The weight of a northern leopard frog ranges from 0.5 to 2.82 ounces (16 to 80 grams).

The northern leopard frog is a smooth-skinned green, brown or sometimes yellow-green frog covered with large, oval dark spots that have a light halo or border. Northern leopard frogs have a white stripe on the upper jaw and the dorsolateral folds, which are paired, glandular ridges that run along each side of the back from behind the eyes to the rear, are light cream to yellow and are continuous and not broken posteriorly. The belly is white to cream-colored and the posterior thigh has a light background color with dark spots. Tadpoles, the larval stage in the lifecycle of the frog, are dark green to brown above with metallic flecking, and a cream to white translucent underside.

The typical breeding call is a prolonged snore, followed by a series of stuttering croaks or chuckles that tend to accelerate towards the end. These vocalizations may be interspersed with chuckling sounds.

The northern leopard frog is a slim frog with a pointed snout, or nose, and the tympanum, or eardrum, is round and approximately equal in diameter to the eye. During the breeding season, males have enlarged or swollen thumbs, which is their innermost digit, on their forefeet and vocal sacs are not apparent except when the frog is calling. Females average slightly larger than males.

Measurements

Adult body lengths (snout-vent): 2 to 4.5 inches in length (5 to 11 centimeters)
Subadult or recently metamorphosed frogs: 1 to 2 inches in length (2 to 5 centimeters)
 

Northern leopard frogs breed in a variety of aquatic habitats that include slow-moving or still water along streams and rivers, wetlands, permanent or temporary pools, beaver ponds and human-constructed habitats like earthen stock tanks and borrow pits. Successful breeding areas typically do not contain predaceous fish or other predators. Emergent vegetation, like sedges and rushes, are important features of breeding and tadpole habitats and tadpoles occur in backwaters and still pools.

Subadult northern leopard frogs typically move from breeding areas to feeding sites along the borders of larger, more permanent bodies of water, as smaller frogs are closely tied closely to water. Recently metamorphosed frogs will move up and down drainages and across land in an effort to disperse from breeding areas. As they move, these frogs may disperse more than 0.5 miles (800 meters) from their place of metamorphosis. Streams are an important corridor for dispersing juvenile frogs and vegetated drainage ditches may facilitate connectivity between seasonal habitats. In some areas of the western United States, subadults may remain in the breeding habitat within which they metamorphosed.

In addition to the breeding habitats, adult northern leopard frogs require stream, pond, lake or river habitats for overwintering and upland habitats adjacent to these areas for summer feeding. In summer, adults and juveniles commonly feed in open or semi–open wet meadows and fields with shorter vegetation, usually near the margins of water bodies and seek escape cover underwater. Post-breeding summer habitats do not include barren ground, open sandy areas, heavily wooded areas, cultivated fields, heavily grazed pastures or mowed lawns.

During winter, northern leopard frogs hibernate underwater in ponds, lakes or on the bottom of deeper streams or waters that do not freeze to the bottom and that are well-oxygenated. Northern leopard frogs are intolerant of freezing and of waters that have severely reduced or complete loss of dissolved oxygen. If these conditions occur during hibernation, death of northern leopard frogs is likely.

Northern leopard frog tadpoles are predominantly generalist herbivores, or plant eaters, typically eating attached and free-floating algae; however they may feed on dead animals. Adult and subadult frogs are generalist insectivores, or insect eaters, that feed on a variety of terrestrial invertebrates like insect adults, larvae, spiders and leeches. In addition, adult northern leopard frogs will also prey upon small northern leopard frogs, birds and snakes.

Northern leopard frogs generally feed on land, out of the water, and are active in the nighttime, as well as during the daytime. They dive in the water to avoid dangers when startled or scared. During the summer months, these frogs frequently roam around grasslands and woodlands that are located far away from their main habitat regions

Temperature plays an important role in both the springtime migratory and breeding behaviors of northern leopard frogs. When ambient air temperature is greater than or equal to 50 degrees Fahrenheit (10 degrees Celsius), northern leopard frogs move from their overwintering sites to their breeding sites.
 

The northern leopard frog is an amphibian, a cold-blooded vertebrate that spends some time on land, but must breed and develop into an adult in water. As such, northern leopard frogs are ectothermic, or incapable of generating their own body heat. They have highly permeable skin, which allows for rapid passage of water and gases so that they can use their external environment to regulate body temperature and moisture loss. 

The eggs of the northern leopard frogs take about two to 17 days to hatch into small tadpoles. Time taken for hatching depends on the temperature. The length of time required for metamorphosis, or the development of the aquatic tadpole to a frog, is variable. Depending upon temperature, it may take three to six months from time of egg-laying to complete their life cycle. The female frogs reach sexual maturity by the time they are 2 or 3 years of age. The males attain reproductive maturity by around 1 or 2 years of age.
 

As soon as males leave overwintering sites, they travel to breeding ponds and call in shallow water. Breeding typically occurs during a short period in the spring beginning in early April; at higher elevations and more northern latitudes, the onset of breeding is late-April to early May. Male frogs attract females by calling from specific locations within a breeding pond when temperatures are close to 68 degrees Fahrenheit (20 degrees Celsius) or more, with several males typically calling together to form a chorus. Frogs lay eggs within breeding habitats two to three days following the onset of chorusing in non-acidic, shallow, still water that is roughly 4 to 26 inches deep (10 to 65 centimeters). These breeding areas are exposed to sunlight and are usually attached to emergent vegetation just below the water surface. Egg masses may include several hundred to several thousand eggs and the female frog deposits them in a tight, oval mass. The eggs hatch into tadpoles, which are the ephemeral, feeding, non-reproductive, completely aquatic larvae in the life cycle of the frog.

The average life span in the wild of a northern leopard frog is two to four years.

The northern leopard frog is in the family Ranidae, the true frogs, and is one of about 28 species within the genus Rana, that occur in North America.