Amphibians
Northern Leopard Frog
Lithobates pipiens · also called northern leopard frog, meadow frog, grass frog
The northern leopard frog is the frog a generation of students dissected — once so abundant across North America that nobody imagined it could become scarce. Then, through the 1970s and 80s, it quietly vanished from most of the American West.
Overview
Northern leopard frogs live in wetlands, wet meadows, and grassland across southern Canada and the northern and central US east of the Rockies. They are bright green or brown with rows of large pale-ringed oval spots. Unusually for a frog, they hunt far from water, ranging into damp fields at night to ambush beetles, flies, spiders, slugs, and small vertebrates — hence the old name “meadow frog.”
The classroom frog
For most of the 20th century this was the default dissection and physiology frog in North American schools and universities, collected wild by the million, and it was also used in early frog-based pregnancy tests.
The western collapse
Eastern and Midwestern populations remain strong, but the leopard frog has disappeared from most of its former western range since the 1970s. The likely causes stack up: wetland loss, the chytrid fungus, introduced fish and bullfrogs, the limb-deforming parasite Ribeiroia, pesticide runoff, and UV exposure.
Status
The northern leopard frog is globally Least Concern but decreasing, and locally Endangered or gone across the western US and parts of Canada.
Northern Leopard Frog: common questions
Why has the northern leopard frog disappeared from so much of the West?
There is no single cause; researchers point to a combination that hit western populations especially hard from the 1970s onward. Draining and degradation of the shallow wetlands and wet meadows it needs removed habitat. The amphibian chytrid fungus spread through mountain populations. Introduced sport fish and American bullfrogs, stocked widely in the West, eat the tadpoles and adults. The parasitic flatworm Ribeiroia, whose cycle is boosted by nutrient runoff, infects tadpoles and causes severe limb deformities. Add pesticide and fertiliser runoff, road mortality, and possibly increased ultraviolet exposure at altitude, and many isolated western populations simply could not persist. Eastern populations, in larger and better-connected wetland networks, have mostly held on.
How do you tell a leopard frog from a pickerel frog?
They are very similar, both slender green-brown frogs with dark spots and prominent ridges along the back, and they overlap in range. The key differences are the spots and the inner thigh. A northern leopard frog has rounded or oval spots scattered somewhat irregularly, each ringed with a pale halo, and plain white or pale skin on the inner thighs. A pickerel frog has more squarish spots arranged in two neat parallel rows down the back, and bright yellow or orange wash on the inner thighs and groin, visible when the legs are extended. The pickerel frog also has mildly toxic skin secretions that make it distasteful, which the leopard frog lacks.
Were leopard frogs really used for pregnancy tests?
Yes. Before modern chemical pregnancy tests, one method (the "frog test," developed from work on the African clawed frog and applied to other frogs including leopard frogs) involved injecting a woman's urine into a female frog; if she was pregnant, hormones in the urine would cause the frog to release eggs within about a day. Male frogs of some species could be used similarly, releasing sperm. Leopard frogs and clawed frogs were kept in large numbers in hospitals and labs for this purpose from the 1930s into the 1960s. The African clawed frog became the standard, and its global shipment for this trade is one hypothesis for how the chytrid fungus was spread worldwide.
Are northern leopard frogs a good pet or classroom animal?
They can be kept, and captive-bred individuals are available, but wild collection for the pet and biological-supply trade is now discouraged and in some jurisdictions illegal because of the species' decline in the West. They need a semi-aquatic setup with clean, cool, dechlorinated water and a land area, plenty of cover, and a varied live-insect diet, and they are active, nervous frogs that can injure their snouts jumping into enclosure walls. For dissection and teaching, most institutions have moved to captive-bred frogs, alternative species, or non-animal models, partly for conservation reasons and partly because wild-caught frogs carry parasites and disease.