small fish, and even young leopard frogs may be eaten. Most feeding activity occurs on land and at night. However, this does not prevent a frog from eating something that swims or flies past during the day!
Limiting Factors
The major problems for leopard frogs in Alberta are the small population size and the fragmented habitats in which they are found. Small populations are always at greater risk. What appears to be a minor change (such as draining a small wet area, allowing domestic animals to increase their use of a slough, or perhaps a small chemical spill) may result in catastrophic effects on a small population barely able to maintain itself.
In addition, most northern leopard frogs in Alberta now remain in small isolated wetlands surrounded by large dry areas that prevent successful dispersal of frogs. Because of this, the establishment of new populations in suitable habitat currently unoccupied by the species is unlikely. It is also difficult for current populations of leopard frogs to mix and for large numbers of males and females to find each other. They need a variety of habitats throughout their life cycle and the lack of diversity in the surroundings available to them further limits frog populations.
For reasons we do not understand, very few adult leopard frogs are seen in Alberta and large numbers of young are produced at only a few sites. Currently, we also do not know what role climatic changes play (or played) in the decline of leopard frogs. However, there is little doubt that the recent drought conditions experienced across southern Alberta contributed to loss of habitat suitable for frogs. Low oxygen levels during the winter or solid ice to the bottom of a pond can also cause significant losses to overwintering frog populations. In other words, changes or interference with critical habitats can limit the ability of any species to expand its range to new or formerly occupied habitat. For leopard frogs, permanent water bodies are required for successful overwintering. Spawning sites must remain wet long enough for tadpoles to develop and transform. Predation may be an important factor in limiting the number of frogs, particularly in the tadpole stage. Waterfowl, fish, aquatic insects, spiders, leeches, turtles, snakes, and other tadpoles, all are reported to eat leopard frog tadpoles. Similarly, adult frogs may be eaten by garter snakes, great blue herons, mink, otter and, perhaps, striped skunks. Currently, food for the frogs themselves is not a limiting factor.
Humans are also a major predator of frogs. Although it is illegal in Alberta, large numbers of leopard frogs have been and continue to be collected for food, bait, or for sale to biological supply houses. Even in Alberta, casual collecting, particularly by curious children, takes frogs away from the local population, often at the critical breeding time. Small pockets of isolated habitat are especially vulnerable to disturbance by humans. Scientists, naturalists and curious onlookers can trample vegetation and reduce cover along spawning ponds and thus contribute to deterioration of habitat for frogs.
It is not yet known whether disease may have contributed to the decline of leopard frogs in Alberta. It is unlikely, since the frogs disappeared so quickly. However, a bacterial disease condition called "red leg" can kill leopard frogs and has been reported in Alberta.
Management and Outlook
Without knowing the cause(s) of the decline, it is difficult to manage to avoid further problems. Current efforts are focused on finding existing populations of northern leopard frogs and trying to estimate their numbers and reproductive success. Since 1989, a poster campaign was initiated to ask the public to help determine the distribution and status of northern leopard frogs in Alberta. Information gained from this and other programs has helped to locate the few remnant populations. The information also will provide a key element in developing management programs needed to protect the species and, hopefully, allow its recovery. In order for northern leopard frogs to continue to survive in Alberta, protection of remaining populations and their habitat is critical. Removal of even small numbers of frogs can lead to the disappearance of a local population. The relationship between cattle and northern leopard frogs should be investigated so that destructive levels of cattle grazing and use of waterholes can be prevented. Draining of wetlands or other interference with groundwater levels should be avoided as well.
Survival of leopard frogs may be improved by captive breeding programs. This practice would increase the number of eggs and young produced and would also provide protection for the adults. In addition, young frogs (from one or two of the few populations where large numbers are produced) could be raised in protected facilities and released as adults into their former or new habitat. Reintroductions to key sites may provide the basis for natural recolonization of large areas of suitable habitat.
Research into the cause(s) of the losses in the species is necessary to avoid further decline. However, this must be done without undue disturbance of current populations.
In response to the decline in provincial populations, Natural Resources Service and the World Wildlife Fund (Canada) are working together to prepare a Provincial Management Plan for Leopard Frogs. This plan will outline a comprehensive management program aimed at conserving existing populations and habitats as well as identifying the actions necessary to restore populations to healthy viable levels. Natural Resources Service is also reviewing the legal status of leopard frogs to determine if additional protective regulations are needed.
A genuine concern over the decline of this species has resulted in increasing public and political awareness of the problem. Similarly, a worldwide concern over the disappearance of many amphibian species is fuelling the need to find out what happened (or is still happening).
With successful management it should be possible to increase both the number and range of leopard frogs in Alberta. Current populations are large enough to provide a nucleus from which the species can increase in the province. By working together, government agencies, nongovernment groups and private individuals can ensure a secure future for the northern leopard frog in Alberta.
For further information on the northern leopard frog, check out the Status of the northern leopard frog in Alberta report, contact your local Fish and Wildlife Division office or write to:
Wildlife Management Branch
Fish and Wildlife Division
Alberta Sustainable Resource Development
9915 -108 Street
Edmonton, Alberta
T5K 2G8
