Nature Notes: Northern Leopard Frog
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In early March many readers would like to be ensconced in warm climates well to the south of Maine. You toughed it out during January and February and you need a break. Suppose you were stuck under the ice and relief was still weeks away! For aquatic creatures like the northern leopard frog, there may be no escape until ice out.
The only contact some of you have had with the handsome northern leopard frog was in high school science class when you had to dissect one of the little creatures. That probably ruined your lunch, helped along by the lingering smell of formaldehyde on your hands! If you have a taste for the finer things in life, you certainly have enjoyed a tasty plate of “cuisses de grenouilles” or frogs legs lightly cooked in butter and garlic.
Of 39 amphibians known to inhabit Maine, seven are frog species. Some of these live on land and spend the winter dug into the earth to survive. The northern leopard frog is commonly found hibernating or swimming slowly on the bottom of a pond, marsh or slow stream. Not surprisingly, the frog favors waters that have sufficient depth to ensure open water under the ice cap. As a cold-blooded creature, the frog has little choice but to endure bitter cold for months on end. It even has the amazing ability to survive despite its organs freezing solid, thanks to a high glucose concentration that prevents fatal damage until the animal thaws out and resumes respiration and blood flow.
The northern leopard frog (Lithobates pipiens) is considered one of the family of true frogs, amphibians with slender bodies, pointed snouts, powerful legs and smooth, moist skin most often found in or near water. It is a fairly large frog, up to four inches in length not counting the legs. The background color varies from light green to dark brown, with two to three unevenly spaced rows of irregular dots on the back. Each dot is ringed in a lighter color. At the back of each eye is a dorsolateral fold in light colors that runs all the way to the hip. The eye is colored gold and the belly is white or pale green. The legs end in large webbed feet that the frog uses to jump on prey and to swim rapidly away from predators such as herons, snakes and raccoons.
These frogs favor permanent bodies of water such as ponds, swamps, marshes and slow-moving streams but will make use of seasonal vernal pools for breeding. They are normally found in areas with plentiful aquatic vegetation so they have cover from predators. Their home territory includes southern Canada through Kentucky and much of the southwest but for arid regions. As a result of pollution, invasive fish species, drought and the loss of millions of acres of wetlands in recent decades the population of leopard frogs has dropped by 50 percent or more in some areas, particularly in the more arid western parts of its range. They are now listed as a species of special concern in Maine, while some provinces and states rank them as threatened or endangered.
Once the warm weather of spring has arrived, you will hear the rasping snore-like call of male frogs hoping to attract a mate. When a willing female arrives, the male grabs onto her back and rides around in the water with her. In a couple of days, the female releases up to 6,000 eggs in a gelatinous mass several inches in diameter, which causes the male to release his sperm for fertilization. Within a few days the tiny tadpoles begin to emerge from their egg sacks, swimming around and feeding to sustain their rapid growth. The tadpoles are light brown in color with black spots, and are fully developed within three to four months, having absorbed the tail of their infancy.
While tadpoles are mostly vegetarian, living off algae and other aquatic plants, they are also cannibalistic. Mature frogs are opportunistic feeders and will take a wide variety of food as it comes within reach of the long sticky tongue. The menu includes moths, flies, crickets, worms, snails, small birds, salamanders, snakes and frogs. In summer, the frogs can be found in grassy areas feeding on grasshoppers, mosquitoes, spiders and beetles.
While the northern leopard frog has long been popular as a pet, its historic availability has made it a favorite for numerous scientific studies of potential importance for human health. Thanks to those long, powerful legs it has served as a model for biomechanical and muscle physiology investigations. Neuroscientists have made discoveries about neurons by studying the frog’s sciatic nerve fibers. In recent years two enzymes from the frog’s eggs (ranpirnase and amphinase) have entered clinical trials as potential cancer therapies.
This delicate frog serves to warn us about the importance of water quality. Studies are ongoing in the hope of understanding the causes of the population decline of the northern leopard frog and other creatures that use similar habitats. Some actions we can take are well documented. Avoiding the runoff of agricultural fertilizers helps to prevent excess levels of phosphates and nitrogen in fresh water. The prohibition on certain lawn chemicals in Harpswell can prevent water pollution, along with limiting the use of road salt near wetlands and streams. The efforts of the Maine Department of Inland Fisheries & Wildlife to stop the introduction of invasive fish species to our waters is also important since large mouth bass and northern pike are known to feed on frogs when available.
The next time you have the chance to walk along a pond or marsh listen carefully for the call of the northern leopard frog and hope that they will survive for the long haul in healthy wetland environments. That is an imperative all of us can support.